GMPV8.2

Magmas are particle-fluid mixtures and as such governed by the physical laws that determine flow and deformation in granular slurries. However, developing quantitative models that combine conduit-scale flow properties with the local generation of flow instabilities that lead to pattern formation, for example layering, segregation or clumping/jamming of crystals during transit, remains a challenge.

Here we provide a detailed theoretical analysis of the lateral flow of a granular magmatic slurry, with application to flow differentiation and layering in mafic sill complexes. The slurry rheology is decomposed into scalar and vector components of the fluctuations in the time-dependent configuration of the particles, which although operating on different scales, together give rise to fluctuations in velocity and particle concentrations that may impart considerable heterogeneity during flow.

A key determinant in the development of geological features of interest is the ratio of flow velocity to the gravitational settling rates of crystals in suspension.  Equations are derived that explore the relative contribution of lateral, pressure gradient or volume-driven lateral conduit flow (G) to rates of crystal settling (H). The key ratio G/H ~ D is defined for both symmetrical and non-symmetrical flow as a function of particle pressure, the latter key in controlling crystal-liquid segregation. Two regimes are identified, D < 1 (crystal settling/sedimentation dominates) and D > 1 where differentiation and layering are emergent properties intrinsic to the flow.  Sensitivity analysis reveals the upper and lower boundary conditions at the magma-country rock interface play a critical (and unique) role in controlling velocity fluctuations that impact on local flow segregation and layering.

Lack of experimental evidence, or real-time observations of magmatic intrusions, means critical open questions remain concerning the precise thermo-mechanical conditions (density contrasts, crystallinity and pressure gradients), needed to match theory with the natural world. However, theoretical treatment sets the scene for follow-up numerical work and experimental verification while providing new insight into factors contributing to chemical diversity and textural heterogeneity in igneous rocks.

How to cite: Petford, N. and Koenders, C.: Critical dynamics governing the lateral flow of magmatic slurries, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1330, https://doi.org/10.5194/egusphere-egu22-1330, 2022.

Increasingly, volcanologists model mature volcanic systems as being fed by stratified magma reservoirs, that is, small lenses of eruptible magma suspended within a larger volume of non-eruptible, crystal-rich mush. Erupted lavas record geochemical evidence for long-term deep storage of distinct magma bodies followed by their ascent and coalescence shortly before eruption. Conditions and flow mechanisms that allow deep-seated magmas to rise quickly in reservoirs despite the high viscosity and low permeability of crystal-rich mushes are a subject of debate. We present results of melt migration experiments conducted in a triaxial, gas-medium apparatus. We prepared multiple crystal-rich mushes by hot pressing crushed borosilicate glass mixed with different amounts of subrounded quartz sand (44-106 μm diameter). Prepared mushes have crystal fractions from 0.59 to 0.83. A single disk of mush is stacked on a disk of soda lime glass, representing the intruding crystal-free magma, then heated to 900°C (above the glass transitions) and pressurized to 100-300 MPa. The bottom and circumference of the mush experience the confining pressure, but the top is at room pressure, resulting in a pore pressure gradient (~33-100 MPa/mm) that could drive the underlying melt into the mush. After several hours samples are cooled, decompressed, cut and imaged to determine the distribution of the soda lime glass that migrated in to the mush while at the experimental conditions. High crystal fraction samples (>0.80; Hi X) have glass filling intergranular space. In low crystal fraction samples (<0.70; Lo X), glass forms finger-like intrusions in the mush, indicating melt migration displaced crystals in the mush. Samples with intermediate crystal fractions (Int X) have both morphologies. Mush crystal fraction significantly influences the amount of melt migration, quantified as the measured area fractions of soda lime glass in mush normalized to the available intergranular space (1 - crystal fraction). Glass fills ~10%, ~60% and ~30% of the intergranular space for Lo X, Int X and Hi X samples, respectively. Glass area fraction is not correlated with the imposed pressure gradient, indicating melt migration is moderated by viscosity contrasts between the melt and mush (melt to mush viscosity ratio: 10^-1.7 to 10^-8.6). The observed increase in the amount of melt migration with decreasing crystal fraction is coincident with the onset of mush deformation. Applied to natural systems, these results suggest small changes in mush crystal fraction significantly influence the amount of melt migration that occurs and that melt migration in magma reservoirs peaks near the transition from deformable mush to partially-molten rock (i.e., at the rheologically critical melt fraction).

How to cite: Ryan, A., Hansen, L., Zimmerman, M., and Pistone, M.: An experimental study of melt migration in crystal-rich mushes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1543, https://doi.org/10.5194/egusphere-egu22-1543, 2022.

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, the processes aiding melt segregation still need to be further constrained.

Deformation-assisted compaction aided by melt buoyancy is usually referred to as the main process involved in melt extraction. However, buoyancy alone is not sufficient and a number of cumulative rocks are lacking any compaction evidence, opening the potential for the involvement of other processes. In addition, our view of magmatic systems has shifted in the last decades from large melt-rich bodies to crystal-rich mushy reservoirs. This paradigm shift challenges some of the long-established first-order igneous concepts. The idea that melt differentiation at depth is solely governed by (fractional) crystallization processes is now debated.

We propose a novel igneous process, consistent with the mushy nature of oceanic igneous reservoirs, their continuous/cyclic replenishment by primitive melts, and the widespread occurrence of reactive porous flow (RPF) during magma differentiation identified in a growing number of magma systems. The “melt flush” process relies on reactions between the primitive recharge melt(s) and crystal mush at decreasing porosities and the continuous extraction of more evolved interstitial melt by buoyancy, both participating in the acquisition of the cumulate signature.

This model is consistent with the widespread occurrence of RPF in oceanic igneous systems, and matches the petrographic (e.g., olivine & plagioclase dissolution evidence) and geochemical constraints (trace element signatures) brought by natural oceanic samples. We tested different RPF scenarios on which the melt flush model relies to account for their thermodynamic feasibility with the Magma Chamber Simulator*. The first results show that one-step equilibration of primitive melt with primitive to moderately differentiated crystal mush triggers assimilation. The results of the thermodynamic models are consistent with the constraints established from the natural rock record, and strengthen the idea that RPF is a key process for magma differentiation in mushy reservoirs at different evolution stages. The proposed "melt flush" model eventually adds to other processes involved in cumulates formation like magma compaction or crystal settling, and is likely to apply to any other magma system from various settings sharing similar reservoir characteristics.

*Bohrson et al. 2014, Journal of Petrology (doi: 10.1093/petrology/egu036)

How to cite: Boulanger, M. and France, L.: Cumulate formation and melt extraction from mushy reservoirs: the "melt flush" model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2932, https://doi.org/10.5194/egusphere-egu22-2932, 2022.

