Displays

Magmatic reservoirs located in the upper crust have been shown to result from the repeated intrusions of new magmas, and spend most of their dwelling time as cristal-rich mush. Despite advances in our understanding of the physical processes that may occur in a magma reservoir, the architecture of the intrusion into a mush remains poorly constrained. The geometry of such intrusions, however, may greatly affect the thermal and compositional evolution of the magmatic reservoir. We performed numerical simulations coupling computational fluid dynamics with the discrete element method to identify the geometry and emplacement dynamics of an intrusion in a mush, and the relevant physical parameters controlling it. Our results show that the intrusion geometry is to first-order controlled by the density contrast between the melt phases of the intruded and resident materials rather than the bulk density contrast as usually considered. When the melt phase of the intruded materials is denser than that of the host, the intrusion ponds at the base of the mush and is emplaced as a horizontal layer. However, when the intruded melt is lighter, the intrusion rises through the mush forming a Rayleigh–Taylor instability. In the absence of density contrast between the two melts, the intrusion fluidizes the host crystal network and slowly ascends through the mush. The presence of a viscosity contrast between the intruded and resident materials as well as the intrusion injection velocity were found to have less of an influence on the final geometry and intrusion dynamics in mush. In addition, we analyzed the eruptive sequence of well documented eruptions involving an intrusion as a trigger, and found good agreement with our modeling results. This study sheds light on the importance of explicitly considering granular mechanisms and the relative motion between the crystals and the melt phase when studying the physical processes of magmas and mush.

How to cite: Carrara, A., Burgisser, A., and Bergantz, G.: The architecture of an intrusion in magmatic mush, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1984, https://doi.org/10.5194/egusphere-egu2020-1984, 2020.

Chemical differentiation requires the relative motion of melt and crystals during multicomponent phase change.  Compaction is often invoked as the mechanism that allows this in crystal rich ‘mush’ reservoirs.  Compaction is a term used broadly to describe the coupled processes of buoyancy-driven melt flow through permeable crystalline matrix and matrix deformation in response to the extraction or accumulation of melt.  One key challenge to melt segregation models that invoke compaction is that textural evidence for crystal deformation in the residual material left after melt extraction is largely absent (Holness, 2018).

Here, we test the relative contribution of compaction and reactive flow to melt fraction change in crustal mush reservoirs using a modified version of the reactive flow model of (Solano et al., 2014).  Reactive flow changes melt, solid and bulk composition and is essential to chemical differentiation in crustal mush reservoirs but has been largely neglected in models of melt segregation.  We find that melt fraction changes in response to reactive flow can be as important as those caused by compaction, irrespective of the phase behaviour tested.  That compaction may account for only half the melt fraction change observed in mush reservoirs could help to explain why textural evidence for mush deformation remains enigmatic.

References
Holness, M. B. (2018). Melt segregation from silicic crystal mushes: a critical appraisal of possible mechanisms and their microstructural record. Contributions to Mineralogy and Petrology, 173(6):48.
Solano, J. M. S., Jackson, M. D., Sparks, R. S. J., and Blundy, J. (2014). Evolution of major and trace element composition during melt migration through crystalline mush: Implications for chemical differentiation in the crust. American Journal of Science, 314(5):895–939.

How to cite: Hu, H. and Jackson, M.: Compaction versus reactive flow: How does melt fraction change in crustal mush reservoirs?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5715, https://doi.org/10.5194/egusphere-egu2020-5715, 2020.

The Takidani pluton is a Pleistocene intrusion representing a nearly 2 km-thick shallow level magma reservoir located in the Central Japan Alps. The pluton, which is associated with caldera-forming eruptions, is vertically zoned and composed of six distinct lithological units ranging from hornblende-bearing granodiorite to biotite granite, with silica content varying from ca. 65 to 76 wt.%. In its upper part, the intrusion is characterized by the gradual transition between equigranular and porphyritic granodiorites. Textural and geochemical evidence indicates that the porphyritic unit represents a lens of residual melt extracted from the underlying equigranular granodiorite (Hartung et al., 2017).

The time and tempo of melt extraction is determined using both high precision and high-spatial resolution U-Pb zircon geochronology, performed by CA-ID-TIMS and SIMS respectively. High precision ²⁰⁶Pb/²³⁸U zircon ages for the two units are similar, with grains from both rocks exhibiting an age spread as large as 200-300 kyr, from ca. 1.2 to 1.5 Ma. In-situ U-Pb dating obtained by SIMS using a spot size of 20 μm reveal systematic age difference between cores and rims, highlighting two events of zircon crystallization with no substantial difference between the two units. Zircon cores from the porphyritic and equigranular granodiorites give identical ages at ca. 1.45 ± 0.06 Ma. Spot U-Pb ages from magmatic rims range between 1.29 and 1.07 Ma, with a peak of the distribution density at around 1.20 Ma.

This information, combined with Zr saturation temperatures and phase equilibria modelling, suggests that zircon cores crystallized from the magma reservoir before rheological locking and melt segregation were achieved. The segregation of the interstitial melt from the mush took place in the ca. 250 kyr between the two events of zircon crystallization. The extracted residual melt was depleted in Zr and carried entrained crystals of plagioclase and zircon from the mush. The low Zr content of this melt hindered zircon crystallization that was only possible after a time lag of 250 kyr. The youngest event of zircon crystallization at ca. 1.2 Ma was contemporaneous in the segregated melt and in the underlying mush.

Reference: Hartung, E., Caricchi, L., Floess, D., Wallis, S., Harayama, S., Kouzmanov, K., Chiaradia, M., 2017. Evidence for residual melt extraction in the Takidani Pluton, Central Japan. J. Petrol.58, 763–788.

How to cite: Farina, F., Rubatto, D., Hartung, E., and Caricchi, L.: Time and tempo of melt segregation from a magma mush: evidence from the Takidani pluton (Japan), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8115, https://doi.org/10.5194/egusphere-egu2020-8115, 2020.

The mush-magma transition (MMT) marks a profound change in rheological properties between two of the principal magmatic reservoirs that constitute a magmatic system. Mush behaves as a solid and its rheology is largely dominated by the deformation of the crystals network whereas magma is a liquid and has a rheology dominated by melt.

To better understand the solid-liquid transition in such crystal-rich systems, we present here an experimental study using mixtures of aqueous superabsorbent polymers (SAPs). SAPs are constituted of polymer grains that in water can swell up to 100 times and form gel grains whose size can be controlled by controlling the size of the initial powder. Particle fractions between 60% and 80% are easy to reach, making this system a promising analog of mush. The non-Newtonian rheology of the mixture of water and touching grains combines viscous, elastic and plastic aspects and can be characterized using the free-fall of spheres of different diameters and densities. 

We observe five different regimes of motion for the settling of a sphere: (1) A linear regime where the sphere has a rapid and linear fall and reaches a constant terminal velocity. (2) An irregular regime where the sphere’s velocity fluctuates around a constant value. (3) A stop&go regime where periods of no-motion and periods of irregular falls follow one another. (4) A slow fall regime where the sphere’s velocity progressively decreases in a logarithmic way. And (5) a no-motion regime when spheres are not buoyant enough to overcome the yield stress of the mixture or are too small compared to the grain size. So, the Yield number (ratio of the yield stress to the sphere buoyancy-induced stress), critical value Yc above which there is no motion decreases as the sphere to grain diameters ratio becomes smaller than 2. This enlarges the domain of conditions under which the mush strength will lead to the entrapment of the intruder. Moreover, the mixture structure strongly affects the path that a buoyant melt pocket can follow through the mush, and the time it spends motionless. The latter will increase the time available for reactions between melt and surrounding crystalline matrix.

How to cite: Sgreva, N. R., Davaille, A., Kumagai, I., and Kurita, K.: Probing the characteristics of mush-magma transition: insights from laboratory experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9113, https://doi.org/10.5194/egusphere-egu2020-9113, 2020.

Crystals and their hosted melt inclusions are key witnesses of the processes occurring in the magma plumbing systems. Crystal zoning patterns can inform us of the range of magmatic environments, magma interactions, timescales, and of the likely processes that lead to eruption. A complementary view of the plumbing system is provided by detailed studies of melt inclusions. The variability of major, trace, and volatile element concentrations of the inclusions informs us of the heterogeneity and architecture of the system (e.g. minimum depth of magma storage). Coupling of the storing depth with the times of magma movement allows us to propose links with the eruptive behavior and different phases of eruptions, and especially with the monitoring data describing the unrest preceding the eruptions. Here we report the magmatic processes associated with caldera collapse and lateral magma transport recorded in olivine crystals and their melt inclusions on selected samples from the largest historical eruption of Piton de la Fournaise (April 2007).

The olivine crystals and melt inclusions from the 2007 Piton de la Fournaise eruption record shallow storage depth and re-equilibration related to lateral magma movement towards the surface. Most crystals we have studied likely grew during the pre-eruption and eruption period (e.g. in about 3 months) from the basalt that migrated from, and was temporarily stored at shallow depths (about 0.5-1 km a.s.l.), although deformation source modeled from tilt data indicates an initially deeper magma source (ca. 3 km b.s.l.). These observations suggest that magma arrival at shallow depth pressurized the pre-existing melts and led to eruption, but we have no evidence of physical interaction between the two. The zoning and timescales derived from olivine crystals record fast crystal growth, creation of a crystal mush and subsequent lateral transport, eruption, and caldera collapse. Timescales deduced from Fe/Mg, Ni and Ca in olivine rims and towards melt inclusions are relatively short (from 3 to 60 days) and agree with the changes in monitoring data. Degassing and re-equilibration of H⁺ of melt inclusions via diffusion occurred during the depressurization of the shallow system and caldera collapse and vary between a few hours to six days (depending on the crystallographic direction).

How to cite: Albert, H., Costa, F., Di Muro, A., Herrin, J., Métrich, N., and Deloule, E.: Crystal mush formation, timescales, and unrest: a combined study of olivine crystals and their hosted melt inclusions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9649, https://doi.org/10.5194/egusphere-egu2020-9649, 2020.

The Ciomadul in eastern-central Europe is a high-K dacitic volcanic complex characterized by long quiescence (several 10’s of kyr) periods between eruptions and a long-standing (over several 100’s of kyr) magmatic plumbing system. Following intermittent lava dome extrusions from 1 Ma to 360 ka, a more intense eruption stage occurred between 160 ka and 30 ka with initial lava dome building period followed by dominantly explosive eruptions. The volcano has been again in a long quiescence stage since 30 ka, although results of geophysical studies suggest presence of a subvolcanic magma body with significant melt fraction. In order to constrain better the rejuvenation mechanism of such long-dormant volcanic complex, a more thorough understanding of the nature and dynamics of the magmatic plumbing system and the reason of eruption triggers is required. In spite of the homogeneous dacitic bulk rock composition and similar mineral assemblage, each eruption product shows subtle differences in mineral-scale features. Here, we present examples showing how basaltic magma played a role in the genesis of dacite as well as triggered eruption in a timescale of days to weeks.
The Ciomadul dacites are crystal rich and contain plagioclase, amphibole and biotite as main phenocrysts in addition to accessory phases of apatite, titanite and zircon. Several dacitic lava dome rocks formed between 90 and 160 ka in Ciomadul contain also high-Mg minerals such as olivine, clinopyroxene and orthopyroxene. Cr-spinel inclusions occur in olivine, orthopyroxene and also in high-Al amphiboles. Textures and the high Mg-numbers (0.85-0.91) of these mineral phases suggest that they can be considered as antecrysts with magmatic origin having crystallised from primitive mafic magmas, which were involved in the evolution of the subvolcanic magma storage system. Zoning pattern and trace element content of plagioclases and amphiboles clearly show interaction between dacitic and basaltic magmas. In addition, these high-Mg minerals allow us to have an insight into the origin of the primary magmas as well as the dynamics of the mafic magma body stalled at the crust-mantle boundary.
The compositions of the Cr-spinel inclusions significantly differ from those of the spinels in the 600-1200 ka alkaline basalts of the nearby Perşani Volcanic Field and their high Cr-numbers indicate a depleted mantle source for the parental magma. Such Cr-rich spinels are common in high-K magmas originated in metasomatized lithospheric mantle with depleted, harzburgitic lithology. Compositional zoning of clinopyroxenes indicates several recharge events by high-Mg and high-Cr basaltic magmas. Mafic magma batches repeatedly ascended and emplaced below the upper crustal felsic crystal mush and formed a dynamic interface between these two magmas. The mafic components remained in plastic state allowing thorough mixing of the mafic and felsic mineral cargos during turbulent convective magma stirring, whereas farther from this interface, remelting of the crystal mush occurred due to reheating and volatile transfer into the interstitial melt. The series of mafic magma injections increased the eruptible magma volume and occasionally led to eruptions.
This research was financed by the Hungarian National Research, Development and Innovation Fund (NKFIH) within K116528 project.

How to cite: Harangi, S., Petrelli, M., Kiss, B., Bachmann, O., Seghedi, I., Ntaflos, T., Jankovics, É., and Lukács, R.: The role of basaltic magma in the petrogenesis of the Late Pleistocene Ciomadul dacite, Romania, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19199, https://doi.org/10.5194/egusphere-egu2020-19199, 2020.

The crustal-scale magmatic systems of Andean-style subduction zones produce thick volcanic deposits and abundant plutons emplaced into the upper crust. They can also result in the formation of spatially- and temporally-restricted, economically-important porphyry Cu deposits. Understanding the magmatic and tectonic processes acting within an arc segment, including changes in the fractionating assemblage, subduction angle, chemistry of slab-derived melts or water content, is essential to develop and refine quantitative models for the formation of these deposits. Specific geochemical signatures (e.g. elevated Sr/Y) are associated with magmas that source the metals and volatiles to form porphyry deposits based on empirical studies. However, it is unclear whether this geochemical signature is the result of geologically rapid processes resulting in sudden shifts in magma chemistry or whether they are the result of protracted changes within the crustal-scale magmatic system over extended timescales.

