GMPV8.1

The heterogeneous presence of ephemeral magmatic systems below the ridge axis and their complexity mostly account for the heterogeneous character of the oceanic crust accreted at (ultra) slow-spreading ridges. In order to better understand the magmatic processes involved in slow-spreading lower oceanic crust formation, we studied a drilled section of an oceanic core complex (OCC) interpreted as an exhumed portion of lower crust close to the ridge axis. We focused on ODP Hole 735B which presents the most primitive lithologies sampled at Atlantis Bank OCC (Southwest Indian Ridge) in a ~250 m thick section previously interpreted as a single crustal intrusion.

We combined detailed structural and petrographic data with whole-rock and in situ mineral analyses to determine the processes of emplacement and differentiation of melts within this section. The lower half of the unit is comprised of alternating troctolites and olivine gabbros showing intrusive contacts, and both magmatic and crystal-plastic fabrics. Such features are lacking in the upper half, rather uniform, gabbroic sequence. Whole-rock compositions highlight the cumulative character of both lower and upper units, and a great compositional variability in the lower sequence, whereas the upper sequence is rather homogeneous and differentiates up-section. In situ analyses of mineral phases document magma emplacement processes and provide evidence for ubiquitous reactive porous flow during differentiation. Comparison between both units' geochemistry also led us to strongly favor a model of formation of the reservoir that genetically links melts from the lower and the upper unit.

We show that the whole section, and related geochemical units, likely constitutes a single magmatic reservoir, in which the lower unit formed by emplacement of primitive sills related to the continuous recharge of primitive melts. Recharge led to partial assimilation of the crystallizing primitive mush, and related hybridization with interstitial melts. Hybrid melts were progressively collected in the overlying mushy part of the reservoir (upper unit), whereas the sills' residual melt differentiated by reactive porous flow processes under a predominantly crystallization regime. Similarly, hybrid melts’ evolution in the upper unit was governed by upward reactive porous flow and progressive differentiation and accumulation of evolved melts at the top of the reservoir. Our results provide the first integrated model for magma reservoir formation in the lower slow-spreading oceanic crust, and have potential implications regarding the lower crust structure and the composition of MORBs.

How to cite: Boulanger, M., France, L., Deans, J., Ferrando, C., Lissenberg, J., and von der Handt, A.: Formation and evolution of a magma reservoir at a slow-spreading center (Atlantis Bank, Southwest Indian Ridge), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-880, https://doi.org/10.5194/egusphere-egu21-880, 2021.

The Sumail Ophiolite at the northeastern coast of the Sultanate of Oman provides an ideal field laboratory for studies on fast-spread oceanic crust on land. Based on numerous campaigns in the past, the Oman Drilling Project (OmanDP) of the International Continental Scientific Drilling Program (ICDP) obtained nine 300 to 400 m long drill cores covering sections from the upper mantle to the dyke/gabbro transition zone. Drill core GT1 is located in the layered gabbros between ~1200 and ~800 m above the Moho transition zone (m.a.M.) and comprises of modally layered gabbro with cm-scale coherent bands of troctolite, anorthosite, and wehrlite. We prepared thin-sections with a small average spacing of <2 m and analyzed them by petrological, microstructural and geochemical methods. Clinopyroxene reveals Mg# (where Mg# = Mg/Mg+Fe x 100; molar basis) between 74 and 86, with some heavily altered olivine relicts between 70 and 83, and Ca# (where Ca# = Ca/Ca+Na x 100; molar basis) of plagioclase range from 68 to 87. The plots of these data show clear and consistently decreasing trends from the base of the drill core up section to a crustal height of 1070 m.a.M. where all fractionation indices show significant minima. Above 1070 m.a.M., the indices increase to their maxima. Clinopyroxene shows core/rim zonation in Mg# and TiO₂ content with more primitive core compositions. However, distinct zonation is only observed above the minima mentioned above. Besides this general fractionation trend from the core base to 1070 m.a.M., individual fractionation trends on the scale of several decameters can be defined along the core (e.g., 820 to 895, 890 to 970, and 1085 to 1110 m.a.M.). As a quantifier of the plagioclase fabric symmetry, we used the BA index which ranges from 0 for a purely foliated to 1 for a purely lineated fabric. We found that the rock fabric changes parallel the observed fractionation trend with significant lineation at the base of the core and evolving towards almost purely foliated fabrics up section to 1070 m.a.M., indicating either an intense compaction or weaker shearing, or both at 1070 m.a.M. A possible scenario creating the observed trends is an evolved melt entering the more primitive crystal mush at 1070 m.a.M.  and crystallizing primary phases with significantly more evolved compositions. In such an environment, where the liquid/solid ratio is increased, minerals may be more sensitive to compaction and less affected by shearing which is possibly induced by convection of the upper mantle. Magmatic deformation would therefore lead to a strong foliation with only a limited lineation component. Moreover, we interpret the observed decameter-scale fractionation trends, also being accompanied by slight changes in the fabric, as results of individual magma reservoirs crystallizing in-situ and leading to the accretion of the lower gabbros in Oman (e.g., [1]).

[1] Kelemen, P. B., Koga, K., & Shimizu, N. (1997). Geochemistry of gabbro sills in the crust-mantle transition zone of the Oman ophiolite: Implications for the origin of the oceanic lower crust. Earth and Planetary Science Letters, 146(3-4), 475-488.

How to cite: Mock, D., Neave, D. A., Müller, S., Garbe-Schönberg, D., Ildefonse, B., Koepke, J., and Science Team, O. D. P.: Accretion of fast-spread lower oceanic crust: drill core GT1 from the ICDP Oman Drilling Project, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6157, https://doi.org/10.5194/egusphere-egu21-6157, 2021.

Ciomadul is the southernmost eruptive centre of the post-collisional Călimani-Gurghiu-Harghita andesitic-dacitic volcanic chain (SE Carpathians, Romania) and represents the latest manifestation of the Neogene to Quaternary volcanism in the Carpathian-Pannonian Region. Ciomadul consists of older, peripheral shoshonitic to dacitic lava domes formed episodically between 1 Ma and 300 ka and a voluminous, central volcanic complex developed within the last 200 ka. Although several lines of evidence (based on petrology, geophysics and gas monitoring) suggest a long-lived magmatic plumbing system holding a potentially active magma storage (“PAMS” volcano) beneath Ciomadul, the pre-eruptive conditions of the upper crustal magma reservoir (including temperature, oxygen fugacity and TiO₂ activity) are not completely explored so far. In this study 23 rock samples, representing the whole volcanic activity of Ciomadul in time, were involved. Fe-Ti oxide (magnetite-ilmenite) grains were selected from magnetic heavy minerals, but only a few of the samples contained both magnetite and ilmenite crystals. Equilibrium between Ti-magnetite and ilmenite was tested by their chemical composition (Mg/Mn ratios).

Various geothermobarometer calibrations, including Andersen and Lindsley (1985, 1988) as well as Ghiorso and Evans (2008), were applied to calculate temperature and oxygen fugacity from Fe-Ti oxide compositions. Our results show that, in case of dacitic pyroclastic rocks, temperature values gained by the method of Ghiorso and Evans are significantly lower (640–780 °C) than those obtained by the geothermometers of Andersen and Lindsley (1985, 1988), showing 750–830 and 710–790°C temperatures, respectively. On the other hand, andesitic lava dome rocks of Dealul Mare show higher, 800–900 °C temperature according to all of these methods. The obtained temperature was compared with amphibole-plagioclase thermometry results and this shows a better agreement with the values yielded by the Andersen and Lindsley (1985) Fe-Ti oxide thermometry, particularly for the pumice samples.

In case of oxygen fugacity, the Ghiorso and Evans (2008) and Andersen and Lindsley (1985) methods showed fairly similar values (fO₂=0.9–1.8) whereas the Andersen and Lindsley (1988) calculations gave higher oxygen fugacity (fO₂=1.1–2.5). Nevertheless, these results, irrespective the applied calculation methods, suggest relatively oxidized conditions (ΔNNO>1) what is comparable with many other andesitic to dacitic volcanic systems (e.g. Mount St. Helens, Mount Unzen, Santorini). Values of TiO₂ activity was calculated and obtained a range between 0.76 and 0.98 what is consistent with the common presence of titanite.

This study was financed by NKFIH K135179 project.

Andersen, D.J. & Lindsley, D.H. (1985). EOS Transactions of the American Geophysical Union, 66, 416.

Andersen, D.J. & Lindsley, D.H. (1988). Amer Miner 73:714–726.

Ghiorso, M.S. & Evans, B.W. (2008). Amer J Sci 308:957–1039.

How to cite: Szemerédi, M., Mészáros, K., Lukács, R., Kovács, Z., and Harangi, S.: Constraints on the pre-eruption thermal and fO2 conditions in the magma reservoir of Ciomadul (SE Carpathians, Romania) based on Fe-Ti oxide geochemistry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14613, https://doi.org/10.5194/egusphere-egu21-14613, 2021.