Understanding the conditions and timescales of storage and remobilization of magma bodies in the upper crust is key to interpret the signals of potential reawakening of the volcanic activity at active volcanic systems. In this study we provide the first volcanological and petrological characterization of the Singkut volcanic system located in northern Sumatra, ~35 km N of the Toba caldera and ~40 km SW of the major city of Medan. Singkut is a ~9 km diameter caldera delimited by ~300 m-high rims where pre-caldera lavas are exposed. The inner part of the collapsed structure is occupied by three post-caldera volcanoes and currently hosts an active geothermal field. We utilize field observations and correlation with a distal marine tephra layer to map the extension and thickness of the tuff erupted during the caldera-forming eruption and use these data to estimate the erupted magma volume. We use major and trace element data of bulk-rock, matrix glasses and minerals to characterize the pre-eruptive conditions of pre- and post-caldera lavas and caldera-forming tuff and ¹⁴C and U/Th-He zircon dating to determine the eruption ages. In addition, a combination of U/Th and U/Pb in-situ zircon dating and zircon trace element geochemistry provides insights into the mechanisms and timescales that led to the Singkut caldera-forming eruption and those that controlled the post-caldera activity. Our data show that Singkut caldera formed ~50 ka during a large explosive eruption that deposited ~60 km³ of pyroclastic material. The cataclysmic eruption was preceded by at least 200 ky of mostly effusive pre-caldera activity and followed by effusive and mildly explosive post-caldera activity, with the last eruption reported at 1881 AD. The lavas and pumices have high crystallinity (24-62% crystals) and contain pl+amph+bt+opx+Fe-Ti ox+ap+zr±qtz. Notably, large and strongly resorbed quartz crystals are abundant in the pre-caldera lavas and scarce or absent in the caldera-forming tuff and post-caldera lavas. Bulk-rock composition of pumices and lavas varies from andesitic to dacitic, while the matrix glass in the pumices is rhyolitic. Trace element composition of glass (e.g., positive Eu anomalies) indicate resorption of feldspars. Crystallization ages of the youngest zircons in pre-caldera lavas overlap with eruption ages (~250 ka) while crystallization ages of the youngest zircons in the caldera-forming tuff and post-caldera lavas are significantly older (~100 ka) than the eruption ages (~50 and ~16 ka, respectively). Ti-in-zircon thermometry combined with zircon geochronology show that the Singkut magma body experienced a heating phase which started approximately upon eruption of the pre-caldera lavas and continued at least until the eruption of the post-caldera lavas. Such prolonged heating event determined progressive melting of the least refractory mineral phases (mostly quartz and feldspars) and hampered zircon crystallization for ~50 ky before the caldera-forming eruption and ~80 ky before the effusion of the post-caldera lavas. Heating was likely due to an increase of the recharge flux in the magma reservoir which reduced the crystallinity of the crystal mush and promoted remobilization and eruption of the Singkut magma body.

How to cite: Forni, F., Oalmann, J. A., Fellin, G., Eisele, S., Phua, M., Bernard, O., Guillong, M., Rifai, H., and Bouvet de Maisonneuve, C.: Remobilization and eruption of an upper crustal cumulate mush: the Singkut caldera (North Sumatra, Indonesia), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9525, https://doi.org/10.5194/egusphere-egu22-9525, 2022.

We analyzed the mineral composition of Giant Plagioclase Basalt flows from the Deccan large igneous province. In these flows typically occurring at the transition between geochemically distinct volcanic formations, plagioclase occurs as mm-to cm-seized crystals and crystal clots, which sometimes show evidence of high-temperature deformation, as constrained by EBSD analyses. Plagioclase have anorthite contents generally in the range An65-An60, sometimes showing relatively high-An cores (ca. An75). Clinopyroxene has mainly augitic and rarely pigeonitic composition and generally crystallized at shallow depth. Minor and trace element compositions of plagioclase crystals are correlative to those of their host-rocks. Most core-rim profiles of rapidly diffusing elements (e.g., Mg) are flat, suggesting diffusive re-equilibration. On the contrary, slower diffusing elements like K and Fe are zoned in some samples. In particular, Fe shows a marked increase at the crystal rims. Sr isotopic compositions of plagioclase cores are slightly but significantly different from those of the crystal rims and of the surrounding matrix in several samples.

These geochemical and textural characteristics of the analyzed plagioclase suggest that they derived from a crystalline mush crystallized and stored for variably long periods (years to centuries) in the shallow magmatic plumbing system, which was repeatedly flushed and partially deformed by magma rising from deeper levels. Finally, plagioclase-rich magmas became cargos of Fe-rich, dense magmas possibly mobilized by increased CO2 contents heralding the arrival of e mantle-derived magmas.

How to cite: Marzoli, A., Tholt, A., Renne, P. R., Andreasen, R., Spiess, R., Chiaradia, M., Ruth, D. C., and Pande, K.: Plagioclase megacrysts from the shallow crystalline mush of the Deccan Traps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11127, https://doi.org/10.5194/egusphere-egu22-11127, 2022.

Crystal bearing magmas have long been known to be non-Newtonian, exhibiting clear shear-thinning features when exposed to shear stresses. However, the micromechanical origin of this shear thinning remains enigmatic and attempts to describe how shear thinning arises in magmas have been equivocal. Here, we demonstrate that in controlled experimental systems, shear thinning is a non-local and scale-dependent artefact of crystal organisation during flow, and is not therefore an intrinsic property of the crystal-bearing magmas sensu stricto. Furthermore, we show that most experimental approaches to crystal-bearing magma rheology sit in a regime in which crystal migration effects will be dominant, explaining why the experimental evidence is that non-Newtonian effects are observed. We use a numerical conduit model for crystal migration physics to demonstrate that in nature, volcanic systems sit in another regime altogether and will not organise crystal cargo on the time- and length-scales of magma ascent. This leads us to tentatively conclude that crystal-bearing magmas on Earth are Newtonian, and that the only non-Newtonian effects of concern relate to bubbles at moderate capillary number, and the melt phase at moderate Weissenberg number. Finally, we note that this goes some way to unify the mismatch in effective viscosity between low-temperature analogue experiments and experiments on natural crystal-bearing melts.

How to cite: Vasseur, J., Wadsworth, F., Wayne, L., and Dingwell, D.: Are all crystal-bearing magmas actually Newtonian?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11714, https://doi.org/10.5194/egusphere-egu22-11714, 2022.