In this study we examine the magmatic evolution of the Rio Blanco-Los Bronces district, ~30 km northeast of Santiago, Chile, which is host to the Earth’s largest resource of Cu. Eocene to Early Miocene volcanic rocks were intruded by the Miocene San Francisco Batholith that, in turn, partially hosts intrusions related to the Late Miocene to Early Pliocene Rio Blanco-Los Bronces porphyry deposit cluster. We apply a combination of whole-rock and zircon geochemistry, isotopic tracing and LA-ICP-MS U-Pb geochronology to the intrusive rock suite of the district to provide temporally- constrained geochemical information over the entire duration of batholith assembly and ore formation.

U-Pb geochronology reveals incremental assembly of the San Francisco Batholith by individual magma batches over >13Myr (~17 – 4 Ma), with ore formation occurring in discrete pulses in the last 3 Myr before cessation of intrusive activity within the district. Temporally-resolved whole-rock major element chemistry shows that the progressively-emplaced magmas were not sourced from a common, continuously differentiating, lower crustal magma reservoir. Evolving trace element signatures over the recorded timescale indicate that magmas were sourced from progressively deeper fractional crystallisation reservoir(s) that exhibited increasing water contents. The geochemical evolution recorded over the entire investigated 13 Myr timescale could reflect geodynamic changes linked to the ingression of the subducting Juan Fernandez ridge from the north. However, within this continuous evolution, the most prominent geochemical shifts occur over a much shorter timescale of a few Myr, directly preceding economic ore-formation, implicating an additional mechanism for controlling the metallogenic potential of the magma source.

How to cite: Large, S., Buret, Y., Knott, T., and Wilkinson, J.: Petrochronology resolves the multi-Myr crustal scale magmatic evolution of an arc segment resulting in porphyry copper formation , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20757, https://doi.org/10.5194/egusphere-egu2020-20757, 2020.

Steady-state volcanic activity implies equilibrium between the rate of magma replenishment and eruption of compositionally homogeneous magmas, lasting for tens to thousands of years in an open conduit system. The Present-day activity of Stromboli volcano (Aeolian Islands, Southern Italy) has long been recognised as typical of a steady-state volcano, with a shallow magmatic reservoir (highly porphyritic or hp-magma) continuously refilled by more mafic magma (with low phenocryst content or lp-magma) at a constant rate and accompanied by mixing, crystallisation and eruption. The lp-magma is erupted only during more violent explosive events (paroxysms), which usually occur at intervals of a few years. However, the two most recent paroxysms occurred at very short timescales on 3 July and 28 August 2019 offering the unique opportunity of obtaining crucial information on the current magma dynamics of Stromboli.

Albeit the plumbing system shows such uniformity, clinopyroxene phenocrysts exhibit marked chemical heterogeneities and complex textures caused by continuous lp-hp magma mixing as well as antecryst recycling from different mush portions. The compositional zoning in clinopyroxene provides essential information on pre-eruptive magma dynamics, indicating multi-stage crystallization across the lp-hp-reservoirs, where diopsidic compositions are markers of more primitive, high-T magmas injecting into shallow, low-T domains of the plumbing system. By comparing clinopyroxene texture, chemistry and residence times from the Present-day eruptions with the previous Post-Pizzo activity, we conclude that a distinct phase in the life of Stromboli volcano commenced after the violent 2003 paroxysm. Our observations suggest there are more efficient mechanisms of mush disruption and cannibalization, in which old diopsidic antecrysts are continuously remobilized and transported by the lp-magmas permeating the mush. The disappearance of diopsidic recharge bands within augitic overgrowths indicates that over time, magmatic injections feeding the persistent Present-day activity are more intensively mixed and homogenized prior to eruption.

How to cite: Petrone, C. M., Di Stefano, F., Gertisser, R., Mollo, S., Tommasini, S., Del Bello, E., Andronico, D., Scarlato, P., Giacomoni, P., and Coltorti, M.: Significant changes in the magma dynamics of Stromboli steady-state volcano recorded by clinopyroxene crystals., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18109, https://doi.org/10.5194/egusphere-egu2020-18109, 2020.

Plinian eruptions are the most hazardous yet enigmatic style of volcanism at basaltic systems. The low viscosity of basaltic magma should preclude its fragmentation; however, there are several recognised examples of basaltic Plinian activity. Historical eruptions of Masaya caldera, Nicaragua; Etna, Italy (122 BC); and Tarawera, New Zealand (1886) have ejected > 1 km³ of material. The Las Sierras-Masaya volcanic complex (Masaya caldera) has produced several basaltic Plinian eruptions, yet currently exhibits low explosive-effusive activity. This volcano has erupted chemically homogeneous magmas over at least the past 6000 years, which suggests that this significant difference in eruptive style is not attributable to a compositional change. Therefore, the cause of increased explosivity at Masaya caldera remains uncertain. 

We present new measurements of major, trace and volatile elements in basaltic Plinian eruption products from the Fontana Lapilli (60 ka) and Masaya Triple Layer (2.1 ka) eruptions of the Las Sierras- Masaya volcanic complex. We use our data in rheological and thermometric models to define the pre- and syn-eruptive conditions that favour highly explosive activity. We then combine our petrological data with a numerical conduit model to constrain the pre-eruptive condition of the magma reservoir and simulate the conduit processes, to understand the magmatic conditions that promote fragmentation during magma ascent. The common physico-chemical magmatic conditions that promote basaltic Plinian activity at Masaya are high microlite crystallinity, moderate storage temperatures and a low initial H₂O concentration. Our combined approach greatly improves our general understanding of explosive basaltic activity and provides new insight into the effusive-explosive transition of the highly hazardous Las Sierras-Masaya system.

How to cite: Bamber, E. C., Arzilli, F., Polacci, M., La Spina, G., Petrelli, M., Hartley, M. E., Di Genova, D., Fellowes, J., Chavarría, D., Saballos, J. A., De' Michieli Vitturi, M., and Burton, M.: Understanding basaltic Plinian activity at Masaya caldera, Nicaragua, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9513, https://doi.org/10.5194/egusphere-egu2020-9513, 2020.

Activity at Santiaguito volcanic dome complex started in 1922 with the continuous eruption of crystal-rich dacitic-andesitic lavas, which over the course of the last century, constructed a series of four domes and were host to frequent minor explosions. In 2016, a drastic shift in activity occurred with an 8-months period of heightened explosion intensity. We present records of textural and compositional variations in plagioclase, orthopyroxene and plagioclase-hosted melt inclusions of a series of ash and ballistic samples erupted and collected in-situ between 2015 and 2019 to reconstruct the magmatic processes associated with such shifts in activity.

Plagioclase phenocrysts show a wide range of compositions (An_90-35) and can be grouped into three populations based on compositional and textural variations: crystals with resorbed albite-rich cores (An_35–40), anorthite-rich cores (An_85–90) and patchy zoned cores (An_50–85). All plagioclase crystals contain homogenous rims of An₅₀ that are marked by an increase in Fe content from about 3000 to 5000 ppm and a higher Mg content (of up to 300 ppm) towards the rim. Orthopyroxene phenocrysts show constant enstatite compositions from core to rim (En_68-70). However, rims are relatively enriched and depleted in Ti and Mn contents respectively. Plagioclase-hosted melt inclusions are found in reversely zoned crystals, in crystal rims and between glomerocrysts. Irregularly shaped melt pockets are frequently observed in patchy zoned cores. Melt inclusions overall range in silica content from 71 to 78 SiO₂ wt.% (anhydrous) and are marked by relatively high TiO₂ and K₂O contents.

Melt and mineral compositions and textures suggest that a shallow magma storage zone currently exists below Santiaguito volcanic dome complex. Pressure estimates of plagioclase-hosted melt inclusions yield an average of about 150 MPa (± 50 MPa) using rhyolite-MELTS indicating magma storage at depth of about 4 to 8 km. The observed increase in Fe, Mg and Ti contents in the rims of the plagioclase and orthopyroxene phenocrysts and microlite crystals are consistent with recharge of new magma into the upper crust, which was likely responsible for the drastic shift in eruption dynamics at Santiaguito volcanic dome complex in 2015-2016.

How to cite: Hartung, E., Wallace, P. A., Von Aulock, F. W., Hornby, A., and Lavallée, Y.: Shifting eruption dynamics: Constraints from mineral chemistry and plagioclase-hosted melt inclusions at Santiaguito volcanic dome complex, Guatemala, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11154, https://doi.org/10.5194/egusphere-egu2020-11154, 2020.

Lithium (Li) is one of the fastest diffusing elements in most geological media and so has the potential to provide information about processes occurring on timescales too short to be captured by other proxies.  These processes may be of fundamental importance both in terms of understanding what happens during explosive volcanism and for defining where lithium, an element of increasing economic importance, ends up.  To investigate the fate of Li, we studied in detail the 1.30 Ma Mesa Falls Tuff (MFT) from the Yellowstone volcanic field (USA).  MFT is a typical rhyolite of the Yellowstone system containing an anhydrous mineral assemblage of sanidine, quartz, plagioclase, clinopyroxene, fayalite, orthopyroxene and accessory phases.  We focussed on plagioclase crystals that have a strong gradient in Li contents from cores at ~25 ppm to rims with ~ 5 ppm.  This notable decrease in Li abundance is decoupled from changes in other major and trace elements.  δ⁷Li values measured by fs-LA-MC-ICPMS in the plagioclase crystals reveal that cores are about 5 ‰ lower than rims.  Taken together, the Li abundance and isotopic data make a compelling case for the plagioclase attempting to react to a sudden change in Li abundance in the surrounding melt.  Diffusion modelling of these gradients indicates that this sudden Li drop in the melt occurred over timescales of tens of minutes prior to quenching.  The volatile behaviour of Li implied by this result finds support in Li concentrations measured in quartz-hosted melt inclusions that reach 400 ppm while groundmass glass Li contents are much lower (36-55 ppm).  While equilibrium fractionation of stable isotopes is minimised at high temperatures, the large-magnitude, rapid loss of lithium from the melt phase may allow kinetic isotopic fractionation to occur, as recorded in the plagioclase crystals.  With glass / groundmass both volumetrically dominant and the main repository of Li in virtually all volcanic deposits, further consideration of how syn-eruptive processes may affect the bulk Li identity of a sample is warranted.        

How to cite: Ellis, B., Neukampf, J., Laurent, O., Steinmann, L., Weyer, S., Magna, T., Ubide, T., and Bachmann, O.: Plagioclase as a witness of syn-eruptive degassing in rhyolitic magmas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8886, https://doi.org/10.5194/egusphere-egu2020-8886, 2020.

Aiming to improve the current knowledge about amphibole growth kinetics at deep crustal levels, new amphibole growth rate data are provided. Our findings, indeed, may be useful to correctly interpret the textural features of amphibole-bearing mafic cumulates and rocks, and for a better constraining of the timescales of magmatic processes at upper mantle-lower crustal depths. Experiments were performed to determine the amphibole growth rates in a primitive alkaline basalt from Procida island (Campi Flegrei Volcanic District, southern Italy) at the following conditions: temperature of 1030 and 1080 °C, pressure of 0.8 GPa, water content in the range 3.3-4.2 wt%, and variable dwell time from 0.25 to 9 h. Amphibole growth rates range from 1.5·10^-7 to 2.9·10^-8 cm·s^-1 with increasing the duration of the experiments. It is observed that, keeping a constant dwell time, an increase of the experimental temperature or of the water content results in comparable growth rate increase. Coexisting synthetic amphibole and clinopyroxene show a faster growth rates in favour of amphibole regardless of the dwell time, since the chemical and structural similarities of these minerals cause kinetic competition. Moreover, the chemical composition of amphibole is influenced mainly by the experimental time; in detail, in the shortest (≤3 h) and low temperature runs edenite is the prevailing composition whereas the magnesiohastingsitic term becomes dominant at higher temperature and longer run duration. Based on the interpretation of the Fe-Mg exchange coefficient values between amphibole and coexisting liquid, the magnesiohastingsitic amphibole is considered to be the stable term at the investigated run conditions. Finally, the resulting growth rates have been applied to constrain the crystallization time of natural amphiboles and clinopyroxenes from the Oligo-Miocene cumulates of north-western Sardinia (i.e., Capo Marargiu  Volcanic District, Italy), yielding crystallization times in the range 1.46-3.12 yr.

How to cite: Bonechi, B., Perinelli, C., Gaeta, M., Tecchiato, V., and Fabbrizio, A.: Experimental time constraints on the kinetic and chemistry of amphibole at deep crustal levels, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4895, https://doi.org/10.5194/egusphere-egu2020-4895, 2020.

Persistently active volcanoes are often closely monitored, yielding a rich archive of observational data. The availability of varied observations provides a unique opportunity for improving theoretical models of magma dynamics, but data and model can be difficult to compare directly. Geophysical observations like seismicity or geodetic measurements often operate at similarly large scales as many models, but they only provide indirect and non-unique testimony of the processes occurring at depth. In contrast, crystals in erupted tephra or scoria samples record at least some aspects of the pre-eruptive condition in the volcanic conduit directly, but refer to spatial scales that are much smaller than most models resolve.

The goal of this paper is to demonstrate the potential of crystalline-scale data for distinguishing directly between different conduit-flow models. As a proof of concept, we focus on the preferential alignment of olivines crystals from tephra erupted at Kilauea Iki in 1959. Prior petrographic analysis suggests that these olivine glomerocrysts formed through synneusis of individual crystals. To evaluate the fluid-dynamical conditions under which both crystal synneusis and preferential crystal alignment would occur, we compare two broad classes of conduit flow models, unidirectional and bidirectional models.

We hypothesize that the observed preferential alignment of olivine crystals is created by a pronounced, nearly stationary wave at the interface that separates the ascending and descending magmas in bidirectional flow models. Crystals in bidirectional flow are hence exposed to a superposition of wave and shear, while crystals in a unidirectional, laminar flow experience approximately constant shear strain during ascent. To test our hypothesis, we quantify the crystal alignment resulting from a pure shear flow and from the superposition of a stationary wave on shear flow through two complementary model approaches. We first derive an analytical model for when crystals align under the joint influence of a wave and shear flow. We then use direct numerical simulations to quantify how crystal-crystal interactions modulate the analytically predicted preferential alignment of crystals.