Haramul Mic is a ~0.15 km³ volume, crystal-rich, homogeneous, high-K dacite lava dome, which is one of the oldest ones in the Ciomadul Volcanic Complex (Romania, eastern-central Europe). The eruption that formed the lava dome occurred after about 200.000 years of quiescence. 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). The apparently continuous crystallization of zircon between the eruption age and the 306 +/- 37 ka oldest zircon core date records a long-living magmatic plumbing system.

The Haramul Mic lava dome rock has 35-40% average crystal content 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 and sheared vesicles. The dacite contains sparse mafic enclaves with K-rich, shoshonitic bulk composition, composed of plagioclase and biotite in addition to less amount of amphibole. Felsic crystal clots are more common and they comprise plagioclase, amphibole, biotite and interstitial vesicular glass.

Trace element content of the mineral phases and the groundmass glass was determined by LA-ICP-MS. All of the studied phases show homogeneous trace element compositions and along with the textural characteristics these imply general equilibrium state in the magma storage system before the eruption. Amphibole-plagioclase geothermometer and geobarometer calculations result in 700-800 ^oC crystallization temperature and 200-300 MPa crystallization pressure.

In order to reveal the magma chamber processes that triggered the eruption and formed the Haramul Mic lava dome after long quiescence time, it is necessary to understand better the behaviour of trace elements as the most sensitive indicators of magma reservoir mechanisms. We determined mineral-liquid trace element partition coefficients and evaluated the result in the context of crystal lattice strain model. They show many similarities with those proposed for the Fish Canyon Tuff dacite except for Li and Sc. The anomalous behaviour of Sc is clearly expressed by the elevated concentration in the glass phase and many times, there are some zonation in Sc from crystal core to rim. This could be explained either by inherently higher Sc content of the melt reflecting the nature of the primary magmas or by partial remelting of biotite just before the eruption. Significant positive anomaly of Li content can be observed in biotite crystals of the mafic enclave compared with the dacitic host rock. Li content of plagioclase varies between 15-30 ppm with slight rimward depletion.

Eruption initiation cannot be explained by physical mixing of mafic recharge magma, but rather by volatile transfer or second boiling. The water-rich nature of the melt is reflected by the abundant vesicles in the glassy groundmass. Furthermore, the amphibole phenocrysts have sharp margin without resorption rim, which suggest hydrous melt phase and relatively fast magma ascent.

This research belongs to the NKFIH-OTKA K135179 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.: Trace element partition coefficients and petrogenesis of the 154 ka dacitic Haramul Mic lava dome (Ciomadul, Romania), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13667, https://doi.org/10.5194/egusphere-egu21-13667, 2021.

The Colli Albani volcano is an ultrapotassic caldera complex located 30 km to the SE of Rome and has displayed a wide range of eruptive behaviors, ranging from effusive activity to highly explosive and large volume eruptions (up to 63 km³ dense rock equivalent per eruption) despite its mafic nature.

We combine physical volcanology, petrology, and geochemistry to focus on the mildly explosive to effusive products of two sections (Tuscolo and Artemisio) which are located on opposite sides of the main caldera and stratigraphically between the last large ignimbrite, Villa Senni. The target of this study is to identify the processes responsible for the transition from the smaller explosions to the larger caldera-forming ignimbrite eruptions, and eventually trace how the magmatic system rebuilds in the interim.

Whole rock analyses, mineral chemistry, and petrography of fall deposits from both field localities are compared with an existing dataset for the Villa Senni ignimbrites. We will use unsupervised and supervised machine learning approaches to identify similarities and differences between large caldera-forming eruptions and mild-explosive to effusive activity and identify the processes modulating the transition between these two behaviours.

How to cite: Ágreda López, M., Caricchi, L., Jorgenson, C., Musu, A., and Giordano, G.: The transitions from mildly explosive to caldera-forming eruptions at Colli Albani volcano (Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10054, https://doi.org/10.5194/egusphere-egu21-10054, 2021.

Crystal clustering influences the formation of crystal mushes and the rheology and differentiation of magmas. Heterogeneous nucleation is known to be an important cluster-forming mechanism, but there has been little systematic experimental study of cluster formation and evolution.

In this study, we analysed dynamic crystallization experiments from Pontesilli et al. (2019), focusing on clusters of clinopyroxene (cpx) and titanomagnetite (tmt). These experiments aimed to reproduce the crystallisation behaviour of dry (nominally 0 wt.% H₂O) and hydrous (2 wt.% H₂O added) Etnean trachybasalt at mid-crustal storage conditions (400 MPa, 1100°C, NNO+1 oxygen buffer, corresponding to undercooling of 120°C and 80°C respectively). After superheating at 1300°C for 30 minutes, samples were cooled at 80°C/min to 1100°C and annealed for dwell times ranging from 0.5h to 8h.

Electron backscatter diffraction (EBSD) maps and image analysis were used to quantify clustering parameters such as tmt number density, “shared perimeter fraction” (“SPF”, the fraction of total tmt boundary length shared between cpx and tmt), “fraction of touching tmt” (“FTT”, the fraction of all tmt grains that are touching cpx), and the crystallographic orientation relationships (CORs) between cpx and tmt. Dry samples generally show a higher number density of tmt crystals than wet samples. SPF and FTT are highest (≥ 0.40 and ≥ 0.93 respectively) in the 0.5h duration dry experiments. Both parameters fall to ≤ 0.25 and ≤ 0.75 respectively after 4h of annealing. In wet experiments, SPF and FFT are lower (≤ 0.33 and ≤ 0.79 respectively) at 0.5h annealing time and do not decrease strongly with annealing.

EBSD maps reveal that > 70 % of tmt grains are in contact with cpx in all analysed samples. Tmt exhibits two closely related CORs to cpx. More than 60% of total tmt-cpx boundary length in all samples follows COR 1 ([-110]_tmt[010]_cpx, [111]_tmt(100)*_cpx, [-1-12]_tmt[001]_cpx) or  COR 2 ([-110]_tmt[010]_cpx, [-1-11]_tmt(-101)*_cpx, [112]_tmt[101]_cpx). COR frequencies suggest a strong influence of water content and annealing time on their formation. In the 0.5h duration dry experiment, tmt-cpx boundaries following COR 1 are twice as frequent by length as those following COR 2, whereas in the 0.5h duration wet experiment, COR 2 boundaries are 5 times more frequent by length than COR 1 boundaries. In both wet and dry experiments the length ratio of COR 1 : COR 2 boundaries approaches 1 with longer annealing times.

The degree of undercooling (as imposed by the different water contents) is the most important influence on the microstructural clustering parameters, leading to lower overall number densities of tmt as well as affecting the SPF and FTT values at short durations and the subsequent evolution of these parameters with increasing annealing time. The high frequency of tmt-cpx CORs is consistent with heterogeneous nucleation. However, the mechanisms controlling which CORs develop are unclear. Annealing does not fully erase CORs or microstructural signatures of clustering, suggesting that crystal clusters erupted in volcanic products could still preserve signs of their formation.  

Pontesilli et al. (2019), Chem Geol 510:113-129. 10.1016/j.chemgeo.2019.02.015

Funded by the Austrian Science Fund (FWF): P 33227-N

How to cite: Peres, S., Griffiths, T., Masotta, M., and Pontesilli, A.: The effect of melt water content and isothermal annealing time on the formation and evolution of clinopyroxene-titanomagnetite clusters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10936, https://doi.org/10.5194/egusphere-egu21-10936, 2021.

The magnitude of forces at play in active magmatic systems is poorly constrained because direct observation is difficult. Additional complications include short time scales and the likelihood of overprinting signatures of deeper processes by the catastrophic nature of eruption. Deformation of crystal lattices is one signature of magmatic force common to all crystals that survive eruption. Quartz crystals have documented residual elastic stresses in the hundreds of MPa measured using synchrotron µXRD. These stresses may be caused by several processes: crystal-crystal impingement in a crystal mush, explosive fragmentation, or shear in flowing lavas. To better unravel when these stresses were imparted relative to the ultimate eruption, we combine µXRD with new EBSD measurements. EBSD helps constrain subgrain and twin boundary relationships, geometrically-necessary dislocation density (GND), and plastic deformation.

