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Much of Earth’s magma is stored as extensive crystal mush systems, yet the prevalence of physical processes operating within mushes and their importance in volcanically active regions remain enigmatic. In this Review, we explore the physical properties and key processes of crystal mush systems. The initiation, evolution and decline of volcanic systems, modulated by heat supply and loss, could generate differences in the prevalence of mush processes through space and time. Additionally, regional tectonics alter mush properties, with mushes in cool wet settings having persistent residual melt, permitting more effective melt segregation than in hot dry settings. Disaggregation of mushes results in crystal mush material being mobilized or entrained into lavas and erupted, presenting opportunities to define the timescales and chemistry of some mush processes in volcanically active regions. Mush systems can be observed on length scales ranging from kilometres (using geological mapping) to micrometres (using crystal textures). Therefore, it is difficult to integrate data and interpretations across different fields. Improved integration of thermodynamics, textural analysis, geochemistry, modelling and experiments, alongside inputs from adjacent fields such as porous media dynamics, engineering and metallurgy will help to advance understanding of mush systems and ultimately improve hazard evaluation at active and dormant volcanic systems. 

The debate about early Earth differentiation focuses on the processes responsible for the formation of protocrust(s) and continental crust of felsic (SiO₂ ≥ 55 weight %) composition. One aspect of this debate is how Hadean zircons fit into an ultramafic environment. On the basis of experiments, thermodynamic modeling, and elemental partitioning, we show that felsic melts could have been generated by shallow interaction between primordial serpentinized peridotite and basaltic magmas on Earth and Mars. On the basis of the hafnium isotopic evolution of Hadean detrital zircons worldwide, we infer that these interactions allowed for the formation of extensive Hadean felsic crust (4.4 to 4.5 billion years ago), which, in turn, would account for up to 50% of the present continental crustal mass. A similar process may have occurred on Mars. The serpentinized protocrust had a dual role in the primitive planetary environment: to provide ingredients for the continental crust and to enable life to emerge on water-bearing terrestrial planets. 

Documenting the processes and timescales of magma formation and diversification and defining the locations, shapes, volumes, and phase states of magma storage and transport zones rely on data produced by novel analytical techniques and state-of-the-art experimental methods. Computational modeling effectively links these critical tools. The Magma Chamber Simulator (MCS) is an internally consistent thermodynamic open system model that uses experimental constraints from rhyolite MELTS (Gualda et al. 2012, Ghiorso & Gualda, 2015) to compute paths of open system magmas that evolve via processes including crystallization, magma mixing, cumulate/mush entrainment, and host-rock assimilation. MCS results yield elemental, isotope, mass, and thermal characteristics of melt ± crystals ± volatiles in "resident" magma, crustal wallrock (melt and solids), recharge magma, and entrained material. To model the petrochemical evolution of igneous rocks related by open-system processes, one typically runs 200+ models that vary initial compositions, pressures, and temperatures of magma, wallrock, etc. Comparison of model results with whole rock, mineral, and melt inclusion chemical data and other constraints (e.g., thermobarometry) yield interpretations about igneous processes at a range of scales—from how crust forms and evolves to processes responsible for in situ geochemical records of crystals—and allows assessment of epistemic and aleatoric uncertainties. Two examples of computational studies will illustrate MCS's utility and flexibility. (1) Modeling of historical basalts at Mt. Etna (Italy) provides evidence for variable degrees of melting of metasomatized mantle, followed by magma recharge and assimilation of partial melts of carbonate-flysch crust. (2) MCS models reproduce whole rock and mineral data of plagioclase-rich basalts at Steens Mountain (USA) through entrainment of gabbroic mush that likely formed in early stages of Columbia River Basalt magmatism. To enhance understanding of trans-lithospheric magma systems, future work on MCS will prioritize (i) building a post-processing environment that utilizes select statistical methods to inform "best-fit" models and to quantitatively assess uncertainty, and (ii) increasing modeling efficiency by adding automated modeling capabilities. 