We find that the formation of glomerocrysts with preferential aligned olivine crystals is consistent with bidirectional flow models, but unlikely to form in a unidirectional model. We emphasize that the imprint of the conduit flow on the crystals is subtle, suggesting that both clustering or alignment in isolation would be compatible with a much wider range of flow conditions than the observed conjunction of both attributes in the Kilauea Iki olivines. To our knowledge, these observations provide the first direct evidence of bidirectional flow in volcanic conduits.

How to cite: Suckale, J., diBenedetto, M., and Qin, Z.: Leveraging crystal-scale data to constrain the conduit flow regime in persistently active volcanoes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22411, https://doi.org/10.5194/egusphere-egu2020-22411, 2020.

Kīlauea Volcano emits large quantities of sulfur and other chalcophile elements into the troposphere as gas and aerosol particles, with widespread implications for regional air quality. The concentration of these elements in erupting melts is controlled by sulfide saturation, as well as the interplay between the exsolved volatile phase, silicate and sulfide liquids on eruption. Analysis of sulfur and other chalcophile elements (e.g, Ni, Cu, Se, As, Bi, Cd) in melt inclusions and matrix glasses allow deconvolution of these various processes. Olivine-hosted melt inclusions have significantly lower Ni and Cu concentrations than matrix glasses, defining trajectories consistent with sulfide saturation. These observations, when interpreted with the latest generation of sulfide saturation models, demonstrate that sulfides saturate at high MgO contents (10-14 wt%), in contrast to the traditional interpretation that sulfide saturation occurs relatively late at Kīlauea (~ 2 wt% MgO). This apparent discrepancy may be reconciled by considering the behaviour of sulfides during syn-eruptive degassing. The release of ~90% of dissolved sulfur into the vapour phase at low pressures leads to previously sulfide-saturated magmas becoming sulfide-undersaturated, driving the resorption of sulfides in contact with the degassing silicate melt. Sulfide resorption releases Cu, Ni, S and other chalcophile elements into the vapour-melt-(sulfide) system. Comparisons of melt inclusion and matrix glasses reveals that significant quantities of S, Se, Bi and As partition into the exsolved volatile phase. Other elements, such as Ni and Cu, remain largely in the melt. The contrasting behaviour of Se and Cu demonstrates that chalcophile element degassing is largely controlled by fluid-melt, rather than sulfide-melt partitioning. Crucially, sulfide resorption obscures the textural and chemical record of sulfide saturation in matrix glass and whole-rocks, but not in melt inclusions, which are isolated from the late-stage release of chalcophile elements from sulfide breakdown. Sulfide resorption during degassing and eruption provides a significant, but previously unquantified flux of sulfur to the atmosphere (as SO₂) at Kīlauea. Careful evaluation of melt inclusion Cu-Ni-S systematics reveals that the total S release during eruptions is ~1450 ppm (1.8 x previous estimates).

How to cite: Wieser, P., Jenner, F., Edmonds, M., Maclennan, J., and Kunz, B.: Tracking sulfur and its chalcophile allies at Kīlauea Volcano, Hawaii: A story of sulfide saturation, sulfide resorption and magmatic degassing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-355, https://doi.org/10.5194/egusphere-egu2020-355, 2020.

The depth at which magma chamber processes take place below magmatic arcs and the parameters controlling them are highly debated. These questions are fundamental for our understanding of the global magma differentiation as well as the formation of the continental crust at convergent margins, but also for evaluating the risks associated with volcanic eruptions.

In the Central Southern Volcanic Zone (Central-SVZ) of the Chilean Andes, a thin continental crust (30-40 km) and the occurrence of a major fault zone (Linquiñe-Ofqui) likely favor rapid magma ascent. This segment of the arc is as a consequence one of the most active in Chile with several recent eruptions (e.g. Llaima 2009, Cordon Caulle 2011, Calbuco 2015, Villarrica 2015 & 2019). The Central-SVZ is characterized by dominant mafic lavas (basalts, basaltic andesites), few rhyodacitic lavas, a noticeable compositional (Daly) gap in the intermediate compositions (andesites). Noteworthy, amphibole is usually absent, except in a few volcanoes (e.g. Calbuco) or only occurs as microliths in enclaves, which suggests rather low water contents. These observations contrast sharply with the Northern-SVZ where andesitic lavas are dominant and hydrous phases common.

We focused our research on the eruptive products of Osorno volcano (41°S, CSVZ) located between two volcanoes (Calbuco and Cordon Caulle) which recently showed very explosive eruptions and partly overlies an older Pleistocene eroded volcanic edifice (La Picada). A large series of samples were collected in four units spanning 200 kyr. They define a differentiation trend ranging from tholeiitic basalts to calk-alkaline dacites with a Daly Gap between 58 wt. % and 63 wt. % SiO₂. Plagioclase and olivine are dominant before the gap while plagioclase and clino- and orthopyroxene dominate afterwards.

The use of recent thermobarometric models revealed two main storage regions: (1) at the MOHO interface (1-1.2GPa) and (2), at the upper/lower crust interface with rather low pressures (likely ≤0.3 Gpa). While at (1) primary magmas differentiate, (2) is interpreted as the depth of major differentiation and volatile exsolution. Thermodynamic simulations (Gualda et al., 2012; Ghiorso & Gualda, 2015) support these (2) depth estimates and reproduce the main paragenesis by simple fractional crystallization at 0.1-0.2 GPa. Our results may explain the recent seismic unrest below Osorno (from 2015 to 2019) with earthquakes mostly taking place between 0.1-0.3 GPa (4-10km below the summit). We suggest that Osorno is an important target to perform a comprehensive petrological study aiming at characterizing the Central-SVZ magmatic arc and the magmatic storage depths.

How to cite: Bechon, T., Vander Auwera, J., Namur, O., Fugmann, P., Bolle, O., and Lara, L.: What is the magma storage depth under Osorno Volcano (Southern Volcanic Zone, Chile)?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4848, https://doi.org/10.5194/egusphere-egu2020-4848, 2020.

Understanding the origin of intermediate magmas that commonly erupt from subduction zone volcanoes is important to better constrain the mechanisms of continental crust formation. We carried out a detailed mineralogical and petrological study of the eruptive products from the last eruption of Calbuco volcano, Chile. In April 2015, Calbuco produced a 3 phase sub-Plinian eruption with pyroclastic fallouts and flows of andesitic composition. Rocks from Calbuco are made up of a glass phase and a high but variable proportion of minerals dominated by plagioclase, clinopyroxene, orthopyroxene and minor olivine, amphibole and magnetite. Plagioclase is very strongly zoned with highly anorthitic cores surrounded by more albitic rims. Based on thermodynamic calculations and using published experimental data, we estimate that the anorthitic cores crystallized from a basaltic andesite containing 3.5-4.5 wt.% H₂O. Using geochemical modelling, we also estimate that the bulk-rock major and trace element variability of Calbuco is best explained by accumulation of minerals in proportion plagioclase/pyroxene 72/28 in a dacitic melt. Such minerals most likely formed in the crystal mush zone of a magma chamber which, according to pyroxene and amphibole compositions, may have formed at a pressure of 2-3 kbar, corresponding to a depth of 8-11 km. A few weeks to months before the eruption, the crystal mush disaggregated, perhaps due to magmatic underplating, and a crystal-bearing dacitic melt moved upwards into a sub-surface storage region where the anorthite-poor rims formed. The 2015 eruption was probably internally triggered by over-pressurization in the shallow magma chamber.

How to cite: Namur, O., Bolle, O., and Vander Auwera, J.: Petrology of the April 2015 eruption of Calbuco volcano, southern Chile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8591, https://doi.org/10.5194/egusphere-egu2020-8591, 2020.

Magmatic arcs are usually considered to be major sites of new continental crust formation. However, the detailed differentiation processes that produce the characteristic calc-alkaline trends are still controversial. More particularly, the depth of differentiation in the arc crustal column and possible changes during the lifespan of a volcano are current subject of discussion.

The Central Southern Volcanic Zone (CSVZ) in Chile is characterized by a thin crust (~ 35 km; Hickey-Vargas et al., 2016) and by the presence of a major dextral transpressional crustal scaled structure (Liquiñe-Ofqui Fault Zone), two features that favor a rapid ascent of magmas from the mantle wedge to the surface. Recent petrological data acquired on volcanoes of the CSZV further indicate that most of the differentiation takes place at about 0.2 GPa, a depth corresponding to a major intracrustal discontinuity. However, for Villarrica stratovolcano (VR; 39.3°S, 71.6°W), estimates suggest two depths of differentiation, respectively at 0.8 and 0.2 GPa (Morgado et al. 2015, 2017).

VR is one of the most active volcanoes in the Andean Cordilleras. Since the mid 80’s, it has been constantly degasing through an open conduit filled by a summit lava lake. Several Holocene, monogenetic small eruptive centers (SECs) surround VR which forms together with Quetrupillán and Lanin stratovolcanoes a NW-SE oriented chain. It gives thus a perfect opportunity to study how the mentioned features influence the differentiation processes, their corresponding depth and the observed differentiation trends. VR is mainly composed of basaltic andesites and basaltic lavas and pyroclasts with less andesitic lavas and minor dacitic – rhyodacitic domes, while rocks from Quetrupillán and Lanin are compositionally more evolved (e.g. Hickey-Vargas et al., 1989).

Here we present mineral compositions (plagioclase, olivine, clinopyroxene) and whole-rock (lavas, pyroclasts) geochemical data for different units of VR as well as for some nearby SECs (Los Nevados, Chaillupén, San Jorge). The WR data combined with published analyses define a single differentiation trend extending from ~50 – 71 wt.% SiO₂, with a compositional “Daly” gap between 58 – 62 wt.% SiO₂. Moreover, a few VR samples have high Mg# up to 62 (SiO₂ 50.3-52.6, MgO 7.98 wt.%) and a tholeiitic affinity (e.g. AFM, K₂O/Yb vs. Ta/Yb). The most mafic, tholeiitic basalts found in the area where produced by the proximate San Jorge SEC (Mg# 69, SiO₂ 50.6, MgO 9.5 wt.%) and interpreted by McGee et al. (2019) as reflecting a deep, melt-exhausted region of the mantle wedge. Major- and trace elements data together with supportive mass balance modelling and thermodynamic simulations with rhyolite-MELTS imply fractional crystallization as a major differentiation process.

How to cite: Fugmann, P., Vander Auwera, J., Namur, O., Bechon, T., Bolle, O., and Lara, L.: Magmatic processes under Villarrica stratovolcano (Central Southern Volcanic Zone, Chile)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13762, https://doi.org/10.5194/egusphere-egu2020-13762, 2020.

Vran Kamak paleovolcano is formed during the Upper Cretaceous igneous activity along the Panagyurishte strip of Central Srednogorie Zone, Bulgaria, part of the magmatic-metalogenic arc belt Apuseni-Banat-Timok-Srednogorie. It represents a comparatively well-preserved, eroded stratovolcano built of epiclastics, pyroclastics and lava flow (with typical hyaloclastite and peperite formation) succession surrounded by marine environment, as only a part from the volcanic cone was over the sea level. The central (conduit) parts of the paleovolcano are intruded by a volcanic neck in the area of Vran Kamak summit. The volcanic activity was accompanied by sedimentary gravity flows and volcaniclastic debris is dispersed in the Late Cretaceous basin. The present study provides new petrological and geochronological data for Vran Kamak paleovolcano.

The analyzed samples from the lava flows show basaltic andesite to andesite composition with SiO₂ contents ranging from 51 to 55.5 wt %, while the volcanic neck of the Vran Kamak summit is trachydacite (SiO₂ of 61.54 wt % ). The rocks are medium- to high-K calc-alkaline. On a primitive-mantle normalized diagram, the rocks show peaks in LILE (U, Th, Pb) and troughs in Nb, Ta, Ti and P. Weak negative Eu anomaly (0.83–0.94) and La_N/Yb_N (10 to 13) are observed. Fractionation of mafic minerals (amphibole and pyroxene) and plagioclase is visible on the harker diagrams. The ⁸⁷Sr/⁸⁶Sr_(i) ratio of 0.705141 from the volcanic neck shows small degree of crustal assimilation.

The basaltic andesite to andesite lava flows are built of plagioclase (with normal oscillatory zoning, bytownite-labrador, An_88-56), amphibole (tschermakite to magnesiohastingsite) and pyroxenes (mostly augite and rare small enstatite crystals embedded in them). Some of the clinopyroxenes form corona texture around the amphibole, showing processes of dewatering. The trachydacite neck is built of porphyries of plagioclase, sanidine, biotite, amphibole (megnesiohornblende to thermakite), magmatically coroded quartz and accessories of zircon, apatite and magnetite set in a fine-grained groundmass. The calculated depths of crystallization and temperatures of the hornblende from the lava flows are 17–22 km and 930–970 ^oC and that from the neck are 5.9–7 km and 800–830 ^oC, that give evidence for a complex volcano-plutonic system.

An attempt for LA-ICPMS U-Pb zircon dating of one the lava flows is made, but it contains only xenocrysts which fall in several age intervals: 306–314 Ma, 440–450 Ma, 520–530 Ma, 560–614 Ma, 810–830 Ma which represent inherited and recycled component from the local basement. This lava flow has a peperitic contact with sediments faunistically dated at the Turonian/Coniacian boundary (Cremnoceramus deformis erectus, Vangelov et al., 2019). The zircon population of the trachydacite neck is presented mostly by own magmatic grown crystals giving a Concordia age of 91.12 ±0.43 Ma.

Acknowledgements. The study is supported by grant DN 04/9 funded by the National Science Fund, Ministry of Education and Science, Bulgaria.

References:

Vangelov, D., Gerdjikov, I., Dochev, D., Dotseva, Z., Velev, S., Dinev, Y., Trayanova, D., Dancheva, J. 2019. Upper Cretaceous lithostratigraphy of the Panagyurishte strip (Central Bulgaria) – part of the Late Cretaceous Apuseni-Banat-Timok-Srednogorie magmatic belt. – Geol Balc., 48, 3, 11–33.