Changes in melt fraction and local bulk composition in high-crystallinity, crustal mush reservoirs are essential to produce the large volumes of low-crystallinity, silicic magma that are emplaced to form plutons and batholiths, or erupted to the surface.  Heating (and cooling) is well understood and widely invoked in driving melt fraction change, but does not cause chemical differentiation since there is no separation of melt and crystals. Fractional crystallization at high melt fraction is invoked to explain differentiation but is inconsistent with the evidence that large-scale, long-term magma storage and evolution occurs in high-crystallinity mush reservoirs.

Compaction is widely invoked to explain melt fraction change and differentiation at low melt fraction, but compaction (and decompaction) causes simple unmixing (and mixing) of melt and solid crystals: to produce very refractory bulk composition, melt fraction must be driven down to very low values.  Yet microstructural evidence demonstrating widespread compaction in crustal mush reservoirs at low melt fraction is lacking.

Here we show that melt fraction change can be expressed in terms of heating/cooling and compaction, plus an additional term that we term 'reactive flow'. Similarly, composition change can be expressed in terms of compaction and reactive flow.  Reactive flow changes the local bulk composition, which causes ‘chemical’ melting (dissolution) and freezing (precipitation), distinct from ‘thermal’ melting/freezing caused by changes in enthalpy.  

The contributions of compaction and reactive flow in a crustal mush reservoir are similar in magnitude, but reactive flow typically opposes melt fraction and composition changes caused by compaction (or decompaction): if compaction causes melt fraction decrease and creates a more refractory bulk composition, then reactive flow causes melt fraction increase and a more evolved bulk composition, and vice-versa.  In general, compaction and reactive flow cause opposing melt fraction and compaction changes when compaction occurs in a temperature gradient that increases upwards at, for example, the base of a mush reservoir, or decompaction occurs in a temperature gradient that decreases upwards at, for example, the top of a reservoir.

Reactive flow means that very small melt fraction is not required to produce very refractory composition, consistent with the relatively scarce microstructural evidence for widespread compaction.  The apparent lack of compaction in mush reservoirs, as compared to other natural and engineered systems in which reaction does not occur, is also explained by the contribution of reactive flow.  Reactive flow means that melt loss in compacting regions of mush may instead be accompanied by evidence for mineral dissolution, which facilitates ongoing melt fraction loss by preserving connected melt flow paths through the mush pore-space. Reactive flow can also explain why interstitial mineral phases display textures that mimic those of interstitial melt. Chemical differentiation and the evolution of rock microstructure in crustal mush reservoirs should not be interpreted only via the commonly invoked mechanisms of heating/cooling and compaction.

How to cite: Hu, H., Jackson, M., and Blundy, J.: Reactive flow: the hidden mechanism that controls melt fraction change and chemical differentiation in mush reservoirs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12317, https://doi.org/10.5194/egusphere-egu22-12317, 2022.

Late-Permian intermediate-acid magmatic rocks in the Panxi region, inner zone of the Emeishan large igneous province (ELIP) is of great significance for deepening the magmatic process, crustal structure evolution and metal element mineralization under the background of mantle plume.In this contribution, we report contemporaneous (~259Ma) ferrosyenite and granite in the inner zone of the ELIP, and their chronological and geochemical data suggest simultaneous crustal melting at different depths. The ferrosyenites display moderate SiO₂, high Fe₂O₃T and alkaline contents. In combination with their positive zircon ε_Hf(t) values (+1.5 to +12.9), we propose that the ferrosyenites were formed by high temperature melting of Fe-rich refractory juvenile lower crust at H₂O-poor condition, which was induced by underplating of the high-temperature mantle plume. Rhyolite–MELTS modelling results show that ~7% melting of ferrodiorite-like source at ~1120 ℃ and 8 kbar can produce similar compositions with least evolved ferrosyenite under a relatively dry condition. The granites have high SiO₂ contents and 10000*Ga/Al ratios, indicating the affinity to A-type granites. Their evolved zircon ε_Hf(t) values (-8.1 to -0.6) indicate an evolved crustal source region. Rcrust modeling indicates that ~9% partial melting of ancient granodiorite-like crustal material under low pressure condition (4 kbar) can produce the observed granitic compositions. Considering the close spatial and temporal relationship between ferrosyenites and A-type granites, we highlight the significant impact on crustal magma response to the high-temperature mantle plume event in the inner zone of ELIP.

How to cite: Zhang, Z., Qin, J., Lai, S., Long, X., Ju, Y., Wang, X., Zhu, Y., and Zhang, F.: High-temperature melting of different crustal levels in the inner zone of the Emeishan large igneous province, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-31, https://doi.org/10.5194/egusphere-egu22-31, 2022.

The volcanic–plutonic connection plays a fundamental role for magmatic systems, linking crystallising plutons, volcanic activity, volatile exsolution and ore deposits. Nonetheless, our understanding of the nature of these links is limited by the scarcity of outcrops exhibiting clear relationships between the plutonic roots that feed its volcanic counterpart. One way to better characterise the volcanic–plutonic connection is to quantify the amount and rates of melt segregation within a crystallising plutonic body, and to compare the volumes and rates with recent silicic eruptions. Here we investigate the processes of interstitial melt segregation in the calc-alkaline Western Adamello (WA) pluton. The WA tonalite (WAT) is part of the southern Alps and represents an intrusive body emplaced at 2.5 kbar in ~1.2 Myr (Floess and Baumgartner, 2015; Schaltegger et al., 2019). The WAT exhibits a coarse-grained, equigranular texture and is composed of hornblende partially replaced by biotite, plagioclase, quartz, K-feldspar, apatite, zircon, and secondary epidote. K-feldspar, quartz and albite-rich plagioclase (An_25-40) are late and occur as interstitial phases. Several types of igneous structures, constituting <0.5 vol.% of the WA, are found, comprising: (i) hornblende and biotite accumulations (0.1–30 m) with interstitial K-feldspar, quartz and albite-rich plagioclase (An_25-40) representing 25–45 vol.% of the rock; (ii) plagioclase (An_40-70) accumulations with 40–50 vol.% of the same interstitial assemblage; and (iii) quartz-, albite- and K-feldspar-rich domains (0.1–10 m) containing WAT-derived plagioclase phenocrysts which form either zoned aplitic to pegmatitic dikes or schlieren-shaped bodies probably representing in situ melt segregations. The latter are spatially associated with the accumulation zones. Hornblende, biotite, and plagioclase phenocrysts have essentially the same compositional range in accumulations and segregations. This observation indicates that deformation-driven crystal–crystal and crystal–melt segregation operated within the host tonalite. Quantitative modal compositions and mass balance calculations indicate that the hornblende–biotite accumulations lost 60–90 vol.% of their plagioclase phenocrysts and 20–55 vol.% interstitial melt, whereas the plagioclase accumulations lost up to 15 vol.% melt. Such calculations place the maximum efficiency of crystal–melt segregation to 40–55 % in the WAT, as most of the melt remains trapped within the crystal framework. Based on phase relationships and major element modelling, it is proposed that the peritectic relationship hornblende + melt₁ = biotite + quartz + melt₂ and the efficiency of plagioclase–melt separation are linked to the variable composition of the felsic dikes. Such a reaction is known from experimentally derived phase relationships of tonalite (Marxer and Ulmer, 2019) and probably plays a fundamental role linking pluton solidification and extraction of interstitial liquid.