We target quartz grains from a violent Yellowstone super-eruption and from a large-volume rhyolitic obsidian lava flow (Huckleberry Ridge Tuff and Summit Lake lava, respectively). We use ‘Herkimer diamonds’ as a comparative baseline for deformation. Herkimer diamonds are quartz crystals, famous in the mineral specimen community, that grew into vugs and have experienced no tectonic or volcanic stresses. Samples from both Yellowstone eruptions preserve roughly the indistinguishable amounts of elastic residual stresses, ranging from 100 to 150 MPa. EBSD indicates a GND density of ca. 4E12, with slightly higher values in the Summit Lake Lava. Diffraction peak broadening provides a record of plastic deformation using µXRD. Diffraction peaks are significantly more smeared in Summit Lake lava (0 to 0.15 degrees) than in Huckleberry Ridge Tuff (~0.06 degrees). Subgrain formation in both samples is documented by both µXRD and EBSD. By isolating processes we conclude that elastic residual stresses record pre-eruptive magmatic environment. Viscous shear during lava emplacement generates the majority of plastic deformation, which swamps the signal of lesser amounts of plastic deformation produced in the reservoir or conduit. Pre-eruption processes are likely the source of elevated elastic residual stresses, and we favor an interpretation where the stresses arise from force-chain impingements within crystal mushes prior to eruption.

Finally, EBSD and µXRD provide complementary and overlapping results. Because µXRD peak smearing is sampling both geometrically-necessary and statistically-stored dislocations (SSD; dislocations which contribute no net lattice bending but do contribute to strain hardening), and EBSD only GNDs via relative lattice curvature, the relative proportion of both types of dislocation may be calculated. Huckleberry Ridge Tuff grains preserve up to 20% GNDs, and Summit Lake lavas less than 10%, potentially reflecting the greater total stresses of the Summit Lake samples.

How to cite: Orlandini, O. F. and Befus, K.: Volcanic forces inferred from EBSD and µXRD analyses of Yellowstone quartz, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13305, https://doi.org/10.5194/egusphere-egu21-13305, 2021.

Identification of trans-crustal magma reservoir processes beneath volcanoes is a crucial task to better understand the behaviour and possible future activities of volcanic systems. Detailed petrological investigations have a fundamental role in such studies. Dacitic magmas are usually formed in an upper crustal magma reservoir by complex open-system processes including crystal fractionation and magma mixing following recharge events. Conditions of such processes are usually constrained by crystal-scale studies, whereas there is much less information about the petrogenetic processes occurring in the lower crustal hot zone. Here we provide insight into such processes by new results on amphibole crystal clots found in dacitic pumices from an explosive volcanic suite of the Ciomadul volcano, the youngest one in eastern-central Europe.

Amphibole is a common mineral phase of the Ciomadul dacites, occuring as phenocrysts and antecrysts, but occasionally they also form crystal clots with an inner core of either pyroxene or olivine with high Mg-numbers. Olivine is observed mostly in the 160-130 ka lava dome rocks, whereas the younger explosive eruption products are characterised by orthopyroxene and clinopyroxene. Such mafic crystal clots are most common in the pumices of the earliest explosive eruptions, which occurred after long quiescence at 56-45 ka. The most common appearance has high-Mg pyroxene core (mg#: 0.76-0.92) rimmed by amphibole. Two types of amphibole are found in such clots: irregular zone of actinolite to magnesio-hornblende directly around orthopyroxene and high Mg-Al pargasitic amphibole as the outer zone. Several crystal clots contain smaller amphibole crystals with diffuse transition to clinopyroxene at the inner part and complexly zoned amphibole with biotite inclusions in the outer part. These amphibole and pyroxene have lower Mg-number (< 0.80), and higher MnO content (up to 0.52 wt%) than the most common type. In both cases, amphibole could be a peritectic product of earlier-formed pyroxenes, which reacted with water-rich melt at higher and lower temperatures, respectively. Actinolite to magnesio-hornblende at the contact represents a transitional phase between pyroxene and the newly formed amphibole. In a few cases, crystal clots contain amphibole inclusions in pyroxene macrocrysts. These amphiboles have a particular composition not yet reproduced by experiments: they have high mg# (>0.86), but low tetrahedral Al (0.9-1.0 apfu) and usually high Cr content (Cr₂O₃ is up to 0.9 wt%), similar to the orthopyroxene and clinopyroxene hosts (0.26-0.71 and 0.78-0.89 wt%, respectively). We interpret these amphiboles as an early formed liquidus phase crystallized along with pyroxene from an ultra-hydrous mafic magma. Occasionally, crystal clots are complexly zoned amphibole macrocrysts with dispersed clinopyroxene inclusions. The amphibole has a wide compositional range, usually with high Mg-Al pargasitic core. These amphiboles could have formed by peritectic reaction between clinopyroxene and a water-rich melt.

The observed mafic crystal clots in the dacites indicate the presence of strongly hydrous mafic magmas accumulated probably at the crust-mantle boundary. During mafic recharge, volatile transfer may contribute to the crystal mush rejuvenation at shallow depth and triggering explosive eruptions.

This research was financed by the Hungarian National Research, Development and Innovation Fund (NKFIH) within K135179 project.

How to cite: Cserép, B., Kovács, Z., Fehér, K., and Harangi, S.: A message from the depth: the origin of the amphibole crystal clots with pyroxene cores in the Late Pleistocene Ciomadul dacites, Romania, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13439, https://doi.org/10.5194/egusphere-egu21-13439, 2021.

Pegmatites are texturally, mineralogically, and geochemically zoned rocks that show distinctive features such as graphic granite in the wall zones, coarse-crystalline material in the centre, and unusual mineralisation sometimes of economic significance. They are usually considered to be derived from silicate melts, but a significant fluid supply is also required to reproduce their unique characteristics. These fluids are commonly enriched in flux anions such as F^-, Cl^-, CO₃^2-, and BO₃^3- . Many studies have investigated the petrogenetic processes that led to pegmatite crystallisation, yet not all aspects of pegmatite formation have been fully understood. Notably, the nature of the precipitating medium remains uncertain for the different zones of the pegmatite. In order to better understand the transition from a silicate-melt-dominated crystallisation to fluid-dominated precipitation, we aimed to produce a temperature profile across the pegmatite and its host granite. We analysed quartz crystals from the different zones of the Wellington Lake Pegmatite and the host rock, a syenogranite of the Pikes Peak Batholith, in Colorado (USA). This NYF-type pegmatite consists of a fine-grained graphic granite wall zone, a coarse-grained quartz and albite intermediate zone, and pure blocky quartz core zone with REE-dominated mineralisation including fluocerite, bastnäsite, thorite, columbite, zircon, and cassiterite. Quartz trace element data (Al, Ti, Ge) suggest that the granite crystallized over a range of conditions, with Ti-in-quartz temperatures varying from 800 to 550°C. The wall zone of the pegmatite crystallised over a more constricted range, with temperatures on the order of ~660 to 630°C, just below the experimentally determined H₂O-saturated haplogranite solidus. Finally, the intermediate and core zones of the pegmatite show much colder conditions, with fluid inclusion homogenisation temperatures calibrated for typical pegmatite pressures ranging from 450°C (for 300 MPa) or 380°C (for 200 MPa) for the intermediate zone to 380°C (for 300 MPa) or 325°C (for 200 MPa) for the core. These results suggest crystallisation from a range of conditions transitioning from hydrous silicate melt-based mineral precipitation at the high temperature end (in the core of quartz crystals in the granite) to sub-solidus Al-Si-Na-enriched fluid precipitation in the interstitial quartz of the granite and in the pegmatite. Textural and geochemical zoning in the pegmatite records the transition from near-magmatic conditions in the borders to colder and more hydrothermal conditions in the core.

How to cite: Fonseca Teixeira, L. M., Allaz, J., and Bachmann, O.: Evolution of the crystallisation conditions in the Wellington Lake Pegmatite in the Pikes Peak Granite, Colorado (USA), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5970, https://doi.org/10.5194/egusphere-egu21-5970, 2021.

Volcano geodetic observation is a valuable tool to infer location, strength and geometry of magmatic plumbing systems. The availability of high precision and spatial resolution, spanning decades, deformation data from satellite radar observation and Global Navigation Satellite Systems (GNSS) can give us important information for detecting and characterizing their temporal variations as well as other possible geodynamic sources acting in the volcanic area. For this objective inversion techniques are necessary which help us to obtain the maximum of information from these new datasets. We present a new, original methodology to carry out a multi-source inversion of ground deformation data to better understand the subsurface causative processes (Camacho et al., 2020). The methodology uses a nonlinear approach which permits the determination of location, size and three-dimensional configuration, without any a priori assumption as to the number, nature or shape of the potential sources. The proposed method identifies a combination of pressure bodies and different types of dislocation sources (dip-slip, strike-slip and tensile) representing magmatic sources and other processes such as earthquakes, landslides or groundwater-induced subsidence through the aggregation of elemental cells. This approach carries out a simultaneous inversion of the deformation components and/or line-of-sight (LOS) data; and a simultaneous determination of diverse structures such as pressure bodies or dislocation sources, representing local and regional effects. Both things are done in a fully 3D context and without any initial hypothesis about the number, geometry or types of the causative sources is necessary. We show results from the application of this new methodology to synthetic and real test cases (e.g., Mt. Etna).