How to cite: Georgiev, S., Balkanska, E., Peytcheva, I., and Vangelov, D.: Petrology and geochronology of Vran Kamak paleovolcano, Central Srednogorie, Bulgaria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10836, https://doi.org/10.5194/egusphere-egu2020-10836, 2020.

Diffusion profiles in sanidine (Ba) and olivine (Mg-Fe, Ca, Mn, and Ni) were used to track recharge events prior to the eruption of the Laacher See volcano, East Eifel volcanic field, western Germany (12.9 ka). Sanidine crystals were analyzed in samples from cumulates and mafic to intermediate phonolites. Olivine crystals occur only in the final mafic eruption products of the compositionally zoned tephra deposit and represent the hybrids of mixing between differentiated phonolite, crystal cumulates, and intruding basanitic magma at the bottom of the magma reservoir. This mixing event is likely related to the eruption triggering event. Additionally, olivine crystals from ten basanitic scoria and maar deposits in the East Eifel and two locations in the West Eifel (Pulvermaar melilith-nephelinite, Meerfelder Maar ol-nephelinite) were analyzed to represent Quaternary parent mafic magmas in Eifel volcanism.

Olivine from the mafic component that mixed with the Laacher See phonolite are always reversely zoned from cores of variable composition (Fo_83-89). Zoning of all crystals show trends to a common rim composition (Fo_87.5-89). Most crystals show additionally a narrow (<10 μm) normally zoned overgrowth at the outermost grain boundary (Fo_86.5-87.5). Olivine crystals from mafic cones in the East Eifel show similar zoning patterns and core compositions (Fo_80-88) as those from Laacher See hybrids, but their rims are more variable and always less forsteritic (Fo_83-88). The lack of olivine rims with >Fo₈₈ indicates that East Eifel basanites are less primitive than the basanite that intruded into the Laacher See reservoir with olivine rim composition >Fo₈₉. However, olivine in samples from the West Eifel nephelinite maar deposits show rim compositions similar to the olivines from Laacher See (Fo_87.5-90), but are dominantly normal zoned and have high-Fo cores (Fo_88-92).

We interpret these observations to indicate that olivine crystals on Laacher See hybrids probably originate from a cumulate or crystal mush with low melt fraction that was disaggregated by the ascending basanite before hybridization. Diffusion modeling of olivine rims indicate a time scale between mixing and eruption of less than 49 days.

Diffusion times of the sanidine phenocrysts from the intermediate phonolite indicate older recharge events every 1500-3000 yrs that did not result in complete hybridization and eruption. Ba-diffusion times are much shorter for sanidines from the mafic phonolite (4-8 yrs) and the cumulates (months). The reactivation of crystals from cumulates, that can be related to the eruption-triggering recharge event, occurred therefore only months prior to the eruption of Laacher See. These timescales between recharge and eruption are remarkably shorter than the diffusion times calculated for olivine from basanite erupted from scoria cones (up to 500 days).

How to cite: Sundermeyer, C., Rout, S. S., and Wörner, G.: Timescales from mixing to eruption in alkaline volcanism in the Eifel volcanic fields obtained from sanidine and olivine diffusion modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6945, https://doi.org/10.5194/egusphere-egu2020-6945, 2020.

Mt. Etna is one of the most protrusive features of the eastern coastline of Sicily, Italy. As Europe’s most active volcano it has been studied extensively to reveal its geodynamic setting, plumbing system and due to the constant monitoring of the volcano edifice the prediction of the risk future events is sophisticated at Mt. Etna.

The eruptive activity has been divided according to the age into 6 stages: (1) “Tholeiitic Stage”, was active between 600-320 ka ago, (2) the “Timpe Stage” between 220 and 110 ka ago, (3) the “Ancient Alcaline Volcanism”  between 110 and 65 ka ago and (4) the “Ellittico Stage” between 57 and 15 ka ago (5) the “Mongibello Stage” from 15 ka ago until 1971 and (6) the “post -1971 Stage” active since 1971 (Casetta et al., 2019).

The lava propagating through the Etnean plumbing system generated a complex network consisting of sills and dykes responsible for the formation of the summit craters and a plethora of eccentric cones that cover the flanks of the volcano.

We studied whole rock and mineral chemistry of the lavas from three eccentric cones (Monte Spagnolo, Monte Fiori and Monte Rossi) and the 2002/2003 southern flank lava flow. All lavas are characterized by trachytic texture with variable modal composition of olivine, clinopyroxene and plagioclase phenocrysts. Euhedral and skeletal olivine phenocrysts can be distinguished into three main groups; a) normal zoning, b) inverse zoning, and c) patchy appearance with melt inclusions of andesitic and trachytic composition. The Monte Spagnolo whole rock composition has an Mg# ranging between 52-54 and 10.7 wt% CaO , being are the most primitive lavas among the sampled outcrops whereas the Monte De Fiore lavas are the most evolved since the Mg# ranges from 48.6 to 49.2 and the CaO content from 11 to 11.2 wt%. Both, Monti Rossi and the  2002/2003 lava flow are more evolved than the Monte Spagnolo since they have Mg# ~ 50 and 49-49.3 respectively. The CaO concentration in both outcrops is relatively constant ranging around 10.5 wt%.

The olivine compositions follow the same trend as their whole rocks. The most MgO-rich olivine (Fo=88.9 %) was found in the Monte Spagnolo lavas. This olivine is of magmatic origin and cannot be considered as mantle derived xenocryst since the NiO content is low (NiO=0.17 – 0.2 wt%) and the CaO-content high (CaO=0.24 – 0.26 wt%). The most evolved lavas from Monte De Fiore have the lowest Fo-content (Fo=75 - 78 %). Olivine from all samples has a characteristic inverse zonation with, at Monti Rossi and 2002/2003 lava flow, Fo-content in the core ranging from 69% to 75% and in the rim from 77% to 80% respectively.

In conclusion, the studied eccentric cones show extensive magma mixing as can be inferred from the olivine inverse zoning. Monte Spagnolo lavas represent the most primitive magma formed at high temperatures (olivine skeletal growing) and the Monte De Fiore lavas the most evolved magma.

Casetta et al., 2019. International Geology Review, DOI: 10.1080/00206814.2019.1610979

How to cite: Bauer, M., Ntaflos, T., Abart, R., Giacomoni, P.-P., Ferlito, C., and Coltorti, M.: The eccentric cones Monte De Fiore, Monti Rossi, Monte Spagnolo and the 2002/2003 eruption, Mt. Etna: evidence for magma mixing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15464, https://doi.org/10.5194/egusphere-egu2020-15464, 2020.

Mt. Etna is one of the most protrusive features of the eastern coastline of Sicily, Italy. As Europe’s most active volcano it has been studied extensively to reveal its geodynamic setting, plumbing system and due to the constant monitoring of the volcano edifice the prediction of the risk future events is sophisticated at Mt. Etna.
The eruptive activity has been divided according to the age into 6 stages: (1) “Tholeiitic Stage”, was active between 600-320 ka ago, (2) the “Timpe Stage” between 220 and 110 ka ago, (3) the “Ancient Alcaline Volcanism”  between 110 and 65 ka ago and (4) the “Ellittico Stage” between 57 and 15 ka ago (5) the “Mongibello Stage” from 15 ka ago until 1971 and (6) the “post -1971 Stage” active since 1971 (Casetta et al., 2019).

The lava propagating through the Etnean plumbing system generated a complex network consisting of sills and dykes responsible for the formation of the summit craters and a plethora of eccentric cones that cover the flanks of the volcano.

We studied using whole rock and mineral analyses the lavas from three eccentric cones (Monte Maletto, Monte Nuovo and Monte Frumento) and the 2001 eruption on the south flank from the main crater. All lavas are characterized by trachytic texture with variable modal composition of olivine, clinopyroxene and plagioclase phenocrysts. The Monte Maletto whole rock composition with an Mg# ranging between 56-58 and a CaO content of 12.0 wt% are the most primitive lavas among the sampled outcrops whereas the Monte Frumento lavas are the most evolved since the Mg# ranges from 43 to 46 and the CaO content from 9.5 to 10.8 wt%. Both, Monte Nuovo and 2001 eruption are more evolved than the Monte Maletto since they have Mg# ~ 50 and 51.5-52.9 respectively. The CaO concentration in both outcrops is relatively constant ranging from 9.8 to 10.7 wt%.

The olivine compositions follow the same trend as their whole rocks. The most MgO-rich olivine (Fo=87.5 %) found in the Monte Maletto lavas. This olivine is of magmatic origin and cannot be considered as mantle derived xenocryst since the NiO content is low (NiO=0.16 wt%) and the CaO-content high (CaO=0.22 wt%). The most evolved lavas from Monte Frumente have the lowest Fo-content (Fo=64-68 %). Olivine from both, Monte Nuovo and 2001 eruption have a characteristic inverse zonation with Fo-content in the core ranging from 69.9 to 75 and in the rim from 78.2 to 81.7 respectively.

In conclusion, the Monte Maletto lavas represent the most primitive magma formed at high temperatures (skeletal growing of the olivine) and the Monte Frumento lavas the most evolved magma. The Monte Nuovo and 2001 eruption experienced magma mixing as inferred from the olivine inverse zonation. Monte Nuovo can be considered a flank eruption of lava deviated from the central conduit rather than an eccentric cone.

Casetta, Federico, et al. "The evolution of the mantle source beneath Mt. Etna (Sicily, Italy): from the 600 ka tholeiites to the recent trachybasaltic magmas." International Geology Review (2019): 1-22.

How to cite: Hofbauer, B., Ntaflos, T., Abart, R., Giacomoni, P. P., Coltorti, M., and Ferlito, C.: Petrological constraints on the evolution of the eccentric cones Monte Maletto, Monte Frumento and Monte Nuovo – Mt. Etna, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16427, https://doi.org/10.5194/egusphere-egu2020-16427, 2020.

Ancient volcano-plutonic complexes can record the evolution of single- or multi-pulse plumbing systems and thus can be used as proxy to investigate the magma dynamics beneath active volcanoes. The exceptional state of conservation of the Middle Triassic Cima Pape complex (Dolomitic Area, Southern Alps) makes it an ideal snapshot of a ~238 Ma old feeding system of a dominantly effusive volcano. It is composed of a 50 to 300 metres thick gabbroic to monzodioritic sill intruded in the sedimentary cover and overlaid by its volcanic counterpart, made up of basaltic to trachyandesitic lavas and pillow breccias. A detailed investigation of the textural and compositional features of clinopyroxene phenocrysts in the volcanites revealed that complex dynamic processes took place in the feeding system beneath the Cima Pape “volcano”. Although some crystals have normal homogeneous or simple-zoned texture, with Mg# [MgO/(MgO+FeO_tot) mol%] ranging between 71 and 77 (type 1 clinopyroxene), the great majority of them is typified by a peculiar texture, characterized by the occurrence of intermediate high-Mg# (80-84, up to 90), high-Cr₂O₃ (up to 1.0 wt%) and low-TiO₂ (down to 0.1 wt%) bands (type 2 clinopyroxene). These overgrowths, crystallized between low-Mg# cores and rims, likely indicate that the feeding system was affected by frequent mixing between mafic inputs and differentiated batches. An overview of the main textural/geochemical features of clinopyroxene in effusive and intrusive products was put forward in the present study to reconstruct the main chemico-physical parameters and evolution of the feeding systems beneath the Middle Triassic volcanoes of the Dolomitic Area. Afterwards, these results will be used to advance some speculations about the processes recorded by clinopyroxene crystals in lava flows from active volcanoes, such as Stromboli and/or Mt. Etna.

How to cite: Casetta, F., Giacomoni, P. P., Nardini, N., and Coltorti, M.: How to reconstruct the geometry of a Middle Triassic feeding system: clues from clinopyroxene textures in lava flows from Cima Pape (Southern Alps, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18258, https://doi.org/10.5194/egusphere-egu2020-18258, 2020.

The last eruption on Fogo Island (Cape Verde Archipelago) occurred in 2014-2015, with mostly hawaiian and strombolian but sometimes vulcanian activity, with variable emission rates of lava flows, pyroclasts and gases (SO2 and CO2).   Some lava flows, mainly from the first stage of the eruption enclosed small granular ultramafic nodules (1-3cm), with angular to rounded shapes. The host rocks are porphyritic tephrites, with Ti-augite and Ti-magnetite phenocrysts and, sometimes, amphibole xenocrysts in a brown glassy matrix including laths of plagioclase, clinopyroxene and Ti-mgnetite.

The nodules are composed of an early crystallization phase olivine, in subheuedral crystals devoid of kink-bands or in rounded crystals enclosed in clinopyroxene oikocrysts. Clinopyroxene occurs in subeuhedral to anhedral zoned crystals, sometimes partially patchy replaced by late igneous amphibole which also occurs as primary crystals as well as in some xenocrysts. In both cases they frequently show reaction rims with transformation in rhonite, most probably resulting from degassing. Oxide minerals are present as a minor component occurring in sub-euhedral to anhedral crystals as inclusions in olivine and clinopyroxene or interstitially between silicate minerals.

The typical cumulus textures, and the mineral chemistry already obtained for the ultramafic nodules from the 2014 eruption at Fogo strongly suggest that they have a cumulate origin and are cognate with the host magmas. Indeed, the similar composition of Ti- augites from the nodules and phenocrysts (Wo49-51 En42-36 Fs12-10), as well as the olivine Fo contents and high CaO contents in olivine are explained by crystal segregation from the same magma of the host rock.

Geothermobarometric calculations point to crystallization temperatures for the cumulates between 1150 and 1200 ºC and pressures around 7- 10 kbar, while phenocrysts in host rocks crystallized at around 1000 ºC, and pressures of 3-4. These data confirm the existence of a polybaric plumbing system feeding the 2014-15 Fogo eruption, with some of the reservoirs having developed at mantle depths (at least 22 km).   

This research received financial support from FCT (Fundação para a Ciência e Tecnologia) through project FIRE (PTDC/GEO-GEO/1123/2014).