How to cite: Grocolas, T., Toussaint, A., Jossevel, C., and Müntener, O.: Low pressure crystal accumulation and melt segregation within the Western Adamello tonalite (Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4001, https://doi.org/10.5194/egusphere-egu22-4001, 2022.

Since the first hypothesis, evidences of magma mixing processes have been accumulated in the literature allowing this natural phaenomenon to be defined as fundamental petrological processes playing a role in activating volcanic eruptions and generating compositional variability in igneous systems. Here, we review the key concepts to provide the best understanding of the physical and chemical processes occurring during magma mixing. In particular: 1) an historical perspective, recounting the discovery and evolution of our understanding of magma mixing through time; 2) definitions of mixing and mingling and their major geological evidence from the field; 3) scaling rules and numerical modelling, with a deep overview on the kinematics of magma mixing; 4) a synopsis of experimental investigations of the complexity of magma mixing processes; 5) the implications of magma mixing in volcanic systems together with highlights, outstanding issues and possible future developments.

How to cite: Morgavi, D., Laumonier, M., Petrelli, M., and Dingwell, D. B.: Magma Mixing in Igneous Systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6510, https://doi.org/10.5194/egusphere-egu22-6510, 2022.

Silicic magmas categorized "petrogeny's residua system” of Bowen and Tuttle, 1949) are highly polymerized and high in viscosity (> 10^5 Pas depending on water content and other parameters), which is one of the physical properties of magmas that could inhibit magmatic fractionation by making crystal-melt separation difficult (Pitcher, 1997). Diversities of volcanic and intrusive rock suite showing a consistent chemical trend, such as from andesitic to rhyolite in an arc environment and from trachyte to rhyolite in a continental region have been reported from many localities. Some of these cases involve various extent of crustal assimilation through introduction and mingling of silicic melt from partially molten crustal rocks. However, there are many cases, particularly from continental regions, in which extensive fractional crystallization with negligible material input is shown to have played a major role in magmatic fractionation from unequivocal geochemical evidence. One mechanism to cause extensive fractionation of silicic magmas is segregation of fractionated melt from highly crystalline crystal-melt system so-called crystal mush. This mechanism requires compositional convection or a pressure gradient in the interstitial melt. The latter may be attributed to compaction driven by deformation of the melt-crystal system, such as gravity-driven compaction and convection of the mush as a whole. However, actual mechanisms and controlling factors for their operation are still unclear. We address this issue by examining an alkaline ring complex in the continental region, where extensive fractional crystallization without crustal assimilation took place to form diverse rocks from trachybasalt to rhyolite. The Wadi Dib ring complex (WDRC), Eastern Desert in Egypt, consists of multiple circular rings of volcanic and plutonic units. The plutonic rings show zoning progressively more fractionated inwards from the syenite periphery to the central granitic core through the intermediate zone of quartz syenite. The progressive fractionation from the margin to the center, pyrometamorphism in the country rocks neighboring the ring complex and their enclaves only in the periphery of the outer ring, pyrometamorphism in the overlying volcanic unit and the occurrence of their enclaves only in the inner ring, systematic grain size reduction from the outer ring to the granitic core, and high-temperature shear deformation in the outer ring closer to the inner ring suggest that the ring complex formed at a very shallow crustal level under effective and progressive cooling from the surface accompanying localized brittle and ductile deformation. The significant fractionation of acidic rocks of the WDRC is attributed to the development of roof mush zone, which was later collapsed by surpassing strength of the overlying crust and roof mush to induce fractionation of the upper zone of a magma body followed by its intrusion into the shallow-level.

How to cite: Ozawa, K., Saad, E., Kuritani, T., and Khudeir, A. A.: Fractionation of a shallow silicic magmas body through interaction of collapse of volcanic structure and roof boundary layer documented in the Wadi Dib ring complex, Eastern Desert of Egypt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13434, https://doi.org/10.5194/egusphere-egu22-13434, 2022.

The fate of many economically important elements is controlled at the magmatic-hydrothermal transition, where the co-occurrence of melt and magmatic fluid may significantly change partitioning. With increasing usage of batteries across a range of appliances the requirement for lithium (Li) is growing. Despite this, the behaviour of Li in silicic magmatic systems remains poorly known. Here, we illustrate how compositionally unusual biotites from the Bishop and Kos Plateau tuffs may contain a magmatic volatile phase trapped between layers of biotite crystals. These biotite crystals come from pristine volcanic deposits, appear fresh under the binocular microscope and return oxygen isotopic compositions that, taken together with other phases in the same sample, indicate high-temperature equilibrium. Biotite separates show expected XRD specta indicating the absence of other crystalline phases and yet these biotites return low (< 95 wt.%) analytical totals via electron microprobe (EMP) consistent with the presence of considerable amounts of light elements (non-measurable by EMP). Lithium abundances of these biotites are remarkable with values reaching >2,300 ppm with similar results from both spots and line analyses. Lithium isotopic compositions in these biotites are exceptionally light (δ⁷Li as low as -27.6‰) and large isotopic fractionation between biotite and corresponding bulk samples (Δ⁷Li_bt–bulk as low as −37.3‰). Groundmass glasses, melt inclusions and other mineral phases from the Kos and Bishop systems do not support an extremely Li-rich melt prior to eruption. In contrast, biotites from the phonolitic systems of Campi Flegrei and Tenerife do not exhibit such extreme compositions with bioites and melts having approximately equivalent Li contents with Δ⁷Li_bt–bulk to a maximum of −10.9‰. We infer this difference in behaviour to the appearance of biotite in alkaline systems occurring prior to the widespread exsolution of a magmatic volatile phase in the magma reservoir. In the rhyolitic suites, biotite crystallises at lower temperatures and so most biotite growth occurs in the presence of an exsolved fluid phase allowing such a fluid to be trapped within the biotites.  

How to cite: Ellis, B., Neukampf, J., Bachmann, O., Harris, C., Forni, F., Magna, T., Laurent, O., and Ulmer, P.: Biotite as a recorder of an exsolved Li-rich volatile phase in upper crustal silicic magma reservoirs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2530, https://doi.org/10.5194/egusphere-egu22-2530, 2022.