This research has been primarily supported by the Spanish Ministerio de Ciencia, Innovación and Universidades research project DEEP-MAPS (RTI2018-093874-B-I00) and is part of the CSIC-PTIs TELEDETEC and POLARCSIC activities.

References
Camacho, A.G., Fernández, J., Samsonov, S.V., Tiampo K.F., Palano, M., 2020. Multisource 3D modelling of elastic volcanic ground deformations. Earth and Planetary Science Letters, 547C, 116445. https://doi.org/10.1016/j.epsl.2020.116445.

How to cite: Fernandez, J., Camacho, A. G., Samsonov, S. V., Tiampo, K. F., and Palano, M.: Interpretation of volcanic surface deformation using a 3D multi-source approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2237, https://doi.org/10.5194/egusphere-egu21-2237, 2021.

Phreatic and phreatomagmatic eruptions are difficult to predict with accuracy on volcanoes due to complex interactions at depth between heat, water, and magmatic fluids. To better understand such multifaceted interactions, we present here a multidisciplinary geophysical approach performed on Miyakejima, a 10-km wide stratovolcano in the Izu Bonin arc. Its plumbing system was highlighted by combining four geophysical methods: magnetotellurics, seismicity (hypocenters), self-potential, and thermal image (remote sensing). We thus propose the first large-scale interpretation of the volcanic structure in terms of rock properties, temperature, fluid content, and fluid flow. Our findings indicate that hot volatiles released from a deep magmatic reservoir (> 350°C, 2.5–4.5 km depth) rise through a narrow permeable path, interact with the conductive hydrothermal system beneath the 2000 A.D. caldera (<250°C, 0–2 km depth). This mixture of fluid is finally released in the fumarolic area in the southern part of the caldera at 181°C. This combined approach allow us to: 1) delineate the water table of the volcano (300–700 m depth), 2) determine the general fluid flow circulation beneath the island, 3) characterize seismic signatures of long-period and volcano-tectonic events, and 4) elucidate the origin of the high water content of fumaroles developed since the last eruption in A.D. 2000.

How to cite: Gresse, M., Uyeshima, M., Koyama, T., Hase, H., Aizawa, K., Yamaya, Y., Morita, Y., Weller, D., Rung-Arunwan, T., Kaneko, T., Sasai, Y., Zlotnicki, J., Ishido, T., Ueda, H., and Hata, M.: Anatomy of a volcanic island inferred from a multiphysics approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3591, https://doi.org/10.5194/egusphere-egu21-3591, 2021.

The extraction of melt from a mush in a magma reservoir is of wide interest. All models for melt extraction from a mush require knowledge of mush permeability, and yet this remains poorly constrained. This permeability is typically calculated using the Kozeny-Carman model or variants thereof, which require a priori knowledge of the microstructural geometry. Such models are not calibrated or tested for packs of crystals of a range of shapes found in natural mush piles, leading to the potential for oversimplification of complex natural systems.

Essentially, a magma mush with minimal crystal-crystal intergrowth is composed of packed crystals where the pore space is filled with interstitial melt. Therefore, this can be studied as a granular medium. We use numerical methods to create domains of closely packed, randomly oriented cuboids in which we keep the short and intermediate axes lengths equal (i.e. square cross section) and vary the long axis magnitude. Our synthetic ‘crystals’ therefore cover the range from oblate to prolate, passing through a cubic shape. We supplement these with 3D numerical packs of spherical particles in cubic lattice arrangements or random arrangements. For the sphere packs we use various polydispersivity of sphere sizes. The permeability of all of these pack types is calculated using a numerical simulation (both LBflow and Avizo-based algorithms) with imposed periodic boundary conditions. The preliminary results suggest that the permeability of a granular medium scales with the specific surface area exclusively, without requiring prior knowledge of the geometry and size distribution of the particles.

We suggest that the model toward which we are working will allow magma mush permeability to be modelled more accurately. If our approach is embedded in existing continuum models for mush compaction and melt extraction, then more accurate estimates of melt accumulation rates prior to very large eruptions could be found.

Keywords: melt segregation, compaction, granular media, fluid flow, numerical simulation

How to cite: Bretagne, E., Wadsworth, F. B., Dobson, K. J., Vasseur, J., and Coumans, J. P.: The permeability of magma mush assembled from anisotropic tabular crystals, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16010, https://doi.org/10.5194/egusphere-egu21-16010, 2021.

Melting of the continental crust and subsequent melt transport has been most thoroughly described in the case of metasedimentary rocks. In these rocks segregation and migration of melt occur either through an interconnected network of veins and melt-rich layers (leucosome) or in form of diapirs. For these rocks, porous flow of melt at grain scale is mostly regarded only as a transient stage of separation of melt from the solid rock.

An entirely different style of melting and melt transport occurs in the case of felsic metaigneous rocks. We use the example from the Bohemian Massif, the eastern European Variscan belt, where metaigneous migmatites were studied in large detail. Here, melt did not segregate from the solid rock but migrated pervasively along most of the grain boundaries and equilibrated with the host rock. This equilibration resulted in formation of a continuous sequence of texturally, geochemically and compositionally different migmatites.

The question arises, what are the conditions and driving forces for this unusual behavior. We attempt to address this question by means of numerical modeling of two-phase flow (i.e. flow of porous solid matrix and melt), using the open-source finite-element ASPECT code (aspect.geodynamics.org). Most previous numerical studies of this process were either purely generic or focused on the melting of the mantle. In order to study this process in crustal conditions, we set up a 2D crustal-scale thermo-mechanical model that includes melting and freezing. We investigate the role of material properties (viscosity, solidus and liquidus temperatures, solid matrix permeability, melt composition) and thermal and velocity boundary conditions, as well as the effect of grid resolution. The results are discussed in terms of realistic parameter values and possible styles of melt migration and deformation of the matrix.

How to cite: Maierová, P., Hasalová, P., and Schulmann, K.: Felsic melt migration via porous flow – a numerical modeling approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5703, https://doi.org/10.5194/egusphere-egu21-5703, 2021.

Volcanism is the surface expression of extensive magmatic systems, with their intrusive counterpart representing ~80% of the total magma budget. Our knowledge of igneous processes therefore largely relies on our understanding of deep plutonic processes. In continental or oceanic environments, most of the intrusive igneous rocks bear geochemical cumulate signatures (e.g., depletion in incompatible elements, enrichment in compatible ones) that are commonly explained by minerals-melt segregation during differentiation. Nevertheless, in many cases the processes aiding melt segregation still need to be further constrained.

In oceanic environments, deformation-assisted compaction aided by melt buoyancy is the main process involved in melt extraction. However, a number of cumulative rocks are lacking any clear compaction evidence, opening the potential for the involvement of other processes. Here, relying on current descriptions of melt dynamics within oceanic magma reservoirs, i.e. the mushy nature of the reservoirs and inferred cyclic replenishment by primitive melts, we propose the involvement of a new igneous process. In the "melt flush" model, repeatedly injected fresh melts hybridize within the injected mush triggering mineral dissolution and crystallization, and concurrent partial extraction of the former interstitial melt forced out of the system by the incoming melts aided by buoyancy.

This model is consistent with the widespread occurrence of reactive porous flow (RPF) identified in oceanic igneous systems, and matches the petrographical (e.g., olivine and plagioclase dissolution) and geochemical constraints (trace element signatures) brought by natural oceanic samples. More specifically, it has been shown that RPF proceeds following melt consuming reactions that ultimately result in a progressive closure of the mush porosity. The extraction of the evolved interstitial melts replaced by more primitive ones, and the porosity closure are here proposed to account for some of the cumulative signatures observed in igneous rocks. The "melt flush" model we describe eventually adds to the other processes involved in cumulates formation from various settings like magma compaction or crystal settling.

How to cite: France, L. and Boulanger, M.: A new perspective on cumulate formation and melt extraction from mushy reservoirs: the "melt flush" model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-912, https://doi.org/10.5194/egusphere-egu21-912, 2021.

Understanding the geometry of magma chambers plays a critical role in determining the igneous petrogenic processes that occur as intrusions cool. Quantitative fabric analysis methods, such as anisotropy of magnetic susceptibility (AMS), are routinely used to measure magma flow dynamics and determine the mechanism of magma transport and emplacement. However, magma mushes typically experience multiple flow events; e.g. emplacement, convection, and interstitial melt percolation. There is thus a need to develop a more a sophisticated approach to unravelling complex rock fabrics that record more than one magmatic state process. This study uses novel rock magnetic datasets to untangle the evolution of the 1163 Ma Younger Giant Dyke Complex (YGDC) of SW Greenland, a multi-sheeted troctolite dyke system that attains widths up to 800 m and encloses several evolved and/or modally layered ovoid pods.