How to cite: Caldeira, R., Mata, J., Martins, S., Madeira, J., Ramalho, R., Silva, P., and Moreira, M.: Insights from ultramafic nodules on the plumbing system of the Fogo Island 2014-2015 Eruption (Cape Verde) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20439, https://doi.org/10.5194/egusphere-egu2020-20439, 2020.

Magma reservoirs are characterized by thermal and chemical gradients producing large variations of the spatial distribution of the physical properties of the magma they contain. Understanding the pre-eruptive thermal, chemical and physical evolution of magma represents an important step to correctly interpret the signs of an impending eruption. In this framework, the chemical zoning of minerals, which provide us a record of these thermal and chemical perturbations, represents an important tool to reconstruct reservoir dynamics. We study the effect of the competition between changing intensive parameters, element diffusion and mineral growth on the chemical zoning of minerals. We grow chemical zoned minerals at the Petro-Volcanology Research Group of the University of Perugia, using tephra from 2002-03 Mt. Etna eruption as starting material. The zonation in minerals is been forced inside a high-temperature furnace by oscillating the temperature under three different conditions: static conditions, using a controlled deformation gradient (concentric cylinder apparatus) and using a chaotic mixing regime (Chaotic Magma Mixing Device – CMMD). We collect major and trace elements distribution maps on a large number of crystals using Electron Probe Micro Analyzer (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), respectively. The data will be analysed using a series of custom built machine learning algorithms to disentangle zoning related to variations of the thermodynamic conditions of crystal growth from the effects of the competition between diffusion and growth. Our data will help deciphering the zoning patterns observed in natural crystals, improve our understanding of magma reservoir dynamics and help the interpretation of monitoring signals in the period preceding a volcanic eruption.

How to cite: Musu, A., Caricchi, L., Perugini, D., Corsaro, R. A., Vetere, F., and Petrelli, M.: Analysis of Experimentally Zoned Crystals to Investigate The Thermo-Chemical Evolution of Magma Reservoirs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10696, https://doi.org/10.5194/egusphere-egu2020-10696, 2020.

Pantellerites are strongly peralkaline rhyolites occurring mainly in extensional tectonic setting, from oceanic islands (Ascension Island) to continental rift zones, as for example Pantelleria Island in the Sicily channel rift zone, Kenyan and Ethiopian Rift Valleys and Mayor Island (NZ-Taupo Volcanic Zone). Peralkaline magmas are noted for their ability to shift between explosive and effusive eruptive styles, which is strictly associated with viscosity, degassing kinetics, the initial temperature and crystal content of the magma.

The present experimental work aims at unravelling pre- and syn-eruptive crystallisation dynamics and time-scales of most explosive eruptions that are so unforeseeable and not yet systematically studied, but essential to assessing volcanic hazards of the Pantelleria system.

Crystal nucleation and growth of alkali feldspars in pantelleritic melts have been investigated by cooling and decompression experiments conducted at the T-P range more plausible for the Pantelleria system (T=680-800 °C,  [1]; 25-100 MPa). The studied melt composition belongs to the Fastuca pumice fall eruptive unit of Pantelleria which is rich in Na and Fe and it presents a peralkalinity index of 1.4.

Textural analysis on the hydrous samples reveal that crystal fraction (φ) varies from average pre-eruptive values of 0.02  to 0.2 during magma ascent from magma chamber depths (ca. 3-4 km, at 100 MPa) to shallower depths (corresponding to pressures of 50 MPa), leading to an increase of viscosity of 1 log unit (value estimated using the equation in [2], starting from the pre-eruptive low viscosity of the pure pantelleritic liquid calculated by [3]), which may contribute to enhance a more explosive magma eruptive behavior. Also considering fast decompression rates (DP/Dt) (in the range of 0.2-0.6 MPa/s), it results a large decrease in pressure along the conduit, promoting volatile exsolution and higher magma accelerations, which along with increasing viscosity, crystallinity, and velocities, could lead to magma brittle behavior and trigger explosive eruptive events.

A better understanding of how these explosive pantelleretic eruptions work will lead to improved volcano monitoring and disaster mitigation in high-risk volcano-tectonic areas as for instance is Pantelleria Island, where about 10.000 inhabitants live permanently.

References:

[1] Di Carlo I. et al. (2010) Journal of Petrology 51 (11), 2245–2276. [2] Vona A. et al. (2011) Geochimica et Cosmochimica Acta 75(11), 3214–3236. [3] Di Genova D. et al. (2013) Volcanol. Geotherm. Res. 249, 201–216.

How to cite: stabile, P., Appiah, E., and Carroll, M. R.: The role of syn-eruptive crystallization on pantelleritic eruptive dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-488, https://doi.org/10.5194/egusphere-egu2020-488, 2020.

Clinopyroxene based thermometers and barometers are widely used tools for estimating temperature and pressure conditions under which magmas are stored before eruptions.

Several studies reported the development and the application of Clinopyroxene–liquid geothermobarometers in many different volcanic environments, also warning on the potential pitfall in using overly complex models [e.g., 1 and references therein]. The main drawback in the use of models with a large number of parameters is the potential overfitting of the calibration data, yielding a poor accuracy in real-world applications. On the other hand, simpler models cannot account for the complexity of natural magmatic systems, requiring different calibrations for different magma chemistries [e.g., 2, 3].

In the present study, we report on the development of Clinopyroxene and Clinopyroxene-liquid thermometers and barometers in a wide range of P-T-X conditions using Machine Learning (ML) algorithms. To avoid overfitting and demonstrate the robustness of the different methods, we randomly split the dataset into training and validation portions and repeating this procedure up to 10000 times to trace the performance of each of the used algorithms. We compared the performance of ML algorithms with classical and established Clinopyroxene and Clinopyroxene-liquid thermometers and barometers using local and global calibrations. Finally, we applied the obtained thermometers and barometers to real study cases.

[1]      K. D. Putirka, Thermometers and barometers for volcanic systems, Minerals, Inclusions and Volcanic Processes, 69. 61–120, 2008.

[2]      D. A. Neave, K. D. Putirka, Am. Mineral., 2017, DOI:10.2138/am-2017-5968.

[3]      M. Masotta, S. Mollo, C. Freda, M. Gaeta, G. Moore, Contrib. to Mineral. Petrol., 2013, DOI:10.1007/s00410-013-0927-9.

How to cite: Petrelli, M., Caricchi, L., and Perugini, D.: Unravelling pre-eruptive P-T conditions by machine learning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19028, https://doi.org/10.5194/egusphere-egu2020-19028, 2020.

Basaltic volcanism is strongly influenced by magmatic viscosity, which, in turn, is controlled by magma composition, crystallisation, oxygen fugacity and vesiculation. We developed an environmental cell to replicate the pressure and temperature during magma ascent from crustal storage to the surface, while capturing crystallisation using in-situ 4D X-ray computed microtomography. Crystallisation experiments were performed at Diamond Light Source, using monochromatic 53 keV X-rays, a pixel size of 3.2 μm, a sample to detector distance of 2000 mm, 1440 projections per 180 deg, an acquisition time of 0.04 s, and a rotation velocity of 3.125 deg.s-1. The redox conditions were controlled using an oxidised nickel disk for each experiment. Our starting materials were samples made of crystal-free glass cylinders (Ø 3 mm) from the 2001 Etna eruption with 0.9 and 0.8 wt. % water content. In the experiments, samples and crucibles were sealed initially by applying ~10 N loads. All samples were then heated up above glass transition (between 800 °C and 900 °C) in order to allow sample homogenisation while preventing volatiles exsolution. We then pressurised each sample by applying uniaxial loads (between 80 and 380 N), using high-degree alumina pistons, in order to generate enough internal pressure to maintain bubble-free samples when the desired high temperature was reached. Once at the initial high temperature, we began experiments via dropping the temperature to different target isothermal (from 1210 to 1130 °C or 1180 to 1110 °C) and isobaric conditions (8 and 10 MPa, respectively). For the whole duration of the experiments, we were able to observe directly and record pyroxene crystal nucleation and growth. Specifically, we were able to observe pyroxene nucleation on bubbles at small undercooling (∆T) and epitaxial growth of pyroxene at large ∆T. An increase of ∆T (up to 50 °C) can be associated with a decompression of a magma chamber or a decompression during magma ascent in the conduit. As ∆T = 30 - 50 °C can be reached in most of the basaltic volcanic systems on Earth, our results provide a feasible explanation of which mechanisms control nucleation and growth of pyroxene crystals in hydrous basaltic magmas. In addition, epitaxial growth promotes a faster increase of the crystal volume. As a larger crystal content translates into a higher viscosity, our results have important implications for magma rheology, and are extremely important to improve our understanding of magma ascent dynamics during volcanic eruptions, and our capacity to predict eruptions and mitigate volcanic hazards.

How to cite: Polacci, M., Arzilli, F., La Spina, G., Le Gall, N., Torres Orozco, R., Hartley, M., Di Genova, D., Atwood, R., Llewellin, E., Brooker, R., Mader, H., Lee, P., and Burton, M.: Time-series experiments of pyroxene crystal nucleation and growth in basaltic magmas and implications for magma rheology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9688, https://doi.org/10.5194/egusphere-egu2020-9688, 2020.

Crystal clustering impacts rheology and differentiation in magmatic systems, and also offers insights into nucleation processes. Electron backscatter diffraction (EBSD) is ideal for studying interactions between crystals at interfaces. Clinopyroxene (Cpx) – titanomagnetite (Timt) clusters formed in time series experiments on synthetic trachybasaltic melt were studied using EBSD to understand the cause of clustering. Experiments were performed at 400 MPa and the NNO +2 buffer, at both anhydrous and hydrous (2 wt.% H2O) conditions, by cooling from 1300 °C (superliquidus) to 1100 °C with a rate of 80°C/min and holding at the target temperature for 4 – 8 hours before isobaric quenching.

All experiments crystallize dendritic Cpx (Lmax = 50 – 60 µm) and isometric euhedral to hopper-shaped Timt (Lmax = 5 – 6 µm). Infrequent (~ 10 mm^-2) unmelted Cr-oxide crystals are surrounded by polycrystalline Cr-bearing Timt rims (Lmax Cr-oxide + rim = 20 µm). Cpx dendrite “rosettes” radiate from the polycrystalline rims, but many dendrites do not belong to rosettes, at least in 2D. Individual Timt crystals (Cr-free) are strongly associated with the sides and tips of Cpx dendrites. About 75% of Timt grains are in contact with Cpx in 2D. Cpx-Timt interfaces are irregular, and Timt is often attached only by thin necks. Timt grain centers are weakly clustered (R = 0.87 – 0.95, 1 = random).

Timt shows a strong crystallographic orientation relationship (COR) with Cpx, with 75 – 89% of Timt grains in contact with Cpx lying within 6° of a single fixed (“specific”) COR, OR1 = Cpx [010] // Timt <110>; Cpx (100) // Timt <111>; Cpx [001] // Timt <112>. The axes Cpx [010] // Timt <110> show the least dispersion (< 3°) from the ideal alignment. Relative to Cpx, individual Timt may be rotated up to 6° away from OR1, around an axis close to Cpx [010]. There are two peaks in this continuous distribution, corresponding to OR1 (above) and OR2 = Cpx [010] // Timt <110>; Cpx (-101) // Timt <111>; Cpx [101] // Timt <112>. The misorientation between OR1 and OR2 is 5.3°. OR1 and OR2 together represent 68 – 77% of Timt grains in contact with Cpx (tolerance angle 2.6°).

Cpx dendrite branches bend around Cpx [010]. The anhydrous sample with dwell time 4 hours shows continuous bending of up to ~15°, whereas the hydrous sample with dwell time 8 hours shows bending of up to only ~7° and subgrain boundaries (1 - 2°) separating undistorted domains, suggesting recovery of bent crystals during annealing. Initial Cpx nucleation likely occurred heterogeneously as rosettes on Cr-bearing Timt rims around Cr-oxide crystals. Multiple Timt grains touching different branches of the same bent Cpx crystal all maintain a close COR with the Cpx orientation immediately adjacent to the Cpx-Timt interface, indicating that Timt nucleated on (or attached to) dendrite branches during or after their growth.

In conclusion, EBSD is a powerful method for understanding crystallization and cluster formation. Future work will study the effect of annealing time, water content, and undercooling on Cpx – Timt cluster development.

How to cite: Griffiths, T., Habler, G., Masotta, M., and Pontesilli, A.: The origin of clinopyroxene - titanomagnetite clustering during crystallisation of synthetic trachybasalt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10336, https://doi.org/10.5194/egusphere-egu2020-10336, 2020.

Geochemistry of volatiles in active volcanoes provides insights into the magmatic processes and evolution at depth, such as magma evolution and degassing, which can be implemented into volcanic hazards assessment. Deception Island is one of the most active volcanoes in Antarctica, with more than twenty explosive eruptions documented over the past two centuries. Hydrogen and oxygen isotopic variations in the volatiles trapped in the Deception Island rocks (glass and melt inclusions in phenocrysts) provide essential information on the mechanisms controlling the eruptive history in this volcanic suite. Thus, understanding the petrological and related isotopic variations in the island, has the potential to foresee the possible occurrence and its main eruptive features of a future eruption.

Information from hydrogen and oxygen stable isotopes combined with detailed petrologic data reveal in Deception Island (i) fast ascent and quenching of most magmas, preserving pre-eruptive magmatic signal of water contents and isotopic ratios, with local modification by rehydration due to glass exposition to seawater, meteoric and fumarolic waters; (ii) a plumbing system(s) currently dominated by closed-system degassing leading to explosive eruptions; (iii) control on the interactions of ascending magmas with the surface waters producing hydrovolcanic activity throughout the two main fault systems in Deception Island. These results can be considered in further studies of volcanic monitoring to improve the capability to interpret geophysical data and signals recorded during volcanic unrest episodes, and hence, forecast volcanic eruptions and related hazards.

This research was partially funded by the following projects: POSVOLDEC (CTM2016‐79617‐P) (AEI/FEDER‐UE), VOLGASDEC (PGC2018-095693-B-I00) (AEI/FEDER‐UE) and Programa Propio Ib-2019 (USAL). This research is also part of POLARCSIC activities.