Many volcanic eruptions produce pyroclastic deposits that display pronounced internal gradients in composition, mineralogy, and phenocryst abundance. The volcanic succession of the Mogán Group (14.2–13.6 Ma) of Gran Canaria, Spain, provides an exceptional opportunity to study the temporal evolution of a long-lived magmatic system and the associated diverse styles of zoning preserved in the volcanic record.

The Upper Mogán Formation (13.9–13.6 Ma) contains strongly to moderately welded, peralkaline trachytic to rhyolitic ignimbrites erupted from the multicyclic Tejeda Caldera. Many ignimbrites exhibit upward increases in crystallinity and pumice size and abundance associated with decreases in welding intensity and abundance of lithic clasts. A typical characteristic of the deposits is the coexistence of crystal-poor and crystal-rich pumices along with mingled varieties within a given cooling unit. Major element variations between pumice types are commonly small yet some trace elements (Rb, Zr, Sr, Ba) may differ substantially. We focus here on ignimbrite ‘D’ (13.7 Ma, ~15 km³), which contains dominantly crystal-poor trachydacite fiamme (SiO₂ 68–69 wt%, Ba 80–180 ppm, Zr 2250–2460 ppm) in the lower parts that show increased mingling with a crystal-rich comenditic trachyte (SiO₂ 64–66 wt%, Ba 540–2090 ppm, Zr 420–930 ppm) in the upper parts of the deposit. The mineral assemblage of ‘D’ is dominated by anorthoclase and amphibole with minor clinopyroxene, plagioclase, FeTi-oxides and apatite. Phenocryst abundance increases from nearly aphyric at the base to ca. 15 vol% towards the top. The crystal-poor trachydacite is enriched in components that are incompatible with the observed phenocryst assemblage (FeO, Zr, Rb, Nb, REE). Conversely, the crystal-rich trachyte is enriched in typically compatible major and trace elements (Al₂O₃, Na₂O, P₂O₅, Ba, Sr).

We suggest that crystal-poor pumices in ignimbrite D are derived from evolved aphyric melts extracted from highly crystalline portions of an upper crustal magma reservoir and that crystal-rich pumices contain a considerable component of remobilised shallow cumulate. This interpretation is supported by strong enrichments in Ba in the crystal-rich pumices, significantly larger than expected in melts evolving along liquid lines of descent from a trachybasaltic parent. We show that extensive fractionation of a mineral assemblage dominated by anorthoclase is an important petrogenetic mechanism in the Upper Mogán Formation and not only produces the observed depletions in Ba, Sr and negative Eu anomalies in the crystal-poor pumices but also lays the foundation for remelting of a feldspar-dominated cumulate.

This type of zoning strongly contrasts with that of cooling unit ‘P1’ (14.2 Ma, ~45 km³), which is compositionally zoned from a silicic lower part to a basaltic top and the result of mixing between rhyolitic, trachytic and basaltic magma components. We propose that the observed temporal changes in zoning style and decrease in frequency of basaltic eruptions are related to progressive thermal conditioning and increasing ‘mushification’ of the upper crust, which creates a thermal and viscosity barrier suppressing basaltic eruptions.

How to cite: Oesch, S., Ellis, B. S., Cortes-Calderon, E. A., and Bachmann, O.: Contrasting styles of compositional zonation in pyroclastic deposits of the Mogán Group, Gran Canaria, Canary Islands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4350, https://doi.org/10.5194/egusphere-egu22-4350, 2022.

The eruption of fragmental magma of basaltic composition is the most frequent type of explosive volcanism on Earth and in the Solar System, with eruptions spanning from centuries of persistent, weak explosions, through days to months of lava fountains and ash emissions, to rare, global-scale catastrophic events. The mechanism through which continuous magma fragments into volcanic particles is central in governing eruption dynamics and the ensuing hazards. However, the mechanism of fragmentation of basaltic magmas is still disputed, with both viscous and brittle mechanisms having been proposed for different eruptive styles. Here we carry out textural analysis of the products of ten eruptions from seven volcanoes by Scanning Electron Microscope. We find broken crystals surrounded by intact glass and other features that testify to the brittle fragmentation of basaltic magmas during explosive activity differing in style, intensity and magnitude. We then replicated the natural textures of broken crystals in laboratory experiments where variably crystallised basaltic melt was fragmented by rapid deformation. The experiments reveal that crystals are broken by the propagation of a network of fractures through magma, and that afterwards many of the fractures heal by viscous flow of the melt. Fracturing and healing affect gas mobility, stress distribution, and bubble and crystal size distributions in magma. Our results challenge the idea that the grain size distribution of basaltic eruption products reflects the density of the fractures that initially fragmented the magma. Unrecognised broken crystals and accompanying textures appear in previous literature covering many eruptions spanning Hawaiian lava fountains, catastrophic Plinian events, and more. We conclude that brittle fragmentation and subsequent healing are not specific to some eruption style or pyroclast type, but are ubiquitous factors controlling basaltic explosive volcanism.

How to cite: Taddeucci, J., Cimarelli, C., Alatorre-Ibarguenguoitia, M. A., Delgado-Granados, H., Andronico, D., Del Bello, E., Scarlato, P., and Di Stefano, F.: Features of broken crystals reveal the fracturing and healing of basaltic magmas during explosive volcanic eruptions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9204, https://doi.org/10.5194/egusphere-egu22-9204, 2022.

In SW Iberia (namely in the northernmost domains of Ossa-Morena Zone [OMZ]), NW-SE lineaments of peralkaline igneous massifs are found, known to have been emplaced during the Cambrian-Ordovician rift-related magmatic stage of the Variscan extension in northern Gondwana. In the Portuguese counterpart of the OMZ, these rocks can be found in two distinct tectono-stratigraphic segments, namely the Blastomylonitic Belt (BB) and in the Alter do Chão – Elvas Sector (ACES), intruding Neoproterozoic to Middle Cambrian successions (further extending to the Spanish side). Though the peralkaline magmas are coeval, some contrasting geochemical features allow a well-marked distinction between rocks located in the BB and ACES, which may provide sustained inferences on the petrogenesis and geotectonic framework. The BB rocks composition fit within the phonolite-trachyte spectrum, and show affinities with “within-plate” and A1-type granitoids. On the other hand, rocks from the ACES display trachyte to alkaline rhyolite compositions, chemical features of A2-type granitoids and “anomalous ocean ridge granite” tectonic setting of emplacement. However, within the ACES there are exceptions, as three massifs appear to have the same chemical signatures of the BB. Lithogeochemical data suggest the Ossa-Morena Zone alkaline melts (i) could be extracted from distinct sources, and/or (ii) underwent different degrees of fractionation along with variable crustal assimilation. In addition to lithogeochemistry data, further mineralogical and isotopic studies will be addressed to better understand and provide sustained inferences on the development of the northern OMZ peralkaline magmatism and related ore-forming systems.