Field results identify that ovoid pods occur in the thickest dyke segments. Several pods are defined by gently inward dipping modal layers and/or a parallel mineral foliations, and in-phase AMS magnetic foliations lie parallel to the observed field fabrics. Critically, imbricated plagioclase crystals record a magma transport direction toward the center of each pod, and this observation is substantiated by in-phase AMS lineations that plunge down dip of the foliation and shallow toward the center of each pod. These observations are interpreted to show gravitational settling under a convective flow regime.

In addition, 66% of out-of-phase AMS fabrics are non-parallel with in-phase AMS results. Out-of-phase AMS is a relatively new technique and is thought to reflect anisotropy controlled by a restrictive group of ferromagnetic minerals such as single domain magnetite and pyrrhotite. Out-of-phase lineations in layered pods are relatively steeply inclined and do not shallow towards the center, we therefore hypothesize that these lineations record a late stage filter-pressing process within the crystal mush. To test this hypothesis, anisotropy of anhysteretic remanent magnetism (AARM) data were collected from 15 samples. Results show that the AARM and out-of-phase AMS tensor axes are parallel, indicating that the sub-fabric detected by out-of-phase AMS is normal and most likely controlled by single domain magnetite.

Our results show that the application of rock magnetic techniques is effective in unravelling magma convection fabrics from later melt migration fabrics in mushy magmas.

How to cite: Koopmans, L. and McCarthy, W.: Using rock magnetics to resolve composite magmatic state fabrics: a case study from the Younger Giant Dyke Complex, SW Greenland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12041, https://doi.org/10.5194/egusphere-egu21-12041, 2021.

The 3D reconstruction of magmatic, metasomatic and/or ore bodies plays a major role in understanding the emplacement mechanisms for magmas and hydrothermal fluids in the upper crust.

The Gavorrano Intrusive-Hydrothermal Complex (GIHC, Tuscany, Italy) is an excellent case study in which intrusive and hydrothermal rocks, as well as sulphides ore bodies are spatially associated.

The evolution of the GIHC starts in the early Pliocene with the sequential emplacement, at the contact between the Paleozoic basement (metapelites) and the overlying Mesozoic limestone-dolostone formations, of a cordierite-biotite monzogranite and a tourmaline microgranite. The monzogranite is highly porphyritic with megacrysts of K-feldspar and phenocrysts of quartz, plagioclase, biotite, and cordierite. The microgranite is characterised by a huge number of euhedral microliths (10-500 µm) of black tourmaline set in a quartz-feldspars groundmass. The small size of the Gavorrano intrusion (ca. 3 x 1 km) and its shallow emplacement level (ca. 5 km) resulted in a thin contact aureole (< 100 m thick) made of phlogopite-olivine marble and biotite-andalusite pelitic hornfels. Isoclinal folds in marble are indicative of dynamic crystallization during contact metamorphism and point out an outward sense of movement of the aureole rocks with respect to the granite intrusion. At the contact with the intrusion, marbles were overprinted by a discontinuous (0.1-10 m thick) layer of vesuvianite-garnet exoskarn. Exoskarn, contact aureole and undisturbed host rocks, were subsequently affected by hydraulic brecciation. The closing stage of the evolution of the complex is characterized by mineralizing fluid circulation, producing widespread chloritization-silicification and decametric pyrite bodies (with adularia, fluorite, and base metal sulfides). 

Surface and underground mapping integrated by mining reports and drill logs allow us gave way to the reconstruction of the attitude and shape of magmatic and hydrothermal bodies. The NW-SE elongated intrusion is characterised by a pronounced asymmetry: the eastern part is made of sub-horizontal multiple bodies, locally with both roof and bottom contacts exposed; the western part has an overall sub-vertical, west-dipping attitude. Such an asymmetry is shown by each of the two intrusive units and highlighted by second order features: the monzogranite unit reaches its maximum thickness (0.8 km) in the central-western subvertical zone while in the subhorizontal eastern branches is few hundred meters thick, and the subhorizontal microgranite bodies display steep west-dipping offshoots. The GIHC asymmetry is also exhibited by the hydrothermal system: the pyrite orebodies mantle the top and the western flank of the intrusion, with the two main masses displaying, in vertical section, a sigmoidal shape with a steep west-dipping thick portion connecting upper and lower tails gently dipping to the west.

The collected data indicate the west side of the GIHC as the focus zone for both magmas and hydrothermal fluids. The overall geometries of the intrusive units and pyrite bodies suggest a sense of movement top-down-to-the-west. This close spatial and shape relationship between intrusive rocks and hydrothermal bodies suggests a common extensional tectono-magmatic regime capable to produce asymmetric crustal traps (dilational structures) for magmas and fluids.

How to cite: Tinagli, L., Vezzoni, S., Rocchi, S., and Dini, A.: Magma and hydrothermal fluids exploiting similar crustal traps at Gavorrano (Tuscany), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3002, https://doi.org/10.5194/egusphere-egu21-3002, 2021.

A transcrustal mush system has been recognized beneath Dominica (Lesser Antilles) with different magma ponding zones that generated a series of pumiceous eruptions from Morne Trois Pitons–Micotrin volcano. Here, the latest, large, pumiceous eruption (Grand Fond - 24 kyrs cal BP) and four, smaller, Plinian eruptions (18-9 kyrs cal BP) are investigated. Pre-eruptive magma dynamics within the mush are unraveled through orthopyroxene phenocrysts by combining a Crystal System Analysis approach (on unzoned and zoned orthopyroxenes) and timescale estimates derived by intracrystalline Fe-Mg interdiffusion modeling. Two magmatic environments are recognized in the mush and have mixed, more or less vigorously, before the successive eruptions. Few interactions between the two magmas began 15-34 years prior to the small Plinian eruptions, but the sustained mixing occurred in the last 2 years. This contrasts with longer timescales (2-80 years) obtained for the larger eruption of Grand Fond with magmas stored deeper. These magma mixing timescales have significant implications for volcanic risk mitigation, with a growing reactivation signal that could be registered at the surface few years prior to the eruptions.

How to cite: Ostorero, L., Boudon, G., Balcone-Boissard, H., Morgan, D. J., d'Augustin, T., and Solaro, C.: Time-window into the transcrustal plumbing system dynamics of Dominica (Lesser Antilles), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2797, https://doi.org/10.5194/egusphere-egu21-2797, 2021.

Volcán de Colima is an active stratovolcano in western Mexico. Its 2013-17 eruptive phase was characterised by transitions between effusive and explosive events. This persistent activity, comprising vulcanian explosions, pyroclastic flows, lava flows and ashfall present significant hazards to ~750,000 people near the volcano.

Tracing patterns of magma storage, recharge and mixing through volcanic systems is key to accurately interpreting monitoring data and understanding potential future hazards. However, at many volcanoes, including Colima, these patterns are poorly constrained and the link between monitoring data and magmatic processes is unclear. To better understand the magmatic plumbing system at Colima, mineral chemistry and textural studies were undertaken on representative 2013-17 samples to constrain different magmatic environments and mixing between them.

These samples contain plagioclase, orthopyroxene, clinopyroxene, Fe-Ti oxides, and rare resorbed olivine and amphibole, typical of Colima andesites. Pyroxene phenocrysts have varied core compositions (Mg#~69-88), zoning and textural patterns, reflecting crystallisation from melts within a heterogeneous magma mush. Whilst we interpret the bulk of the system to be relatively evolved, the presence of disequilibrium textures and high-Cr mafic bands and rims reflect periodic recharge of mafic melts and remobilisation of both evolved and mafic mush material prior to eruption.

The mineral chemistry and petrography indicate the presence of two broad magmatic environments crystallising these pyroxenes. An evolved end-member, crystallising Mg#69-75 pyroxene at between 980-1000°C, comprises the bulk of the system. By contrast, the mafic end-member crystallises high-Mg# pyroxene at a temperature typically between 1020-1080°C. Pressure estimates typically vary between 4-6 kbar or c. 12-20 km depth, in agreement with geophysical evidence suggesting a melt-rich mushy body at this depth.

Zoning patterns range from diffuse zoning in normal zoned pyroxenes to sharper core-rim boundaries in reverse zoned phenocrysts. We applied elemental diffusion modelling to constrain the timescales of pre-eruptive magmatic processes. The modelling indicates relative differences in residence times with long residence timescales typically of decades to centuries for diffuse, normally zoned phenocrysts versus shorter residence timescales of weeks to months in reverse-zoned phenocrysts.

Most notably, an increased frequency of reverse zoned pyroxenes was recorded in lavas erupted after an intense VEI 3 eruption in July 2015. Timescale estimates suggest a recharge and mixing event occurred at approximately this time and estimates from 2016 lavas indicate multiple injection events leading up to the eruption. This suggests that the July 2015 eruption may have been directly linked to this mafic injection.