How to cite: Álvarez-Valero, A. M., Aulinas, M., Geyer, A., Gisbert, G., Kereszturi, G., Núñez-Guerrero, E., Polo-Sánchez, A., and Sumino, H.: Magma ascent and eruption forecasting at Deception Island volcano (Antarctica) evidenced by δD and δ18O variations , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12700, https://doi.org/10.5194/egusphere-egu2020-12700, 2020.

Analysis of noble gas isotopes is an excellent tool to decipher the origin of the Earth materials due to their particular isotopic ratios for each geochemical reservoir. In addition, they are particularly useful for tracing the evolution of these materials as their elemental ratios record modifications produced by key magmatic processes such as degassing, melting and crystallization (1).

We have analysed noble gas composition in melt inclusions in olivine phenocrysts and glass (bulk-rock) of volcanic ejecta from Deception Island’s volcano with the aim to trace noble gas evolution from its magma source to eruption. Deception Island is one of the most active volcanoes in Antarctica, characterised by three main eruptive episodes, namely pre-, syn- and postcaldera, which magmatic system is widely characterized from the petrologic and geochemical perspectives (2). In pre- and syn-caldera samples, we have extracted the gas from the glass, and the melt inclusions in the olivines, by step-heating (up to 2000ºC) and crushing (hydraulic press) in an ultra-high-vacuum mass spectrometer.

⁴He/⁴⁰Ar* ratios in Deception Island (0.15-0.25), where ⁴⁰Ar* indicates non-atmospheric ⁴⁰Ar,are significantly lower than the mantle ratio (1-5). If this ⁴He/⁴⁰Ar* variation resulted from fractionation during degassing, the residual magma (i.e., olivine melt inclusions)should have higher ⁴He/⁴⁰Ar* ratio than the previous magmas as He is more soluble than Ar within silicate melt. Hence, the previous or primitivemagmashould have ⁴He/⁴⁰Ar* lower than 0.15, due to diffusivity-controlled fractionation in its source mantle by precedent melt extraction stages. However, local pre- and syn-caldera olivines show ⁴He/⁴⁰Ar* values as high as c. 20(with ³He/⁴He R_A= 8, i.e. mantle signal), thus revealing intensivedegassing episodes that led to the pre- and syn-caldera eruptive events, responsible forboth the island formation and the caldera’s collapse, respectively. This is coherent at least with (i) the enormous eruption described in the island of over 60 km³of magma erupted (3)during the caldera event (4); and (ii) the current ⁴He/⁴⁰Ar* values(5)of fumaroles in the island (3-8) that represent degassing of the present magma and are also higher than in the melt inclusions, thus implying significant degassing possibly during the caldera event.

(1) Burnard, 2001, GCA; (2) Geyer et al., 2019. Sci.Rep.; (3) Geyer & Martí, 2008. JVGR; (4) Antoniades et al., 2018. Sci.Rep; (5) Padrón et al., 2015. Antarct Sci.

This research was partially funded by the POSVOLDEC (CTM2016‐79617‐P) (AEI/FEDER‐UE) and VOLGASDEC (PGC2018-095693-B-I00) (AEI/FEDER‐UE) projects and a JSPS Invitation Fellowship (S18113) at the University of Tokyo. This research is also part of POLARCSIC activities. E. N-G very much appreciates the travel grant to attend EGU-2020 funded by ANTVOLC, the European Social Fund and the Youth Employment Initiative of the Consejería de Educación of Castilla y León.

How to cite: Núñez-Guerrero, E., Sumino, H., Álvarez-Valero, A. M., Aulinas, M., and Albert, H.: Massive degassing-derived eruptions at Deception Island (Antarctica): Evidences from noble gas isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4830, https://doi.org/10.5194/egusphere-egu2020-4830, 2020.

Understanding how magmas are transported and collected within the crust is crucial for constraining the dynamic of shallow plumbing volcanic systems and associated hydrothermal activity. This study focuses on the Lessines dioritic intrusion exposed in the SW margin of the Brabant Massif in Belgium. The kilometric subvolcanic body was emplaced around 419 Ma and is thought to result from the emplacement of multiple sills which intruded a lithostratigraphic discontinuity within Upper Ordovician sedimentary units. Our study aims to constrain how magmatic flow is recorded through different fabrics, how this flow varies across the solidified magmatic intrusion and how primary fabrics can be affected by subsequent hydrothermal overprint.

The petrofabric of 40 oriented diorite samples was investigated with a multi-methods approach: (i) Anisotropy of Magnetic Susceptibility (AMS) along with K-temperature curves determined using low field KLY-4S Kappabridge susceptibilimeter (at LIENS lab, University of La Rochelle, France), (ii) Shape Preferred Orientations (SPO) of melanocratic phenocrysts (pseudomorphosed amphibole and biotite) as well as leucocratic phenocrysts (quartz and sericitized felspars s.l.) determined by the Intercepts method applied on optical scans of three adjacent cut faces of each sample, (iii) X-ray micro-CT scanning of five selected samples using the HECTOR device at UGCT lab (Ghent University, Belgium).

AMS and melanocratic fabrics SPO are mainly marked by prolate shaped ellipsoids. Both subsets show similar and homogeneous orientation of their structures through the studied area, with E-W striking foliations dipping 70° to the North to subvertical. Leucocratic petrofabric SPO shows more heterogeneous distribution with a similar E-W to N120-striking foliations but generally subhorizontal to low dipping structures (< 30°). This discrepancy is thought to be due to differential record of the subvolcanic phenocrysts during the ultimate emplacement and solidification of the Lessines magmatic body. These results combined to field observations (e.g., enclave orientations, columnar joints, borehole logs) suggest that the Lessines intrusion is a complex dyke-sill hybrid system, made of a main subvertical dyke-like structure that fed lateral sills bodies.

How to cite: Triantafyllou, A., Baele, J.-M., Diot, H., Cnudde, V., Meftah, R., Vandycke, S., and Van Noten, K.: Tracking the Magmatic Flow in a Dyke-Sill Hybrid System using a Multi-Method Approach (AMS, SPO, X-ray micro-CT) for Petrofabrics Characterization (Lessines, Belgium), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5893, https://doi.org/10.5194/egusphere-egu2020-5893, 2020.

Recent eruptions such as the Kilauea 2018 (fissure) eruption on Hawaii are the result of magma intruding into Earth’s crust and ascending towards the surface. Magma is dominantly transported, through the shallow crust in form of vertical sheet intrusions (dykes). Even though dyke propagation and emplacement has been monitored with geodetic and geophysical methods, direct observations of subsurface intrusion processes remain inaccessible due to the hazardous nature of active volcanic and igneous systems. Therefore, we studied the extinct and eroded volcanic system of the Chachahuén volcanic complex (CVC) in Argentina to investigate the scale and physical mechanisms of magma transport in volcanic and igneous plumbing systems.

The Chachahuén volcanic complex is located in the northern part of the Neuquén Basin, east of the southern volcanic zone (SVZ) of the Andes. A decline in volcanic activity during the Quaternary and erosion have exposed the shallow part of the Miocene CVC’s plumbing system, including two major vertical sheet intrusions: (1) the Great Dyke and (2) the Sosa Dyke.

The objective of this ongoing study is to characterize the mechanisms of magma transport within the two exposed dykes to better understand the physical processes during their emplacement. We apply a multiscale approach combining field work and state-of-the-art analytical techniques, i.e., drone/ground-based photogrammetry, Fourier Transform Infrared Spectroscopy (FTIR), Electron Backscatter Diffraction (EBSD) and Anisotropy of Magnetic Susceptibility (AMS), with traditional geological methods, i.e., microstructural analysis and igneous petrology. Thus, we can investigate the effect of magma rheology (small-scale) on the outer shape and morphology of the dykes (large-scale).

Our results using high-resolution 3D outcrop models show a segmentation of the investigated dykes. Each of these dyke segments shows blunt ends. This suggests either the emplacement of a highly viscous magma or a weak brittle host rock. Flow features identified with AMS analysis indicate a dominantly lateral magma transport within the dykes. To estimate the magma viscosity during emplacement FTIR (H2O content of the initial melt), and microstructural analysis (for crystallinity) are performed at the moment. These analyses in combination with a map of the host rock and, the dyke morphologies, will help to characterize the dominantly controlling mechanism(s) of magma emplacements in the CVC. Finally, the new findings from this project will contribute to the general understanding on how the physical properties of the magma affect the shape of magma bodies and magma flow in the Earth’s shallow crust.

How to cite: Schmiedel, T., Burchardt, S., Guldstrand, F., Mattsson, T., Galland, O., Palma, O., Rhodes, E., Witcher, T., and Almqvist, B.: Magma transport in the shallow crust – the dykes of the Chachahuén volcanic complex (Argentina), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18651, https://doi.org/10.5194/egusphere-egu2020-18651, 2020.

Silicic magmas are the most evolved, most viscous and potentially most explosive melts present on Earth. Despite their importance, the processes leading to accumulation of large amounts of silicic magma in the crust are still a matter of debate. Ignimbrite sheets of large caldera forming eruptions are interpreted to be unique snapshots of upper crustal magma reservoirs just prior to eruption and hence represent an exceptional possibility to study pre-eruptive magmatic conditions within silicic reservoirs.

The Aso System, in Central Kyushu (Japan), is an archetypical example of a multicyclic caldera-forming volcanic edifice; it was built by four catastrophic caldera forming eruptions, with the latest (Aso 4) taking place approximately 90 ka ago. The ignimbrite sheets produced during the Aso eruptions are some of the first ever described compositionally zoned pyroclastic flow deposits and are interpreted to be the result of extensive magma mixing of two compositionally distinct magmas in an upper crustal reservoir.

Here, we propose an alternative view of the Aso 4 ignimbrite sheets based on re-evaluation of whole rock data combined with mineral and glass geochemistry. The relatively scarce presence of mafic pyroxenes and plagioclases indicate recharge of hot, mafic magmas occurring shortly prior to eruption. However, the large amount of crystal-poor, felsic material in early erupted units in combination with late-erupted, crystal-rich basaltic andesite clasts, which are enriched in compatible elements and rich in compositionally highly evolved minerals, lead to the conclusion that magma mixing alone is not able to explain the complexities observed in Aso 4 deposits. Evidence for crystal accumulation in late erupted basaltic andesite clasts implies the formation of melt-rich lenses within a crystal-rich reservoir due to significant crystal-melt separation. We therefore propose an origin of the compositionally zoned Aso 4 ignimbrite largely by erupting a heterogeneous upper crustal reservoir, consisting of crystal-poor rhyodacitic melt pockets within a cumulate mush. The emptying of this heterogeneous magma storage zone was likely triggered by a recharge event from deeper in the system, initiating partial melting of previously-formed crystals (rejuvenation), mingling/ mixing, pressurization, and finally catastrophic evacuation of the eruptible portions of the subvolcanic reservoir, including parts of the cumulate mush.

How to cite: Keller, F., Bachmann, O., Geshi, N., and Miyakawa, A.: The origin of large zoned ignimbrites: the case of Aso caldera, Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17755, https://doi.org/10.5194/egusphere-egu2020-17755, 2020.

Long-dormant volcanoes (quiescence time is several 100’s to 10’s thousand years between eruptions) pose a particular hazard, since the long repose time decreases the awareness and there is mostly a lack of monitoring. The Haramul Mic, a pancake-shaped flat dacitic lava dome is part of the Ciomadul Volcanic Complex in eastern-central Europe (Romania) and serves as an excellent example of such volcanoes. The Haramul Mic lava dome is the earliest product of the Young Ciomadul Eruption Period (YCEP), when the activity recrudesced in the area after about 200.000 years quiescence time. Eruption age of the dome determined by (U-Th)/He dating on zircon gave 154 +/- 16 ka that is in agreement with the youngest zircon U-Th outer rim date (142 +18/-16 ka). In the YCEP zircon crystallization dates record typically long, up to 350-400 kyr lifetime of the magmatic plumbing system, in case of  Haramul Mic the oldest zircon core is 306 +/- 37 ka old.

The 880.7 m high lava dome covers an area of 1.1 km² and has a volume of ~0.15 km³. It is composed of crystal-rich homogeneous high-K dacite. The average crystal content is 35-40% and consists of plagioclase, amphibole, biotite and accessory zircon, apatite, titanite and Fe-Ti oxides. The groundmass is mainly built up by perlitic glass with some microlites. The dacite includes mafic enclaves having plagioclase and amphibole besides a large amount of biotite crystals, that eventuates K-rich, shoshonitic bulk composition. The dacite contains abundant felsic crystal clots which comprise plagioclase, amphibole, biotite and interstitial vesicular glass.

Amphiboles are relatively homogeneous in chemical composition. They are low-Al hornblendes suggesting 700-800 ^oC crystallization condition at 200-300 MPa compared with experimental data. Al-in-hornblende geobarometer and amphibole-plagioclase geothermometer calculations give results reproducing these temperature and pressure ranges. Although the Kis-Haram dacite is fairly rich in 25-45 anorthite mol% plagioclase, no negative Eu anomaly can be observed in the bulk rock and the glass. Similarities between Fish Canyon Tuff and Kis-Haram rocks can be strikingly noted regarding the major and trace element contents of mineral phases, glass and bulk rock that all refer to a wet oxidised calc-alkaline magmatic system. The relatively small volume Kis-Haram lava dome represents a rejuvenated low-temperature granodioritic crystal mush having similar features as the large volume silicic eruption of Fish Canyon Tuff. Their recorded almost similarly long zircon crystallization intervals give an interesting aspect with regard to the thermal evolution of the magmatic system and eruption volumes.

This research was financed by the Hungarian National Research, Development and Innovation Fund (NKFIH) within No. K116528 project and was supported by the ÚNKP-19-1 New National Excellence Program of the Ministry for Innovation and Technology.

How to cite: Pánczél, E., Petrelli, M., Lukács, R., and Harangi, S.: The ’Little’ Fish Canyon Tuff in Romania: Rejuvenation of granodioritic crystal mush resulting in homogeneous dacite recorded by the Haramul Mic lava dome (Ciomadul) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19689, https://doi.org/10.5194/egusphere-egu2020-19689, 2020.