We investigate the implications of prolonging the equilibrium crystallization (EQX) stage of lunar magma ocean (LMO) solidification beyond the oft-modeled 50% volume solids, to 60%. Most models of two-stage LMO crystallization halt the EQX phase once 50% of a molten Moon (post-core formation) solidifies, after which the remaining 50% of the LMO solidifies via fractional crystallization (FRX). We quantitatively show through a simple scaling analysis that compares crystal settling velocity to vertical convective velocity that the early EQX regime can operate up to (and possible even slightly beyond) 60% volume solids. Phases that stabilize during the EQX and FRX regimes are then computed using Perple_X (thermodynamic calculator) along with the hp633ver database and associated activity-composition relations for solid solutions, and consider an adiabat that remains between the liquidus and solidus. Early results show two key findings: 1) only low volumes (~2%) of ilmenite form over ~50-km thick upper mantle layers for both 50% and 60% EQX regimes, suggesting that a mantle overturn may have been sluggish and/or limited in depth (dense ilmenite is thought to have been a critical driver of late-stage mantle mixing); and 2) contrary to most published two-stage LMO models, a refractory-enriched (i.e. high Al2O3) bulk silicate Moon is not required to produce garnet in the lunar mantle, assuming an Earth-like bulk silicate Moon composition with an alumina content of ~4 wt.%. To complement and test these numerical phase equilibria model results, a series of piston-cylinder experiments is underway that simulate the pressures and temperatures experienced by an FeO+TiO2-rich residual LMO in order to assess the volume and distribution of ilmenite produced during LMO solidification. These results are compared to those of the numerical phase equilibria models. Despite the model-dependent nature of these results, they provide a unique insight into potential LMO crystallization that has not been previously considered in the literature. 

Debate regarding early Earth differentiation focuses on the fate, nature, origin, volume, and processes responsible for protocrust(s) and felsic crust formation. One specific aspect of this debate is how Hadean zircons and their felsic parental magmas fit with an expected ultramafic environment. Based on our new experiments, thermodynamic modeling, and elemental partitioning, we infer that felsic liquids could have been generated by shallow (< 20 km) interaction between primordial hydrated peridotite (serpentinite) and basaltic magmas. Felsic melts (SiO2 ≥ 55 wt%) can be generated at a maximum melt fraction of 0.4 when starting serpentinite:basalt mass ratio is high (i.e., higher than 1.5:1). Here we show that felsic melts obtained in our experimental runs can account for the Hf isotope evolutionary array displayed by Hadean detrital zircons worldwide. We propose that open system interactions between serpentinite and basaltic melts at the end of the magma ocean stage after magma degassing and water ocean precipitation allowed the formation of extensive early Hadean felsic crust (4.4 - 4.5 Gy ago). Our calculations indicate that this felsic crust accounts for up to 50% of present-day continental crust mass. The abundant production of primordial felsic crust throughout the Hadean could be due to the impact-induced melting owing to frequent impacts. A similar process could have also occurred on Mars, and other rocky planets, provided that water was abundant at shallow and surficial levels, which would account for the existence of a thick felsic crust. The serpentinised protocrust had a dual role in the primitive planetary environment: to provide the first and most abundant felsic crust and to facilitate the emergence of life in the shallow hydrothermal environments of water-bearing terrestrial planets. 