Despite both eruptions being associated with mafic recharge, the difference in the style of activity between the explosive 2015 and effusive 2016 eruptions suggest other controls on activity. These may include the volume of magmatic recharge, the frequency of injections, ascent rate, or the supply of volatiles from the mafic magmas. Further refinement of the storage timescales and recharge events, and comparison of timescales to monitoring data, also will help clarify the effect of these processes on the eruption timing and style.

How to cite: Hughes, G., Petrone, C., Downes, H., Varley, N., Hammond, S., and Kerr, K.: Insights into timescales of magmatic processes during the 2013-17 eruption at Volcán de Colima, Mexico, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3172, https://doi.org/10.5194/egusphere-egu21-3172, 2021.

       Studying magma reservoir processes is one of the keys to understand the evolution of igneous systems. One of the main processes, magma differentiation, governs the thermal evolution and chemical composition of the melt-crystal assemblage (magma or mush depending on the relative proportions), and therefore exerts a first order control over its physical properties (density, viscosity), and thus on reservoir dynamics. Various approaches have been implemented to model differentiation in an attempt to benchmark all the involved variables like initial and phase compositions, temperature, pressure, and oxygen fugacity (C₀, X, T, P, fO₂). Those approaches are among others mass balance calculations considering partition coefficients (D) values, experimental studies, thermodynamic models or a combination of those. In any of those cases, the evolution of trace elements is governed by the value of the D that is known to be dependent on (P, T, X, fO₂). However, most of the present-day studies still use fixed values of D to provide first order estimates.

      Here, we present an approach combining thermodynamic modeling (relying on Rhyolite-MELTS, Gualda et al., 2012), that integrates X-T-P-fO₂-dependent D for Rare Earth Elements (REE). We applied this new approach to a MORB system, with olivine, clinopyroxene and plagioclase as main mineral phases, and compared results to more classical approaches. D are derived from the models of Sun & Liang (2012, 2013, 2014) and Sun et al. (2017). The resulting model highlights that T & X effects on the D values can add or counterbalance each other depending on the mineral considered. In any cases our results emphasize the gain of using thermodynamic models along with both T- & X-dependent D values to properly model the evolution of igneous systems. Relying on our results, and on the corresponding thermodynamics constraints, we were also able to provide D for any mineral composition crystallized from this MORB system. Results bring to light that an error of ~1 order of magnitude of the D^mineral-melt value could be introduced when considering a fixed value of D.

Gualda et al. (2012) Journal of Petrology, 53-5, 875-890; Sun & Liang (2012) Contrib Mineral Petrol 163-5: 807-823; (2013) Chem Geol 358: 23-36; (2014) Chem Geol 372: 80-91; Sun et al. (2017) Geochim Cosmochim Acta 206: 273-295.

How to cite: Toussaint, A. and France, L.: Modeling differentiation in igneous systems: On the importance of considering temperature & composition dependent partition coefficients, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1222, https://doi.org/10.5194/egusphere-egu21-1222, 2021.

Caldera-forming eruptions are some of the most devastating events on Earth; however, the volcanoes that produce these eruptions frequently have much more minor activity. Knowing if a restless caldera is currently primed for a large eruption, therefore, has important implications for hazard assessment and risk management. Many calderas, including Rabaul in Papua New Guinea, show cycles of activity with multiple caldera-forming eruptions interspersed with more minor activity. We present data that spans an entire cycle, from one caldera-forming eruption to the next and estimate the storage conditions for each eruption. The last complete caldera cycle of Rabaul started at ~10.5 ka, with the eruption of the dacitic Vunabugbug Ignimbrite. Following the Vunabugbug, little volcanic activity was preserved until ~4.4 ka, suggesting either a period quiescence or destruction and burial during the subsequent caldera-forming eruptions of the region. From 4.4 ka, there is an increase in the volume and SiO₂ contents of volcanic deposits that are preserved, which culminated in the eruption of the dacitic Memorial Ignimbrite at ~4.1 ka. The Memorial Ignimbrite was smaller than the Vunabugbug Ignimbrite and Rabaul Pyroclastics and may not have formed a caldera; however, it does appear to have altered the plumbing system and allowed deeper, hotter basalts to reach the surface. Following the eruption of these basalts, the system gradually evolves towards more silicic magmas, until the eruption of the dacitic Rabaul Pyroclastics at ~1.4 ka. After the Rabaul Pyroclastics hotter, more mafic magmas can again reach the surface, both as more mafic lava flows and as hybrid andesites that contain crystal cargos transported from deeper in the system.

Two-pyroxene, clinopyroxene–liquid and plagioclase–liquid thermobarometers suggest that the dacites, including those erupted during the caldera-forming eruptions, were stored at pressures of ~1 kbar (~4 km depth) and at temperatures of ~930 °C. There is a tight relationship between the temperature and the SiO₂ content of the magmas, with the basalts erupted after the large ignimbrites recording temperatures of up to 1100 °C. Some of the more mafic magmas also record deeper storage, at pressures of 3–4 kbar (11–15 km). Plagioclase–liquid pairs suggest melt H₂O contents of ~2.8 wt.% for the dacites, although some of the more mafic magmas have slightly higher melt H₂O contents (3.2–4.0 wt.%)—this may be because the basalts were saturated and stored at greater pressures. Magnetite–liquid pairs record relatively constant oxygen fugacities of ~1.2 log units above the FMQ buffer.

At Rabaul it would take on the order of a few millennia to differentiate or accumulate enough dacitic magma to produce a large explosive eruption. The eruption of highly evolved, crystal-poor, cold, hydrous magmas geochemically similar to those erupted prior to the Memorial Ignimbrite and Rabaul Pyroclastics may provide a warning of an impending large explosive eruption.

How to cite: Fabbro, G. N., McKee, C. O., Sindang, M. E., Oalmann, J. A., and Bouvet De La Maisonneuve, C.: Tracking reservoir dynamics across a complete caldera cycle at Rabaul, Papua New Guinea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13867, https://doi.org/10.5194/egusphere-egu21-13867, 2021.

Kolumbo is the largest of twenty submarine volcanic cones, tectonically aligned in the transtentional Anydros basin, one of the most seismically active zones in the South Aegean Volcanic Arc, whose magmatism is related to the subduction of the African Plate beneath the Aegean microplate. Kolumbo explosively erupted in 1650 CE, causing the death of 70 people on Santorini, which is only 7 km SW of Kolumbo. Explorative cruises employing ROVs discovered a high temperature (220°C) hydrothermal field with CO2-rich discharges and accumulation of acidic water at the bottom of the crater (505 m b.s.l.), increasing the related hazard. A possible magma chamber was recognized below the crater at depth 9-6 km by seismic data [Dimitriadis et al. 2009]. Geochemical data [Klaver et al. 2016] suggest that Kolumbo have a different mantle source and storage system from Santorini. It is fundamental to understand the behaviour of this volcano, and how its storage and plumbing system works, to correctly assess risk for nearby islands.

We present petrographic, geochemical and isotopic data of samples collected during the cruises and by divers. Most samples represent the juvenile products of the 1650 CE activity, characterizing different magmas interacting before the eruption. They consist of white rhyolitic pumices with grey and black bands, also including basaltic-andesitic enclaves. Plagioclase, biotite, pyroxenes are the main mineral phases; olivine is found in the mafic enclaves. Minerals show quite complex zoning and a large compositional variability. Fresh lithic lavas were sampled; they also have amphibole and can be subdivided in three groups with distinctive petrographic textures that are well reflected in their different chemical compositions. They give information on the early history of the volcano and on how the rhyolitic magma could have been generated.

Our data suggest the presence of a complex storage system where the most evolved magma differentiated by assimilation and fractional crystallization, undergoing several inputs of mafic magmas. Early batches of new melts initially mixed with the resident ones, whereas later arrivals only mingled with the rhyolitic magma, thus possibly representing the final trigger of the eruption.

How to cite: Mastroianni, F., Fantozzi, I., Petrone, C. M., Vougioukalakis, G. E., Braschi, E., and Francalanci, L.: Magmatic processes leading to the 1650 CE explosive eruption at the Kolumbo submarine volcano, Greece., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12471, https://doi.org/10.5194/egusphere-egu21-12471, 2021.

The Okinawa Trough (OT) is an incipient continental back-arc basin that extends from Kyushu in the north to Taiwan in the south. The Okinawa Trough can be split in to three segments, the Northern (NOT), Middle (MOT), and Southern (SOT) with active back-arc volcanism restricted to volcanic centres located in en-echelon grabens the MOT and SOT. Previous studies have shown magmatism in the OT is bimodal (basaltic to rhyolitic), with at least two types of silicic melts inferred to form through pure fractional crystallisation from basalt and by fractional crystallisation along with minor crustal assimilation (Shinjo and Kato, 2000).