The presence of a thick continental crust makes Earth a unique planet in the solar system. During post-Archaean times, with the onset of plate tectonics, processes by which continents form is a complex function of juvenile growth and recycling of pre-existing crust. Indeed, post-Archean mantle-derived magmas commonly intrude pre-existing, felsic continental crust. As a result, the origin of upper crustal granitoids, the most accessible products of planetary differentiation, is either accounted for by the melting of the pre-existing mid- to lower crust or the differentiation of mantle-derived mafic magmas. It is therefore critical to identify the relative contribution of these two different granite-forming processes in a given magmatic province, as well as how this relative contribution evolves over time, to assess crustal growth and/or recycling. To shed some light on this question, we used the combination of oxygen, hafnium and uranium-lead isotopic systems in zircons from granitoids of the Ordovician Famatinian Arc (Argentina) representing a typical crust-forming geotectonic setting. While the lower crustal section of Valle Fertíl, representing the basal level of the Famatinian crust, is already well studied, little is known on the timing and nature of igneous processes that built up the mid- and upper crust. 

From our study, we observe a systematic co-variation of the O and Hf isotopic signatures of zircon in the mid- to upper crustal rocks, from a clearly crustal footprint (granodiorites with zircon δ¹⁸O of ca. +8 ‰; εHf_t of ca. –3) to a mantle-like signature (granites and rhyolites: zircon δ¹⁸O of ca. +5 ‰; εHf_t of ca. +5). Moreover, the high-precision (ID-TIMS) U-Pb dating obtained from the same zircons seem to record a progressive building of the Ordovician continental crust lasting for ca. 13Myrs from 483 to 470 Myrs ago. The results overlap with published ID-TIMS U-Pb data for the Famatinian lower crust, clustering at 470 Myrs, which confirms that the Famatinian Arc was a transcrustal magmatic system ultimately fed by mantle-derived magmas. In details, the oldest granitoids (483 Myrs) show the strongest crustal Hf-O isotopic fingerprint while the younger ones define a continuous range from this end-member towards the mantle signature. These results could be explained by (i) continuous ingrowth and “self-shielding” of lower crustal mafic intrusions progressively decreasing crustal melting or contamination of ascending mafic magma from a homogenous mantle source; (ii) progressive defertilization of an enriched lithospheric mantle or a strongly slab-enriched mantle wedge. The fact that the earliest (483 Myr-old) granitoids also show a more significant crustal contribution (ASI >1.1, inherited zircon cores) supports the first scenario. In this case, the combination of Hf-O isotopic studies as well as high precision U-Pb dating for the Famatinian arc comply with a progressive building of a magmatic column where a certain amount of time is needed for the system to mature and eventually reach mantle dominated processes in the formation of granites and so, new continental crust.

How to cite: Cornet, J., Laurent, O., Wotzlaw, J.-F., Otamendi, J., and Bachmann, O.: Evolution of crust vs. mantle contributions to continental arc granitoids within a few Myr: evidence from zircon Hf-O isotopes and high-precision U-Pb dating in the Famatinian Arc, Argentina , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19390, https://doi.org/10.5194/egusphere-egu2020-19390, 2020.

The Erzurum-Kars Volcanic Plateau (EKVP) was formed by volcanic eruptions during the Messinian-Zanclean (~5.5 Ma) period, related to a continental collision event between Eurasia and Arabia, initiated ~15 Ma ago. The EKVP unconformably overlies a series of older sedimentary formations spanning in age from Cretaceous to Miocene. It starts with a ~400 m thick pyroclastic-rich layer at its bottom, named the Akkoz basal tuff, consisting of rhyolitic and dacitic ignimbrites, pyroclastic fall and surge deposits, which are intercalated with andesitic and dacitic lavas. Upper layers of the plateau are dominated by andesitic and basaltic andesitic lavas (~100 m).

In the northwest of the study area, an eroded stratovolcano, named Hamamlı volcano, which is possibly coeval with the plateau volcanism is present. It covers ~280 km² area and consists of a thick sequence of rhyolitic lavas, tuffs, ignimbrites, perlites and obsidians. The best preserved volcanic edifice in the study area is the Greater Aladağ Stratovolcano with a footprint of ~230 km². It is composed of intermediate lavas with andesitic, dacitic, trachy-andesitic compositions, erupted ~3.55 Ma in Piacenzian. A small volcanic cone, named in this study as the Lesser Aladağ volcano, sits on the northern flank of the Greater Aladağ. Lesser Aladağ has an elliptical shape and is composed of basaltic-andesitic and basaltic trachy-andesitic lavas. Three semi-circular shaped rhyolitic domes called the Odalar rhyolite sit on the southern and eastern slopes of the Greater Aladağ. In the N and NE, the Aladağ volcanic sequence is unconformably overlain by a younger (~2.7 Ma) sequence of olivine basalts and basaltic andesites, which is known as the Kars volcanic plateau.

All volcanic products in the study area are calc-alkaline in character with a clear subduction signature. Results from our petrological modelling studies indicate that the magmas that fed the Aladağ volcanic system were evolved in a chamber, which was periodically replenished by fresh and primitive basaltic magma. Our assimilation model results based on the equations of DePaolo (1981) and Aitcheson and Forrest (1994) show that fractional crystallization was more important than crustal assimilation process in evolved lavas of the Aladağ system. Interestingly, EC-AFC model results indicate that some of the youngest basalts from the Kars volcanic plateau contain higher degrees of crustal assimilation relative to more evolved lavas.

Crystal chemistry of amphiboles by EMP from the amphibole-bearing lavas of the Akkoz basal tuff layer indicates that they had experienced crystallization pressures between 5.63 and 6.45 kbar and temperatures between 949 and 1026 °C during their magma chamber evolution. On the other hand, pyroxene thermo-barometry of the Aladağ units has given crystallization pressures between 0.8 and 4.8 kbar, and temperatures from 1025 to 1078 °C, implying polybaric fractionation. Calculated crystallization pressures and temperatures from the younger lavas of the Kars volcanic plateau are ~8.8 kbar and ~1179 °C respectively. Our partial melting models indicate that the primitive basaltic magmas might have been derived from a metasomatised spinel peridotite source with varying melting degrees from 0.7% to 2%.

How to cite: Duru, O. and Keskin, M.: MAGMATIC EVOLUTION of the ALADAĞ VOLCANIC SYSTEM and SOUTHERN EDGE OF THE ERZURUM-KARS VOLCANIC PLATEAU (SARIKAMIŞ, CITY of KARS, NE TURKEY), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21626, https://doi.org/10.5194/egusphere-egu2020-21626, 2020.

In the southern part of the Adamello Batholith (43-33 Ma; Schaltegger et al., 2019) in Northern Italy (Re di Castello superunit), we identified a multi-generational dyke suite with “exotic” chemical compositions intruding quartz-dioritic units surrounding a gabbroic complex. These dykes are characterised by SiO₂ contents between 43 and 46 wt.%, high Al₂O₃ (20-21 wt.%), and low MgO and Ni (below 6.5 wt.% and 40 μg/g, respectively), displaying a nepheline-normative character. Furthermore, they exhibit positive Sr and Ba anomalies. These chemical features exclude a possible primitive character or derivation from a typical calc-alkaline liquid line of descent, as identified for the Adamello Massif (Ulmer et al, 2018). The primocrystic cargo of these dikes (clinopyroxene, anorthitic plagioclase, and low-Si, high-Na pargasitic amphibole) displays striking similarities with cumulate crystals of the contiguous Blumone amphibole gabbroic cumulate, inferring mechanical interaction of these exotic liquids with and/or derivation from the cumulate complex. Amphibole-plagioclase equilibration temperatures of the dikes (875 to 775ºC) are consistent with thermal equilibration with the surrounding quartz-dioritic mush. Sharp contacts and dyke fragmentation are also observed and are thermally congruent with the ductile-brittle transition of a quartz-dioritic to tonalitic mush (Marxer & Ulmer, 2019).

Simple mass balance calculations modelling of the peritectic melting of pargasitic amphibole and high-An plagioclase (major mineral phases of the contiguous amphibole gabbroic cumulates) with simultaneous crystallisation of low-Al clinopyroxene reveal that melt compositions similar to these dykes can be achieved with amphibole-plagioclase proportions ranging between 65:35 and 50:50. To verify if peritectic cumulate remelting represents a possible generation mechanism of these dykes we performed  experiments at 0.2 GPa.

Established phase equilibria of these dyke compositions reveal a lack of near-liquidus olivine, which is a rare phase in gabbroic complex. This is consistent with preliminary experimental results on cumulate melting, where olivine is also absent at high temperatures (> 1075ºC). These observations further disprove the petrogenesis of these liquids via a calc-alkaline liquid line of descent, where mafic magmas would be early saturated in olivine at low pressure further supporting their generation by local remelting of amphibole-plagioclase dominated mafic cumulates.Geochemical as well as experimental results both strongly point towards the petrogenesis of these nepheline-normative, high-Al, low-Mg picrobasalts by low pressure peritectic melting of a pargasite-anorthite cumulate assemblage in an active magmatic system.

Marxer, F. & Ulmer, P. Contrib Mineral Petr. 174(10), 84 (2019).

Schaltegger, U. et al. J Petrol. 60(4), 701-722 (2019).

Ulmer, P. et al. J. Petrol. 59(1), 11-58 (2018).

How to cite: Pimenta Silva, M., Ulmer, P., and Müntener, O.: Self-cannibalisation of an active cumulate system (Blumone complex, Adamello, Italy): Geochemical and experimental evidence , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11109, https://doi.org/10.5194/egusphere-egu2020-11109, 2020.

The Northern Ireland sector of the North Atlantic Large Igneous Province (NIAP) is the biggest onshore exposure of the British and Irish Igneous Province. The Antrim Lava Group is composed mostly of flood basalt sequences (Lower and Upper Basalt formations) with associated acid-basic central complexes, dyke swarms, plugs and sill complexes (Cooper 2004; Cooper & Johnston 2004; Cooper et al. 2012) that display unconformable and cross cutting field relationships. This study has for the first time generated a self-consistent and representative chronology using high-precision CA-ID-TIMS U-Pb zircon isotopic dating across the range of lithologies to provide a fuller picture of how the region was operating during the Paleogene.

Results indicate that punctuated magmatism within the north of Ireland lasted at least c. 5.5 Myrs from c.61.5 to 56 Ma. The 61.5 Ma age comes from magmatic zircons from a paleosoil immediately below the Lower Basalt Formation (LBF), and is believed to represent the onset of magmatism in the region. This age is matched by that of the Killala-Erne Dyke Swarm (c.61.5 to c.61 Ma) which is now considered to have fed melt to LBF lava flows. Dates of c.61 Ma for the Tardree and c.60.7 Ma for Slieve Gullion igneous complexes are the youngest of this grouping which together spans about 1 Myrs and may represent the igneous activity associated with a single pulse of the Icelandic Plume.

Following the initial activity there was a break in magmatism-volcanism which lead to the development of a thick weathering profile referred to as the Interbasaltic Formation. We then see the development of a regionally significant unconformity and deposition of the Coagh Conglomerate Member which includes clasts of Tardree Complex (or similar) rhyolite. This was followed by extrusion of the Causeway Tholeiite Member (CTM) and Upper Basalt Formation (UBF) across the region. A new age for the Portrush Sill Complex at c. 58.5 Ma provides a constraint on this episode of magmatism, however, it the combination of regional unconformity, outpouring of flood basalts and other magmatism that suggest a second pulse may be represented.

The Mourne Mountains Complex at c.56-56.5 Ma is not associated with flood basalts in Northern Ireland, however, in Scotland basalts of the Upper Skye Lava Formation are of similar age to the Mourne granites and together they might represent a third pulse of the plume in the region.

New U-Pb zircon geochronology has allowed for significant reinterpretation of the regional scale geology and stratigraphy of the Antrim Lava Group. Geochronological constraints define an early episode of igneous activity that is separated from the next by a prolonged period of weathering and the formation of a regionally significant unconformity. In summary three temporally discrete episodes of magmatism and tectonics with 1-2 Myr periodicity are observed that we believe resulted from a pulsing Icelandic Plume head.

How to cite: Cooper, M., Tapster, S., and Condon, D.: Feeling the pulse? New high resolution U-Pb zircon geochronological constraints for the Northern Ireland sector of the North Atlantic Igneous Province, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8464, https://doi.org/10.5194/egusphere-egu2020-8464, 2020.

In recent years, technical developments in isotope dilution thermal ionization mass spectrometry technique (ID-TIMS) have pushed the precision of single zircon U-Pb geochronology to new limits. The use of interlaboratory calibrated U-Pb tracer solutions for isotopic dilution [1] paired with using newly developed high ohmic resistors (10¹³Ohm) in Faraday cup amplifiers, allow the determination of single zircon dates with precision and accuracy at the 0.02 % level [2].This level of analytical precision makes the ID-TIMS technique a geochronological tool able to unravel the detailed temporal evolution of magmatic plumbing systems older than the Mesozoic Era.

In the southern Alps, a thick sliver of continental crust, tilted and exhumed during the Alpine orogeny, is exposed as a complete crustal cross-section (Ivrea crustal section). This section preserves a transcrustal magmatic system, developed in an extensional environment in ca. 4 My during the Early Permian [3]. Its upper crustal portion consists of a zoned granitic intrusion (Valle Mosso pluton) overlaid by a dominantly rhyolitic caldera-related volcanic field (Sesia Caldera).

To obtain a time-integrated view of the petrological evolution of this plumbing system, we combine a new ultra high precision ID-TIMS zircon U-Pb dataset with zircon geochemistry from samples collected in compositionally and texturally different units of the Valle Mosso pluton and Sesia Caldera. All the analyzed units are coeval within 700 ky and the overall trends in zircon trace elements (Eu*/Eu, Zr/Hf, Sm/Yb) suggest an evolution of the reservoir dominated by fractional crystallization. The data show a ca. 200 ky gap in zircon crystallization, following the injection of recharge magma that triggered the eruption of the crystal-rich rhyolite followed by caldera collapse [3]. This suggests mass addition and rejuvenation of a partly crystallized mush, which temporarily hindered zircon crystallization. On the other hand, crystal-poor rhyolites, characterized by a younger eruption age and evolved zircon composition, likely represent late stage evacuation of evolved melt lenses extracted from a mostly crystalline framework.