Acknowledgements: JR acknowledges the financial support provided by Fundação para a Ciência e Tecnologia (FCT) through the PhD grant (UI/BD/150937/2021), and by the Society of Economic Geologists Foundation through the Hugh McKinstry Fund. The authors also acknowledge expenses supported by ICT financed by FCT, under the project UIDB/04683/2020. This work is a contribution to the project “ZOM-3D Metallogenic Modelling of Ossa-Morena Zone: Valorization of the Alentejo Mineral Resources” (ALT20-03-0145- FEDER-000028), funded by Alentejo 2020 through the FEDER/FSE/FEEI.

How to cite: Roseiro, J., Moreira, N., Nogueira, P., de Oliveira, D., and Eguiluz, L.: Revisiting geochemical data from the Ossa-Morena Zone peralkaline rocks: New insights on petrogenesis during the Cambrian-Ordovician rift-related alkaline magmatism in the Iberian Massif, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9260, https://doi.org/10.5194/egusphere-egu22-9260, 2022.

Vesuvius is one of the most iconic active volcanoes on Earth. Historic and archaeological records document numerous hazardous eruptions with thousands of fatalities. Today, more than one-million people live around Vesuvius and are threatened by future volcanic activity. Petrologic and geochemical studies of eruptive products provide important insights into the evolution of the eruption-feeding magma reservoir prior to eruption. Here we quantify the duration of shallow crustal storage and track the evolution of phonolitic magmas prior to major explosive eruptions of Vesuvius employing in-situ uranium-thorium dating of garnet phenocrysts in tandem with detailed geochemical and textural characterization. Garnet uranium-thorium dates provide evidence for progressively shorter pre-eruption storage times throughout the lifetime of the volcano, decreasing from ~5,000 years for the pre-historic Mercato and Avellino eruptions to approximately 1,000 years for the historic AD 79 Pompeii and AD 472 Pollena eruptions. These decreasing residence times mirror the progressively shorter repose intervals between eruptions implying that distinct phonolite magma batches were present throughout most of the volcano’s evolution thereby controlling the eruption dynamics by preventing the ascent of mafic magmas from longerlived and deeper reservoirs. Frequent lower-energy eruptions during the recent history sample this deeper reservoir and suggest that future Plinian eruptions are unlikely without centuries of volcanic quiescence. Crystal residence times from other volcanoes reveal that discrete long-lived deep-seated reservoirs and transient upper-crustal magma chambers are common features of sub-volcanic plumbing systems.

How to cite: Wotzlaw, J.-F., Bastian, L., Guillong, M., Forni, F., Laurent, O., Neukampf, J., Sulpizio, R., Chelle-Michou, C., and Bachmann, O.: Garnet petrochronology reveals the lifetime and dynamics of phonolitic magma chambers at Somma-Vesuvius, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8927, https://doi.org/10.5194/egusphere-egu22-8927, 2022.

Most geobarometers use chemical compositions of minerals and their host melt to estimate crystallization pressures. Crystal structural parameters such as cell and site volumes are not usually considered despite their known sensitivity to pressure. Here, we compare two clinopyroxene geobarometers based upon electron microprobe analysis alone and coupled with single-crystal X-ray diffraction data. The case study is the plumbing system of Popocatépetl volcano (Mexico), which consists of three distinct magma reservoirs in upper, middle and lower crustal depths, represented by three compositionally and texturally distinct clinopyroxene populations (T1, T2, and low-Ca). These clinopyroxenes are augites of limited compositional variability, although yielding a significant increase in cell (V cell) and M1 site (V M1) volumes from low-Ca and T2 core to T1 (core and rim) and T2 (rim) clinopyroxenes. This variation is not due to chemical or temperature effects but is linked to their depth of crystallization. The application of the geobarometer based on chemical composition alone is unable to distinguish the three different reservoirs postulated on volcanological and petrological grounds. In contrast, the application of the geobarometer based on both structural parameters and chemical composition yields a remarkable correlation between the calculated cell volume and the estimated depth of crystallization of the different clinopyroxenes, including core to rim differences.

These results have twofold implications. First, the determination of the structural parameters of clinopyroxenes is the only method to resolve the actual distribution of Mg, Fe²⁺, Fe³⁺ in the M1 and M2 structural sites and, given the sensitivity of cell and site volumes to pressure, permits to improve geobarometric estimates in volcanic plumbing systems. Second, the quantitative determination of the crystallization depth of the different clinopyroxenes has permitted to rescale the depth of the three different reservoirs in the plumbing system of the Popocatépetl Volcanic Complex located from ~30 km b.s.l. (low-Ca clinopyroxene) to ~18 km b.s.l. (T2 clinopyroxene core) and ~10-0 km b.s.l. (T1 clinopyroxene core and rim, T2 clinopyroxene rim) within the crustal structure of the Morelos platform. This provides further support to the complex plumbing system of the Popocatépetl Volcanic Complex consisting of polybaric storage layers of variable interconnected and interacting transient magma reservoirs.

From Tommasini et al. (2021), Lithos, https://doi.org/10.1016/j.lithos.2021.106540

How to cite: Petrone, C. M., Tommasini, S., Bindi, L., Lorenzo, S., Mangler, M. M., and Orlando, A.: Critical assessment of pressure estimates in volcanic plumbing systems: the case study of Popocatépetl volcano, Mexico, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11938, https://doi.org/10.5194/egusphere-egu22-11938, 2022.

Volcano monitoring makes it possible to indirectly visualise magma plumbing systems and follow the onset and evolution of eruptions. While geophysical data provide real-time information on magma transfer and storage, the petrology of erupted products is crucial to assess magma composition and eruptive style. However, erupted magmas often carry recycled crystals which affect bulk rock compositions, masking variations in erupted melts. Here, we explore the use of high-resolution geochemistry to resolve subtle variations in melt composition on the scale of days to years. We investigate recent basaltic eruptions in a variety of geodynamic settings and with distinct eruptive frequencies, including Mount Etna in Sicily (Italy) and Cumbre Vieja in La Palma (Canary Islands, Spain). We demonstrate that targeted laser ablation mass spectrometry can rapidly determine variations in the chemical composition of melts within the plumbing system, which may add petrological insight to volcano monitoring efforts.

How to cite: Ubide, T., Márquez, Á., Magee, R., Ancochea, E., Huertas, M. J., Sanz, D., Herrera, R., Coello-Bravo, J. J., MacDonald, A., Mulder, J., Conn, E., Caulfield, J. T., and Galindo, I.: Using high-resolution geochemistry to monitor variations in magma dynamics during eruption, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9743, https://doi.org/10.5194/egusphere-egu22-9743, 2022.