Here we present petrological descriptions, along with major, trace element and Sr–Nd isotopic data for 75 silicic end member samples recovered as both lava and pumice, collected during the R/V Sonne HYDROMIN1 and 2 cruises in 1988 and 1990, respectively. Samples were dredged from various seafloor knolls and ridges located in the Io and Iheya grabens and from Izena Hole in the MOT, and from a single volcanic ridge in the Yaeyama graben and a single isolated knoll in the SOT.

Results show a chemically highly diverse silicic end member magmas, with at least four identifiable groups based on differences in the degree of enrichment of incompatible elements (LREE, K, Rb, Ba, etc.). Each group contains at least one dense lava sample suggesting the chemical diversity is a primary feature of magmatism in the Okinawa Trough rather than a result of the floating in of pumiceous material from various locations.

Using petrological descriptions and the chemistry of samples along with MELTS modelling we plan to calculate magma formation conditions and identify any evidence of magma mixing or crustal assimilation. In doing so we hope to provide a model to explain the diversity of silicic magma chemistry in the MOT and SOT.

Shinjo, R., and Kato, Y. (2000). Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin. Lithos 54, 117–137. doi:10.1016/S0024-4937(00)00034-7.

How to cite: Murch, A., Tani, K., Sano, T., and Yoneda, S.: A highly diverse silicic end member of a bimodal magma suite formed in an active continental back arc, Okinawa Trough, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6713, https://doi.org/10.5194/egusphere-egu21-6713, 2021.

Understanding the causes of major and trace element variations of granite samples as well as their isotopic signatures is central to attempts to place these rocks in the context of broader geologic processes and continent evolution. For the granites of the Lachlan and New England Fold Belts (LFB and NEFB) of Australia there has been great debate between competing petrogenetic models. The open-system view that the isotopic variability and within-suite compositional trends can be accounted for by magma mixing and fractional crystallisation stands in contrast to the restite unmixing model, in which the geochemical features of certain granites are inherited from protoliths that underwent partial melting to produce magmas entraining varying proportions of residual material. Reconciling all aspects of the geochemical data in a mixing model is contingent on a plausible fractionation regime to produce the observed consistently linear (or near-linear) trends on Harker diagrams; however, the plausibility of existing fractional crystallisation models for LFB granites has not previously been tested with consideration of phase equilibria.

The Magma Chamber Simulator (MCS) models fractional crystallisation alone or with assimilation (AFC), constraining phase equilibria using MELTS and accounting for the thermal budget. This sophisticated modelling tool was used to conduct a case study of the I-type Jindabyne Suite of granites from the LFB, testing whether thermodynamically feasible geochemical trends matching the observed linear variations can arise through fractional crystallisation (with or without assimilation of supracrustal material). The results of 112 MCS models show (1) that for major elements liquid lines of descent (LLDs) may be sensibly linear over limited compositional ranges, (2) that the involvement of assimilation extends the range in which trends are relatively simple and near-linear, and (3) that, despite these observations, neither fractional crystallisation nor AFC are able to correctly reproduce the geochemical evolution of the I-type Jindabyne Suite granitoids as an LLD (contrary to existing models), instead persistently producing curved and kinked trends. The output of these simulations were further used to explore models in which: (a) crystal-bearing magmas evolve via fractional crystallisation or AFC (with chemical isolation assumed to be achieved through crystal zoning) and undergo varying degrees of melt-crystal segregation at different stages to produce the sample compositions; and (b) in situ crystallisation occurs via fractional crystallisation within the crystallisation zone, driving the evolution of a liquid resident magma, which the samples represent. These models are able to reproduce the Jindabyne Suite trends reasonably well. The modelling implies that fractional crystallisation, or some variant thereof, is a viable explanation for the linear trends in Jindabyne; however, tendency for grossly non-linear LLDs highlights that it should not be assumed that fractional crystallisation can generally explain linear trends in granites without careful modelling such as shown here.

How to cite: Iles, K. and Heinonen, J.: Modelling the production of linear trends in granitoids using the Magma Chamber Simulator: a case study of the Jindabyne Suite from the Lachlan Fold Belt, Australia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2570, https://doi.org/10.5194/egusphere-egu21-2570, 2021.

Eocene granitoids in NW Anatolia occurred following the continental collision between Sakarya Continent and Tauride-Anatolide Platform and mark the onset of post-collisional magmatism in the region. One of the representative members of the Eocene granitoids, the Tepeldağ pluton crops out as two isolated granitic bodies and is intruded into the Cretaceous blueschist assemblages (Kocasu formation) and ophiolitic rocks within the Izmir-Ankara-Erzincan suture zone (IAESZ). South Tepeldağ pluton (STP) is composed mainly of granodiorite with subordinate quartz diorite, which show transitional contacts. Aplitic dykes crosscut the pluton as well as the country rocks. STP includes a number of mafic microgranular enclaves (MME) of gabbro/diorite composition.

Geochemically, STP shows distinct I-type affinity with a metaluminous to slightly peraluminous (ASI ≤1.02) nature. The samples are medium-K to high-K calc-alkaline in character. They exhibit depletion in HFSE (Ti, Hf, Zr, Nb and Ta) compared to large ion lithophile elements (Rb, Ba, Th, U, K) and presents negative Nb, P, Ti anomalies. STP displays slight negative Eu anomalies (Eu/Eu* = 0.7–1.2), enrichment in LREE and flat HREE patterns in chondrite-normalized spider diagrams. MELTS modeling (with initial parameters of 1–3 kbar pressure, 2–3% water and QFM-NNO oxygen fugacity buffers) indicate that compositional variations in STP samples can be interpreted as a result of open system processes (assimilation fractional crystallization) rather than a reflection of fractional crystallization in the upper crustal magma chamber. All thermodynamic simulations dictate a crustal assimilation, especially in the late stages of the magmatic process, with a MgO, Na₂O and Al₂O₃-rich assimilant similar to the suture zone (IAESZ) rocks.

How to cite: Arık, T., Kamacı, Ö., Güraslan, I. N., and Altunkaynak, Ş.: The role of shallow open system processes in the evolution of South Tepeldag Pluton (NW Turkey): insights and constraints from thermodynamic modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13662, https://doi.org/10.5194/egusphere-egu21-13662, 2021.

The onset of the final stages of the Variscan orogeny in the Central Iberian Zone (CIZ) is marked by the emplacement of several late to post-tectonic granite melts. The following transition into an extensional regime is associated with subvolcanic magmatism, commonly represented by veins and masses of rhyolitic porphyries, dolerites, and lamprophyres. In Portugal and Spain, these hypabyssal lithologies are fairly abundant.

The Lamas de Olo region of northern Portugal is located about 100 km to the ENE of Porto. Here, the most significant geological body is the composite, post-tectonic Lamas de Olo pluton. Several fracture systems, whose average trends are NNW-SSE, NNE-SSW, and WSW-ENE, cut through this pluton. The composing facies are known as the Lamas de Olo (LO), Alto dos Cabeços (AC), and Barragem (BA) granites. To the east of the pluton, there are two veins: a microgranite and a lamprophyre. While the microgranite is E-W trending, the lamprophyre is N53°E trending.

The felsic vein is rich in quartz and K-feldspar, which are frequently intergrown in granophyric texture, while muscovite, apatite, biotite, and ilmenite are accessories. The feldspars are intensely kaolinized and muscovitized, and biotite is mostly altered in chlorite and brookite/anatase. Compositionally, the microgranite is identical to the BA facies. It is subalkaline, highly felsic peraluminous, and associated with post-orogenic to transitional settings.

Biotite, K-feldspar, plagioclase, pyroxene, and amphibole are the main minerals composing the lamprophyre. Quartz, hematite, goethite, apatite, monazite, zircon, and magnetite are accessories. Pyroxene uralitization, amphibole biotitization, and biotite chloritization evidence the altered state of this vein. Geochemically, the pluton and lamprophyre have nothing in common. This lithology is metaluminous to weakly peraluminous, shoshonitic, alkaline, and associated with within-plate and post-collisional uplift settings. Zircon SHRIMP U-Pb analyses yield a concordia age of 295 ± 2 Ma (MSWD = 2.1) and the Nd isotopic signature is εNd = -0.05.

Considering the geochemistry, the microgranite is more evolved than the LO and AC granites. Most likely, it derived from a plagioclase-rich, crustal source, which was uncontaminated by mantle or young crustal materials. The microgranite melt was presumably derived from the same source that generated the BA granite, and its emplacement was controlled by WSW-ENE trending fractures. The mineral assemblage is mostly diamagnetic, and the post-magmatic alterations were mainly triggered by meteoric fluids, thus generating an ambiguous magnetic fabric. The microgranite is also associated with a subhorizontal magma flow and shallow roots. On the other hand, the lamprophyre was presumably derived from the lithospheric mantle and strongly contaminated by lower crustal materials. Geochemically, the lamprophyre is unrelated to the pluton, but structurally the NNE-SSW trending fractures probably influenced its emplacement. The petrophysical results point out a ferromagnetic behavior and influence of hydrothermal fluids. Based on our results, the lamprophyre was seemingly generated and emplaced after the microgranite.