[1] Condon, D. J., et al., 2015, Geochim. Cosmochim. Acta, 164, 464-480.

[2] Wotzlaw, J. F., et al., 2017, J. Anal. At. Spectrom., 32, 579-586.

[3] Karakas, O., et al., 2019, Geology, 47, 1-5.

How to cite: Tavazzani, L., Wotzlaw, J.-F., Economos, R., Sinigoi, S., Demarchi, G., Laurent, O., Chelle-Michou, C., and Bachmann, O.: Detailed timescale of magma-chamber assembly and eruption revealed by ultra-high precision zircon U-Pb geochronology on a Permian caldera plumbing system, Sesia Magmatic System (southern Alps, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21552, https://doi.org/10.5194/egusphere-egu2020-21552, 2020.

Understanding the assembly of eruptible magma in volcanic plumbing systems through time is key to the evaluation of hazard scenarios at potentially active volcanoes. In this respect, zircon geochronology provides a temporally resolvable record of the presence of magma. However, which specific processes and associated timescales are captured by zircon age distributions is not well constrained. Here we use zircon geochronology and geochemistry and thermal modelling of pulsed magma injection in the Earth crust to quantitatively invert zircon ages and recalculate magma fluxes and the rate of accumulation of eruptible magma in time. Zircon crystals have been analyzed from 4 late Pleistocene eruptions of Nevado de Toluca, a long-lived currently dormant dacitic stratovolcano in Central Mexico. ²³⁸U-²³⁰Th and ²³⁸U-²⁰⁶Pb age distributions show a protracted zircon crystallization history of ~900 ka in the magmatic plumbing system, a large fraction of the 1500 ka record of volcanic activity at the surface for this volcano. The 4 studied eruptions show similar broad age spectra, which are overlapping with each other and comparable peak zircon crystallization ages between 150 and 250 ka. Our dataset suggests that interstitial melt extraction (including zircon crystals) from highly crystallized resident magma and mixing thereof with fresh recharge magma surges is very efficient beneath Nevado de Toluca. Zircon trace element data, together with the geochronology show that the observed temporal trends in zircon geochemistry are consistent with tracking long-term assembly processes beneath the volcano operating over more than 1 million years. The combination of these results and thermal modelling allow us to quantify the rate of magma input, intrusive/extrusive ratio and the rate of accumulation of eruptible magma at Nevado de Toluca, which is essential to estimate the maximum potential size of the next eruption from this system.

How to cite: Weber, G., Caricchi, L., Schmitt, A., and Arce, J. L.: Thermochemical modelling of zircon age distributions from Nevado de Toluca volcano, Trans Mexican Volcanic Belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22291, https://doi.org/10.5194/egusphere-egu2020-22291, 2020.

Plutonic rocks in magmatic arcs record variations in composition, thermal flux, and dynamics of subduction through time. In the northern Andes, arc magmatism of Jurassic age registers a complex history, including the fragmentation of Pangea at the end of the Triassic as well as the beginning of a new subduction zone in the Jurassic located at the western margin of South America. Two contrasting models have been proposed by previous researches to explain the evolution of this arc: i) continuous subduction with a slab-roll back that produced a crustal thinning and ii) oblique subduction associated with a crustal thickening.

We characterized the emplacement conditions and crustal thickness variations of the Jurassic and Early Cretaceous arc in the northern Andes from 170 to 130 Ma using a combination of thermobarometers and trace element signatures and reviewed the previously suggested evolution models. The zircon and apatite saturation temperatures indicate that the intermediate magma became Zr and P₂O₅ oversaturated at 695-739 °C and 849-909 °C, respectively. Pressures obtained with the Al-in-hornblende barometer shows that the magma emplacement pressures varied from 1.2 to 7.1 kbar, with two distinct trends. A low-pressure trend (<2 kbar) related to different stock size bodies emplaced through the arc formation and a high-pressure trend (>5 kbar), which is restricted to the southern segment of the arc at the end of the Jurassic. Low Sm/Yb and Dy/Yb ratios show that the magma interacted with an amphibole-rich crust, implying that the Northern Andes was characterized by a thin crust during the Jurassic.

The shallow emplacement pressures and thin crust suggest that the Jurassic magmatic arc record a predominant extensional tectonic style that could be linked with the Pangea breakup and the beginning of the arc magmatism. However, the younger magmatic pulses are characterized by higher emplacement pressures associated with an increase in crustal thickness during convergence. Such variation indicates that the Jurassic magmatism in the Northern Andes experienced significant changes in their tectonic controls and not a single dominant mechanism, as has been proposed.

How to cite: Chavarria, L., Bustamante, C., Cardona, A., and Restrepo, M.: Thermobarometry of Jurassic and Early Cretaceous plutonic rocks from the Northern Andes: tracing magmatic and tectonic changes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-921, https://doi.org/10.5194/egusphere-egu2020-921, 2020.

High precision dating of Large Igneous Provinces (LIP) is not only useful to understand their link to environmental changes and mass extinctions (Courtillot and Renne, 2003), but they also provide insights into the geodynamic setting in which they form (Encarnación et al., 1996). The Drakensberg continental flood basalts of South Africa and Lesotho are part of the Karoo LIP, which is presumably responsible for a phase of global climate change and disturbance of the oceanic ecosystems (the so-called Toarcian oceanic anoxic event T-OAE; Pálfy and Smith (2000)). However, the paucity of zircon or baddeleyite in most continental flood basalts renders is difficult to match the sub-permil age precision and accuracy that is typical for high-precision U/Pb CA-ID-TIMS age determination. Previous attempts to date the Karoo lavas using the ⁴⁰Ar-³⁹Ar method failed to yield sufficient precision and accuracy for resolving the sequential stacking of the different basalt units. For example, ⁴⁰Ar-³⁹Ar  analyses of carefully selected plagioclase separates yielded dates that are inverted relative to their stratigraphic position, with uncertainties that encompass the entire duration of volcanism in the area (Jourdan et al., 2007; Moulin et al., 2017). Here we test the hypothesis that previous, inconsistent ⁴⁰Ar-³⁹Ar dates of plagioclase were a consequence of degassing of primary, metasomatic and alteration phases (mainly zeolites with subordinate sericite and carbonate) within single or multiple crystals. The lavas are mainly tholeiitic basalts that display two distinct sizes of plagioclase, which can be dated separately. Petrological characterization of these two size fractions shows that the larger plagioclase crystals (100-400 μm) are more altered and fractured than the smaller grains and are therefore more likely affected by post-crystallization disturbance of the Ar isotopic system. We present preliminary ⁴⁰Ar-³⁹Ar data from i) untreated plagioclase that hosts visible alteration phases, ii) untreated plagioclase that is devoid of visible alteration phases (2 grain size aliquots), and iii) leached plagioclase that is devoid of visible alteration phases (2 grain size aliquots). The results of this study may enhance the effectiveness of the ⁴⁰Ar-³⁹Ar dating technique to accurately constrain the crystallisation ages of altered mafic lavas, which form the majority of the exposed Karoo LIP flood basalts. Ar isotope data were collected using a multi-collector Argus VI mass spectrometer, and irradiated in an un-shielded reactor position to optimize the formation of ³⁸Ar from Cl to permit identification of different gas reservoirs in the sample through isochemical dating, based on Ca, K and Cl in-situ concentration (EPMA) and Ar isotopic ratios.

How to cite: Antoine, C., Spikings, R., Miletic Doric, D., Marsh, J. S., and Schaltegger, U.: 40Ar-39Ar geochronology of the Karoo flood basalts: tracking disturbance in the isotopic system., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1457, https://doi.org/10.5194/egusphere-egu2020-1457, 2020.

Andean volcanic activity consists in long quiescence periods interrupted by violent explosive eruptions of diverse intensity, magnitude and duration. Anticipating these transitions between low energy eruptions and violent major explosions is a challenge for modern volcanology. Many parameters could play a role in explaining these fast transitions, such as the conduit process (degassing and microlite crystallizations) but also deeper processes (crustal assimilation, nature of the mantle-derived primitive liquids).

Geophysical methods (seismic, acoustic, heat flow, ground deformation, gas emissions) are currently used by volcano observatories in order to identify unrest phases at medium to short time-scale. Furthermore, the evolution of pre-eruptive magmatic reservoirs is often studied via micro-petrological techniques, such as the detailed study of growth-zoned crystals emitted during paroxysmal phases. These zonation patterns are related to changes in composition and/or P-T-XH₂O conditions in the magma reservoir caused by its complex evolution through time (injection/recharge, crystallization, degassing processes). Thanks to these methods, a tight relation between mafic magma recharge and explosive volcanic reactivation has been constrained with delay time estimated from years to months. Yet, such petrologic methods are not efficient as predictive tools.

In this study, we performed micro-geochemical analysis (trace elements, Pb-Sr isotopes) on time-series of juvenile ash samples (80) emitted by Tungurahua volcano over its most recent period of activity (1999-2016). Geochemical time series display an oscillating pattern with good correlations between Pb-Sr isotope compositions evolution and most trace elements as well as between whole rocks and ashes compositions. Pb-Sr isotope cyclic signal seems to reveal that paroxysmal phases first emissions are the most radiogenic of each eruption phase. As the eruption goes on, we observe a rapid decrease in Pb-Sr isotope composition. Finally, it appears that both ²⁰⁶Pb/²⁰⁴Pb and ⁸⁷Sr/⁸⁶Sr ratios of juvenile ash samples decrease from 2013 to the last year of activity in 2016, leaving the final emissions with the less radiogenic Pb-Sr isotope compositions.

The oscillating geochemical pattern of juvenile ash samples has been meticulously compared to the well-known eruptive dynamics of the volcano through time, providing clues on the processes triggering the violent reactivation phases. Indeed, the rapid geochemical evolution of the magma reservoir with a high temporal resolution has been interpreted as the effect of deep magma recharges in the shallow reservoir of Tungurahua volcano.

Coupling continuous geochemical analysis to geophysical monitoring puts in evidence a geochemical precursor for eruptive phases, delivering a better understanding on the global stratovolcano system and assuring an improvement in volcanic monitoring.

How to cite: Sainlot, N., Vlastélic, I., Samaniego, P., Bernard, B., Nauret, F., and Hidalgo, S.: Pb-Sr isotope temporal variations on juvenile ash samples from the last eruptive period of Tungurahua volcano (1999-2016), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3595, https://doi.org/10.5194/egusphere-egu2020-3595, 2020.

Terrestrial analogues are often investigated to get insights into the geological processes occurring on other planetary bodies. The pyroxenitic layer of the 120m-thick magmatic pile Theo’s Flow (Archean Abitibi greenstone belt Ontario, Canada), due to its petrological similarities, has always been regarded as the terrestrial analogue for Martian nakhlites (e.g. Lentz et al. 2011). However, its origin and cooling history and, as a consequence those of nakhlites, have always been a matter of vigorous debate. Did this lava flow originate from a single magmatic event similar to those supposed to occur on Mars or do the different units derive from multiple eruptions?

To answer this question, we calculated the closure temperature and the cooling history for six augite crystals of Theo’s Flow lava sampled at four different stratigraphic depths. These results were then coupled with (i) the low viscosity data by Vetere et al. (2019) on the same composition and (ii) the results from the finite difference method in order to test the possible emplacement mechanisms for Theo’s Flow.

The combination of geothermometric constraints on augite single crystals and numerical simulations in the framework of a multi-methodological approach, allowed us to demonstrate that Theo’s Flow has been formed by multiple magma emplacements that occurred at different times (Murri et al. 2019). Moreover, this discovery also supports the idea that the enormous lava flows with similar compositions observed on Mars could be the result of a process where low viscosity lavas are emplaced during multiple eruptions. This has profound implications for understanding the multiscale mechanisms of lava flow emplacement on Earth and other Terrestrial bodies.

M.M. and M.A. have been funded by the IMPACt project (R164WEJAHH) and by the ERC-StG TRUE DEPTHS under the European Union’s Horizon 2020 Research and Innovation Programme (n. 714936) to M. Alvaro. M.C.D. has been funded by the IMPACt project (R164WEJAHH) to M. Alvaro. D.P. has been funded by the ERC Consolidator Grant ERC-2013-COG (n. 612776) for the CHRONOS project to D. Perugini. The Alexander von Humboldt foundation senior research grant to F.V. is acknowledged. M.A. is also supported by the Ministero dell’Istruzione dell’Università e della Ricerca (MIUR)Progetti di Ricerca di Interesse Nazionale (PRIN)Bando PRIN 2017 - Prot. 2017ZE49E7_005.

Lentz, R.C.F., McCoy, T.J., Collins, L.E., Corrigan, C.M., Benedix, G.K., Taylor, G.J. and Harvey, R.P., 2011. Theo's Flow, Ontario, Canada: A terrestrial analog for the Martian nakhlite meteorites. Geological Society of America Special Papers, 483, pp.263-277.

Murri, M., Domeneghetti, M.C., Fioretti, A.M., Nestola, F., Vetere, F., Perugini, D., Pisello, A., Faccenda, M. and Alvaro, M., 2019. Cooling history and emplacement of a pyroxenitic lava as proxy for understanding Martian lava flows. Scientific reports, 9(1), pp.1-7.

Vetere, F., Murri, M., Alvaro, M., Domeneghetti, M.C., Rossi, S., Pisello, A., Perugini, D. and Holtz, F., 2019. Viscosity of Pyroxenite Melt and its Evolution during Cooling. Journal of Geophysical Research: Planets, 124(5), pp.1451-1469.

How to cite: Murri, M., Domeneghetti, C. M., Fioretti, A. M., Nestola, F., Vetere, F., Perugini, D., Pisello, A., Faccenda, M., and Alvaro, M.: Thermal history and emplacement mechanisms of Theo’s Flow lava: a proxy for Martian lava flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8372, https://doi.org/10.5194/egusphere-egu2020-8372, 2020.