Mt. Etna features an articulated plumbing system characterized by a central open-conduit, culminating with the persistent degassing summit craters, three rift-related lateral systems (S-Rift, NE Rift and W Rift) and the eccentric feeding system, characterized by disperse monogenetic cones.

In the last twenty years, most the eruptive activity occurred at the summit central craters and by consequence most of the recent petrological studies focused on the parametrization of the central open-conduit system. In this study, we move the focus to the NE Rift system, whose last activity dates back to the 2002-2003 eruption. Rift-related events are potentially more dangerous since they are often accompanied by energetic precursor seismicity and increase the probability of lava effusion at low altitude where towns and infrastructures are concentrated. Samples from this last eruption were examined and the new chemical data integrated with a comprehensive whole rock and mineral chemistry dataset from pre-historical and historical events.

Textural observations of the NE Rift products highlight a greater variability compared to magmas erupted from the central craters, in spite of a comparable mineral assemblage made of Ol, Cpx, Plg and Ti-Mt. High and low porphyritic lavas coexist in the same event and appear frequently mingled. Similarly, whole-rock composition varies from hawaiite-trachybasalt to benmoreite, in contrast with the rather homogenous trachybasaltic composition of magmas erupted from the central craters. Plagioclase phenocrysts show partially resorbed rims associated with an increase in An content or alternatively, alignments of melt inclusions near the crystal rim, related to a decrease in An content.

Thermo-barometric estimates based on Ol-Liq and Cpx-liq equilibria suggest that most of Ol and Cpx phenocrysts equilibrated at temperature comprised between 1140 to 1000 °C and pressure ranging from 10 to 2 Kbar, with a remarkably higher DT/DP with respect to magmas erupted at the central craters. This suggests a magma crustal ponding zone between 4 and 2 kbar. These results have been integrated by thermodynamic modelling through the energy-constrained model Magma Chamber Simulator able to compute the evolution of the magma via fractional crystallization in a polybaric and polythermal volcanic plumbing system. Results highlights that fractionation occur along the Ol-Cpx-Plag liquid line of descent in a range of pressure equivalent to those determined by the crystal-melt geobarometry.

How to cite: Giacomoni, P. P., Masotta, M., Costa, S., Lanzafame, G., and Coltorti, M.: Physical constraints and magma dynamics of Mt. Etna rift systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11321, https://doi.org/10.5194/egusphere-egu22-11321, 2022.

Magma crystallization is a fundamental process driving the evolution of magmas in the crust and influencing the style of volcanic eruptions. Crystallization occurs through either (near) equilibrium or kinetically-controlled mechanisms driving the solidification of magmas and the final textural and chemical characteristics of igneous rocks. Among other factors, the degree of undercooling (∆T), expressed as the difference between the liquidus temperature and the actual temperature of solidifying magma, plays a key role. Experimental investigations on the effect of ∆T are extremely important to reconstruct the crystallization of basaltic melts under kinetic conditions which are frequently encountered in open conduit volcanoes.

Stromboli (Sicily, Italy) is a reference example for these types of volcanic systems, due to its persistent activity and periodic changes of eruptive style, from normal, mild strombolian activity to effusive events or sudden, short-lived, more violent explosions (paroxysms). In this study, we examined the effect of ∆T on the crystallization path of basaltic magmas erupted at Stromboli. The starting material is a high-K basaltic glass obtained from a low-porphyritic (LP) pumice erupted during the paroxysm of April 5, 2003. Undercooling crystallization experiments were performed in a non-end loaded piston cylinder apparatus at 350-500 MPa, 1050-1150 °C, anhydrous and hydrous (2 wt.% H₂O added to the experimental charge) conditions, and NNO +1.5 buffer. The degree of ∆T imposed to the system ranges from 10 to 162 °C. Textural features and chemical composition of the experimental charges were investigated by combining synchrotron radiation X-ray microtomography (SR-µCT) for the 3D reconstruction of crystal morphologies, scanning electron microscopy (FE-SEM) and electron probe microanalysis (EPMA).

Clinopyroxene represents the main mineral phase crystallized in all the experimental charges, and shows a remarkable textural and chemical dependence on the degree of ∆T. In particular, as the degree of ∆T increases, clinopyroxene morphology evolves from prevalently skeletal to dendritic, and the crystal composition becomes enriched in incompatible elements (Ti and Al), with a simultaneous depletion in compatible elements (Si and Mg). According to this cation exchange, the degree of ∆T can be parameterized to derive a new predicting model for high-K basaltic melts and based on clinopyroxene composition only. Modeling results using natural clinopyroxene crystals open new perspectives for the interpretation of open conduit dynamics at Stromboli.

How to cite: Costa, S., Colle, F., Masotta, M., Mollo, S., Landi, P., Pontesilli, A., Peres, S., Griffiths, T., and Mancini, L.: Kinetic crystallization of a high-K basalt melt undercooled in laboratory: Implications for modeling open conduit dynamics at Stromboli volcano, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5786, https://doi.org/10.5194/egusphere-egu22-5786, 2022.

The mobility of basaltic magma within the Earth’s crust is controlled by magma viscosity. Crystallization and crystal morphology affect the viscosity, mobility and ultimately eruptibility of magma, by locking it at depth or enabling its ascent towards the surface. However, relationships between crystallinity, rheology and eruptibility remain uncertain because of the challenges associated with documenting magma crystallization in real time. Here we show, for the first time, the results of in situ 3D time-dependent, high temperature experiments performed under water-saturated conditions to investigate crystallization kinetics in a basaltic magma at crustal pressure. This new 4D approach provides unique quantitative information on the growth kinetics and textural evolution of pyroxene crystallization in basaltic magmas, quantifying dendritic growth on initially euhedral cores and revealing surprisingly rapid increases in crystal fraction and aspect ratio at undercoolings ≥30 °C. Such crystallization favours a rheological transition from Newtonian to non-Newtonian behaviour within minutes. We applied a numerical model to quantify the effect of dendritic crystallization on basaltic dike propagation towards the surface. Modelling results show that dendritic crystallization can strongly affect magma rheology during magma ascent with important implications for the mobility of basaltic magmas within the crust. 

How to cite: Arzilli, F., Polacci, M., La Spina, G., Le Gall, N., Llewellin, E. W., Brooker, R. A., Torres-Orozco, R., Di Genova, D., Neave, D. A., Hartley, M. E., Mader, H. M., Giordano, D., Atwood, R., Lee, P. D., and Burton, M. R.: In situ 4D dendritic crystallization in basaltic magmas reveals how magma mobility occurs within the Earth's crust, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5584, https://doi.org/10.5194/egusphere-egu22-5584, 2022.