This work was supported by the Portuguese Foundation for Science and Technology (FCT), through the project reference UIDB/04683/2020 and ICT (Institute of Earth Sciences). The main author is also financially supported by FCT through an individual Ph.D. grant (reference SFRH/BD/138818/2018).

How to cite: Oliveira, A., Martins, H., and Sant'Ovaia, H.: Insights into the petrogenesis and petrophysics of vein magmatism in the Lamas de Olo region, northern Portugal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2749, https://doi.org/10.5194/egusphere-egu21-2749, 2021.

There are many well-known geothermal systems linked to magmatic activity on Earth, many of which eventually express a surface manifestation of the below ground magmatism. The Oligo-Miocene was a period of very active magmatism that took place in Western Anatolia, where granitoidic plutons were emplaced within crust while calcalkaline to alkaline lavas and associated pyroclastics produced by volcanoes under the control of extensional tectonism. Progressive deformation of the crust due to the extension resulted since that time resulting in the development of a E/NE-W/SW trending fault system and of fracture zones that run perpendicular to main faults.

The mineralogical composition of the Hamamtepe and Muratdağı silica sinter deposits is comprised of kaolinite, alunite, and quartz. Microlithofacies of these deposits were defined as, i) massive, ii) laminated, iii) breccia, and iv) porous. δ¹⁸O stable isotope analysis on silicified rocks and δ³⁴S with ⁴⁰Ar/³⁹Ar radiometric age analysis on alunite minerals were performed with the aim of constraining the origin and timing of the silica deposits. We obtained results from δ¹⁸O ranging from 12.3 to 18.4 ‰, δ³⁴S ranging from 9.2 and 16.6 ‰, and radiometric age analysis, which all suggest that the silica sinter deposits formed in a steam heated, low pH, oxidizing epithermal environments., coeval with prominent volcanic activity in the region.

How to cite: Ünal Ercan, H., Ece, Ö. I., Schroeder, P. A., and Gülmez, F.: Origin and Evolution of Silicified Rocks in the Etili - Çanakkale, Turkey, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1496, https://doi.org/10.5194/egusphere-egu21-1496, 2021.

Igneous rocks in magmatic arcs record variations in composition, thermal flux, and subduction dynamics through time. In the Northern Andes, arc magmatism of the Jurassic age registers a complicated history, including the fragmentation of Pangea at the end of the Triassic and the beginning of a new subduction zone in the Jurassic located at the western margin of South America.

We characterized the crustal thickness variations of the Early Jurassic to Early Cretaceous (194-130 Ma) in plutonic and volcanic rocks of the Northern Andes of Colombia and Ecuador, using trace elements signatures and analyzed the implications of the emplacement conditions during the last stage of the magmatism using Al-in-hornblende thermobarometry and mineral chemistry. Moderate rare earth elements (REE) slopes and depleted heavy REE patterns show that the primary residual magma source was amphibole, but plagioclase and pyroxene were also significant residual phases indicating that the magma source was formed in a crust that varied in thickness from 35-50 km. The La/Yb and Sr/Y crustal quantifications variations indicate that the arc underwent two thickening episodes. The first episode (190 to 180 Ma) is associated with a magmatic event. The second episode (165 to 154 Ma) is related to the shift to an oblique subduction setting and a subsequent collisional event that produced medium P-T metamorphic rocks. In the Late Jurassic to Early Cretaceous (154-130 Ma), the crust became thinner and, in this scenario, was emplaced the last stage of plutonism with depths that varied from shallow to deep level (until 25.5 km) in the crust.

How to cite: Chavarria, L., Bustamante, C., Cardona, A., and Bayona, G.: Reconstruction of the Jurassic to Early Cretaceous tectono-magmatic evolution of the Northern Andean Arc from their Crustal Thickness and Thermobarometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10222, https://doi.org/10.5194/egusphere-egu21-10222, 2021.

The Petite Pluton is a Cretaceous intrusion covering an area of nearly 136 km² located in Isla Capitán Aracena, southernmost Patagonia, Chile. This pluton and other stocks are located outside of the margins the Early Cretaceous-Paleogene Fuegian Batholith. The Petite Pluton intrudes the Capitán Aracena ophiolitic complex, interpreted as supracrustal remnants generated during the rifting stage of the Rocas Verdes marginal basin (Late Jurassic- Early Cretaceous; cf. Calderón et al., 2013, Geochem. J.) overlain by hemi-pelagic sedimentary basin infill (Yahgan Formation). These units are locally deformed and exposed in the southern limit of the NW-SE-trending Magallanes fold-and-thrust belt. The satellite plutons consist of amphibole-bearing diorites and quartzdiorites (48-55 wt.% SiO₂) with calc-alkaline compositional trends consistent with their generation in a subduction environment. On N-MORB normalized incompatible elements pattern, the rocks show peaks in LILE (Rb, Ba, Sr) and subtle throughs in Ti, Zr, Nb, Ta and Y. Chondrite-normalized REE pattern is concave upwards with enrichment of LREE relative to HREE without Eu anomaly. The mineral compositions of diorites of Petite pluton consist of amphibole (magnesio-hornblende and tschermakitic hornblende), plagioclase is labradorite and andesine (An_44-59), with Ca-rich composition in small grains included within poikilitic amphibole, biotite (annite), quartz, minor contents of K-feldspar, titanite, magnetite-ilmenite pairs and traces of apatite and zircon. Amphibole composition can be used as a proxy of the amount of H₂O-rich fluids involved in magma evolution and could potentially be used to constrain the crustal depths of pluton emplacement in magmatic plumbing systems (Yavuz & Döner, 2017, P. di Mineralogia; Torres García et al., 2020, Lithos). The calculated pressure and temperature of 3 kbar and 800-850°C, indicate the emplacement and crystallization of magma batches in the upper crust. Oxygen fugacity [log (ƒO₂)] varies between -9.9 and -10.7 (NNO), indicating amphibole crystallization from basaltic-andesitic melts under moderately oxidizing conditions. The moderately Mg# (60-72) of amphibole is consistent with their crystallization from mafic-intermediate melt-dominated crystal mushes with residual melts generated after the fractionational crystallization of olivine and clinopyroxene at deeper crustal depths. The amphibole composition constraint an amount of 6 wt% of H₂O in the residual melts. The subtle negative Eu anomaly in amphibole indicates its partially simultaneous fractionation with plagioclase, suggesting rapid undercooling. The emplacement of the Petite Pluton at ~10 km depth occurred during and/or lately after the tectonic emplacement of ophiolitic complexes within an accretionary wedge, governed by a NE tectonic transport (Muller et al., 2021, Tectonophysics). Late Cretaceous satellite plutons suggest a continentward migration of the magmatic arc, related to the flattening of the subducted oceanic lithosphere of the proto-Pacific Ocean.

Acknowledgements. The study is supported by  Fondecyt grant 1161818.

How to cite: Gregorina, G., Torres García, M. F., Calderón, M., Theye, T., Hervé, F., Ramírez de Arellano, C., and Fuentes, F.: Petrogenesis and emplacement depths of the Petite Pluton during the closure of the Rocas Verdes basin, southern Patagonia - preliminary results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13269, https://doi.org/10.5194/egusphere-egu21-13269, 2021.

The crustal-scale magmatic systems of Andean-style arcs produce thick volcanic deposits and abundant plutons that are emplaced into the crust. They can also generate spatially- and temporally-restricted, economically-important porphyry Cu deposits. These deposits are formed at the magmatic-hydrothermal transition and require significant amounts of volatiles and metals to be concentrated in the sub-volcanic environment. Thus, understanding the magmatic and tectonic processes acting within an arc segment and their effect on the volatile budgets of crustal magmas could be essential for identifying the constraining factors controlling the potential of a magmatic system to produce a porphyry deposit.

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, apatite and zircon geochemistry and zircon 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 >14Myr (~18 – 4 Ma), with ore formation occurring in discrete pulses in the last 3 Myr before cessation of intrusive activity within the district. Progressive changes in the trace element chemistry indicate crustal thickening and deeper magma evolution within the arc segment as a result of the subduction of the Juan Fernandez ridge. A temporal shift to elevated SO₃ and Cl contents is recorded by zircon-hosted apatite inclusions from the intrusions with highest values occurring in porphyry intrusions directly associated with the ore forming events. These data suggest variable volatile budgets of magmas during zircon crystallisation and hint at crustal-scale controls on the porphyry ore-forming potential of an arc segment.

How to cite: Large, S., Nathwani, C., Buret, Y., Knott, T., and Wilkinson, J.: Resolving changes in arc magma volatile budgets over Myr timescales leading up to porphyry Cu formation , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13377, https://doi.org/10.5194/egusphere-egu21-13377, 2021.