Abstract The nearly continuous volcanic eruption record at Mt. Etna dating back ~700 years provides an excellent opportunity to investigate the geochemical evolution of a highly active volcano. Of particular interest is elucidating the cause of selective enrichment in alkali elements (K and Rb) and 87Sr/86Sr observed in various episodes of past activity. More recently, this alkali enrichment trend started to manifest in the 17th century and accelerated after 1971, and was accompanied by an increase in the volume, frequency, and explosivity of eruptions. Explanations for this signature include recharge of alkali-enriched magmas and/or crustal contamination from the subvolcanic basement. This study quantitatively examines the role of crustal contamination in post-1971 Etnean magma compositions via hundreds of open-system phase equilibria and trace element calculations based upon whole-rock major oxides, trace elements, 87Sr/86Sr ratios, and mineral compositional data. Available pre-1971 petrochemical data are satisfactorily reproduced by fractional crystallization of a high whole-rock MgO (12–17 wt.%), Ni (135–285 ppm), and Cr (920–1330 ppm) parental magma composition that is documented in Etna's ~4-ka fall-stratified deposit. Observed post-1971 whole-rock and glass trends and phase equilibria are reproduced via modeled assimilation of a skarn and flysch mixture, lithologies that represent the uppermost 10 to 15 km of sedimentary rocks beneath Etna. Notably, models show that K2O (wt.%) and Rb (ppm) behave incompatibly during partial melting of skarn/flysch. Additionally, the observed elevation of 87Sr/86Sr in post-1971 samples is consistent with the addition of radiogenic Sr from wallrock partial melts. In best-fit models, which yield observed post-1971 K2O, Rb, and 87Sr/86Sr trends, ~17% anatectic melt is assimilated and there may be a subordinate stoped wallrock component of ≤2% (percentage is relative to the starting mass of pristine magma). Previous work has shown that metasomatized spinel lherzolite and garnet pyroxenite can be melted in different proportions to reproduce long- and short-term changes observed in Etna’s geochemical products. We propose that the alkali enrichment signature observed after 1971 can be fully explained through the combination of mantle heterogeneity and crustal contamination. In particular, up to ~20% crustal input coupled with mantle heterogeneity of primitive melts explains the geochemical signals quite well. The influence of crustal contamination on post-1971 lavas is, in part, the result of frequent recharge of magmas that thermally primed the middle to upper crust and enhanced its partial melting. 

Several times in the past 60 ka, Mount Etna has erupted lavas with variable alkaline character. The most recent chemical excursion began in 1971, accompanied by an increase in explosivity and eruption frequency. The origin of the alkaline signature remains enigmatic, with endmember hypotheses involving dominant contributions from mantle vs. crust. For lavas that erupted between 1329 and 2016, we used thermodynamic modeling to test if post-1971 anomalous alkalinity is dominated by mantle processes. First, we assessed mantle melting conditions required to reproduce the chemistry of potential parental magmas. Second, we examined the differentiation of partial melts as they underwent closed-system crustal storage and ascent. The mantle melting conditions explored via ~300 models include source compositions (peridotite + pyroxenite ± metasomatic phases), pressure, extent of melting, and fO2. Best-fit models that reproduce the major element chemistry of volcanics interpreted as parental magmas involve 20 to 30% melting of a peridotite-pyroxenite mantle source that contained phlogopite, pargasite, and CO2 (~1 wt.%) between ~1-1.5 GPa (~30-45 km) along the QFM+1 buffer. Subsequent isentropic decompression (adiabatic and reversible) of mantle partial melts + crystallization at shallower pressures (0.8-0.2 GPa) were modeled to test the effects of closed system ascent and storage. Isentropic decompression models yield no crystallization although temperature decreases ~3°C/0.1 GPa. Decompression + closed system crystallization fail to replicate post-1971 glass samples and do not explain observed post-1971 alkali enrichments. We conclude that partial melting of a metasomatized source produced Etna parental magmas, but closed system crustal ascent and storage cannot fully account for alkali enrichment highlighted in post-1971 products at Etna. Open system modeling suggests that assimilation and crystallization (e.g., Takach et al. 2024) play a critical role, and ongoing modeling is testing the contributions of recharge (magma replenishment) and entrainment of previously formed mushes to the high alkalinity excursions. 

Paleomagnetic, rock magnetic, or geomagnetic data found in the MagIC data repository from a paper titled: Paleomagnetic behavior of volcanic rocks from Isla Socorro, Mexico