Clustering of clinopyroxene (Cpx) and titanomagnetite (Tmt) is commonly observed in magmatic products and crystallisation experiments. The existence of crystallographic orientation relationships (CORs) between Cpx and Tmt is thought to indicate their formation by heterogeneous nucleation. Heterogeneous nucleation is promoted by high degrees of undercooling, and thus associated with disequilibrium microstructures and compositions. We studied the effect of isothermal annealing on Cpx-Tmt clusters exhibiting CORs, in order to examine whether information about cluster formation is preserved during re-equilibration at depth in a magmatic system.

We analysed samples synthesized in experiments of Pontesilli et al. (2019), which aimed to reproduce the crystallisation behaviour of an Etnean trachybasalt, under nominally anhydrous (0 wt.% H₂O) and hydrous (2 wt.% H₂O) conditions, at mid-crustal storage conditions (400 MPa, 1100°C, NNO+1 oxygen buffer), corresponding to a degree of 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.

We employed electron backscatter diffraction (EBSD) analysis to characterise microstructures and detect CORs. In hydrous samples, phase fraction, maximum crystal size, and perimeter/area ratio are unaffected by dwell time. In contrast, anhydrous samples exhibit decreasing crystal fraction with increasing dwell time. Although crystallinity falls overall in anhydrous samples, area fraction of Tmt increases slightly up to 2h dwell time. The increase in Tmt area correlates with an increase in maximum Tmt size and a decrease in Tmt perimeter/area ratio.

Tmt exhibits two closely related CORs to Cpx, COR1 ([-110]_tmt//[010]_cpx, [111]_tmt//(100)*_cpx, [-1-12]_tmt//[001]_cpx) and  COR 2 ([-110]_tmt//[010]_cpx, [-1-11]_tmt//(-101)*_cpx, [112]_tmt//[101]_cpx). The fraction of the total length of Cpx-Tmt boundaries that follow one of the two CORs (F_COR1+2) exceeds 60% in all samples. However, the relative frequencies of the two CORs vary. In hydrous samples with dwell times of 4h and below, F_COR2 (~55%) exceeds F_COR1 (~10%). However, at 8h dwell time, the frequency of both CORs is ~30%. In anhydrous samples at dwell times of 1h and below, the pattern is reversed, with F_COR1 (~40%) exceeding F_COR2 (~20%). The frequency of both CORs is once again ~30% for dwell times of 2h and above. The normalised abundance (total length/map area) of boundaries with a COR does not change in the hydrous samples, and only decreases slightly in anhydrous samples. After 8h, the total abundance of boundaries with a COR is similar, regardless of water content.

The different COR frequencies observed at short dwell times in hydrous and anhydrous samples imply that the Cpx-Tmt clustering mechanism is affected by degree of undercooling. Re-equilibration of COR frequencies progresses faster in the anhydrous samples, correlating with the greater intensity of microstructural re-equilibration observed. In rapidly cooled systems, relative frequencies of different Cpx-Tmt CORs could potentially be used to estimate degree of undercooling. Total abundance of boundaries associated with a COR remains constant, suggesting that Cpx-Tmt CORs preserve some information about heterogeneous nucleation on longer timescales.

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: Griffiths, T., Habler, G., Peres, S., Pontesilli, A., and Masotta, M.: Re-equilibration of clinopyroxene-titanomagnetite clusters: the effect of isothermal annealing time and melt water content, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12189, https://doi.org/10.5194/egusphere-egu22-12189, 2022.

Crystal clusters of titanomagnetite (Tmt) and clinopyroxene (Cpx) are ubiquitous features in undercooled trachybasaltic magmas and can affect the rheology and chemical differentiation of volcanic plumbing systems. Furthermore, the circumstances of cluster formation may provide petrological information about processes occurring at depth.

We carried out isothermal time-series experiments on synthetic trachybasaltic melts in a non-end loaded piston cylinder apparatus at 4 kbar, to investigate the formation and evolution of Tmt-Cpx clusters. Samples were held for 30 minutes above the liquidus, before cooling at 80°C/minute to the experimental temperature. We varied the following parameters: i) the degree of undercooling ∆T (expressed as the difference T _liquidus - T _experiment), ranging from 30 to 80 °C; ii) melt H₂O content (either anhydrous or 2 wt.% H₂O added); and iii) dwell time, ranging from 5 minutes to 8 hours.

Phase-contrast synchrotron X-ray microtomographic analysis (SR-µCT) was used to obtain a high-resolution (0.9 µm voxel size) 3D reconstruction of the experimental samples, allowing us to visualize the 3D geometry of the contacts between Cpx and Tmt crystals.

The overall crystallinity of the samples increases with increasing degree of undercooling. Crystal phases are mainly Cpx and Tmt. Their shapes vary from mostly skeletal in samples with lower undercooling and longer dwell time, to mostly dendritic at higher undercooling and/or shorter dwell time. 3D X-ray imaging reveals three Tmt populations: i) large skeletal to euhedral Tmt (up to 150 µm) isolated in the glass or partially embedded in Cpx crystals (Population 1); ii) small skeletal to anhedral Tmt grains (from 1 µm to 50 µm) touching Cpx crystals at a straight interface (Population 2); and iii) arrow/cigar-shaped Tmt grains (major axis ranging from 10 µm to 60 µm, minor <10 µm) almost completely embedded in larger Cpx crystals, with only a small interface (<5 µm) exposed to the melt (Population 3). All population 3 Tmt within the same Cpx grain share a common shape preferred orientation (SPO).

The coexistence of different morphologies of clustered Tmt suggests the coexistence of at least two Tmt nucleation mechanisms within the samples. Population 1 Tmt are inferred to nucleate homogeneously, as they are also found isolated in the glass. Population 2 and 3 Tmt are both inferred to have nucleated heterogeneously in contact with pre-existing Cpx grains.

The reason for the different morphologies of the two heterogeneously nucleated Tmt populations is unknown. It may be due to different timings of heterogeneous nucleation of population 2 and 3, or simultaneous growth of population 3 Tmt together with Cpx. In the future, studies of compositional zoning and crystallographic orientation will enable us to identify the reasons for the difference.

In conclusion, multiple populations of Tmt crystals with different sizes, morphologies and clustering behaviour can arise even from a single cooling event and subsequent annealing at constant temperature. This has important implications for the interpretations of microstructures and crystal size distributions in natural magmas.

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

How to cite: Peres, S., Griffiths, T., Colle, F., Iannini Lelarge, S., Masotta, M., Pontesilli, A., and Mancini, L.: Titanomagnetite-clinopyroxene clustering in synthetic trachybasalts: Insight into nucleation mechanisms from new experimental samples, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7750, https://doi.org/10.5194/egusphere-egu22-7750, 2022.