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The continental crust is rich in aluminosilicates and formed by the crystallization of arc magmas. However, the magma produced at sub-arc depths is often silica-poor. The chemical evolution of sub-arc magma from silica-poor to aluminosilicate-rich is perplexing. Magnetotelluric (MT) observations in subduction zones and complementary laboratory-based constraints of electrical conductivity (σ) are crucial to understanding this chemical evolution. The σ of a magma is sensitive to pressure (P), temperature (T), and chemistry (X). To date, laboratory-based measurements on the σ of silicate melts have helped to interpret MT observations at P ≤ 2 GPa. Yet, the melting in subduction zones could occur deeper, at P ≤ 6−7 GPa. The σ of melt at such pressures is poorly constrained. To address this, we performed experiments at P ≤ 6 GPa to examine the σ of basaltic to andesitic melts, which are common in subduction zones. We constrained the effects of silica, alumina, alkali, alkaline, and water (H2O) contents on the σ of melt. The activation volume of σ increases with silica contents. Hence, the σ of basaltic melt is overall greater than that of an andesitic counterpart. The σ of basaltic magma is also less sensitive to P than andesitic magma. Water lowers the activation energy and enhances σ for all melt compositions. Our results help constrain how the electrical properties of a magma change with an evolving composition in a subduction zone. 

Abstract Viscosity of silicate melts governs magma transport and influences mantle dynamics, yet effects of pressure and water on melt viscosity remain poorly understood. Here, we report in situ falling‐sphere viscosity measurements on diopside (Di) melts with 0–3 wt.% H₂O along the liquidus up to 7 GPa and 2103 K using synchrotron X‐ray radiography. By incorporating our hydrous melt data into a previously validated model for the dry system, the effects of pressure, temperature, and H₂O contents on Di melt viscosity can be satisfactorily captured by the function: whereT*is the homologous temperature,x_H2Ois the molar % H₂O,η₀ = 8.90 (1.50) × 10⁻⁸ Pa s,b₀ = 3.02 (0.10), andH*(P) = 15.72 (0.03)−0.35 (0.01)·P + 1.07 (0.07) × 10⁻²·P²−1.19 (0.14) × 10⁻⁴ P³, ×10⁻³ GPa⁻¹. Adding 3 wt.% H₂O systematically reduces viscosity by ∼0.7 log units. For both dry and hydrous melts, viscosity along the liquidus decreases monotonically with increasing pressure, suggesting that moderate hydration may not significantly alter the compressional behavior of Di melts. Combining the Di viscosity model with models for feldspar and olivine, we simulated the viscosity of analog basaltic magmas under mantle conditions. Increasing H₂O content from 0 to 3wt.% raises mobility of basaltic magma increases by >1 order of magnitude. In hot plume settings, the mobility further increases by a factor of 30 relative to typical ambient mantle. Assuming a simple percolation model, the increased mobility corresponds to faster melt ascent in mantle plumes that could, in part, explain the voluminous magmatism of large igneous provinces. 

Abstract The continental crust is produced by the solidification of aluminosilicate‐rich magmas which are sourced from deep below the surface. Migration of the magma depends on the density (ρ) contrast to source rocks and the melt viscosity (η). At the surface, these silica‐rich melts are typically sluggish due to highη > 1,000 Pa s. Yet at their source regions, the melt properties are complexly influenced by pressure (P), temperature (T), and water contents (). In this study, we examined the combinedP‐T‐ effects on the behavior of melts with an albite stoichiometry (NaAlSi₃O₈). We usedfirst‐principlesmolecular dynamics simulations to examine anhydrous (0 wt % H₂O) and hydrous (5 wt % H₂O) melts. To constrain thePandTeffects, we exploredP ≤ 25 GPa across several isotherms between 2500 and 4000 K. The melts show anomalousP‐ρrelationships at lowP ∼ 0 GPa and highT ≥ 2500 K, consistent with vaporization. At lithospheric conditions, meltρincreases with compression and is well described by a finite‐strain formalism. Water lowers the melt density (ρ_hydrous < ρ_anhydrous) but increases the compressibility, that is, 1/K_hydrous>1/K_anhydrousorK_hydrous < K_anhydrous. We also find that the meltηdecreases with pressure and then increases with further compression. Water decreases the viscosity (η_hydrous < η_anhydrous) by depolymerizing the melt structure. The ionic self‐diffusivities are increased by the presence of water. The decreasedρandηby H₂O increase the mobility of magma at crustal conditions, which could explain the rapid eruption and migration timescales for rhyolitic magmas as observed in the Chaitén volcano in Chile. 

Abstract Fluids and melts in planetary interiors significantly influence geodynamic processes from volcanism to global‐scale differentiation. The roles of these geofluids depend on their viscosities (η). Constraining geofluidηat relevant pressures and temperatures relies on laboratory‐based measurements and is most widely done using Stokes' Law viscometry with falling spheres. Yet small sample chambers required by high‐pressure experiments introduce significant drag on the spheres. Several correction schemes are available for Stokes' Law but there is no consensus on the best scheme(s) for high‐pressure experiments. We completed high‐pressure experiments to test the effects of (a) the relative size of the sphere diameter to the chamber diameter and (b) the top and bottom of the chamber, that is, the ends, on the sphere velocities. We examined the influence of current correction schemes on the estimated viscosity using Monte Carlo simulations. We also compared previous viscometry work on various geofluids in different experimental setups/geometries. We find the common schemes for Stokes' Law produce statistically distinct values ofη. When inertia of the sphere is negligible, the most appropriate scheme may be the Faxén correction for the chamber walls. Correction for drag due to the chamber ends depends on the precision in the sinking distance and may be ineffective with decreasing sphere size. Combining the wall and end corrections may overcorrectη. We also suggest the uncertainty inηis best captured by the correction rather than propagated errors from experimental parameters. We develop an overlying view of Stokes' Law viscometry at high pressures. 

Abstract The viscosity of magma plays a crucial role in the dynamics of the Earth: from the crystallization of a magma ocean during its initial stages to modern-day volcanic processes. However, the pressure-dependence behavior of viscosity at high pressure remains controversial. In this study, we report the results of first-principles molecular dynamics simulations of basaltic melt to show that the melt viscosity increases upon compression along each isotherm for the entire lower mantle after showing minima at ~6 GPa. However, elevated temperatures of the magma ocean translate to a narrow range of viscosity, i.e., 0.01–0.03 Pa.s. This low viscosity implies that the crystallization of the magma ocean could be complete within a few million years. These results also suggest that the crystallization of the magma ocean is likely to be fractional, thus supporting the hypothesis that present-day mantle heterogeneities could have been generated during the early crystallization of the primitive mantle. 

Abstract The earliest form of continental crust was produced by tonalite‐trondhjemite‐granodiorite (TTG) magmas. Molten albite (NaAlSi₃O₈) is representative of TTGs and also a major component of modern crust‐forming magma. The viscosity of the melt controls the magma ascent rate and hence influences the production of new continental crust. It is well known that the viscosity (η) of albitic melt exhibits an anomalous pressure (P) dependence. However, prior results on the meltηat high‐Pdiffer significantly which limits our ability to predict the movement of crust‐forming magma at depth. In this study, we more tightly constrained theP‐effect onηin anhydrous albitic melt via high‐Pand high‐temperature (T) falling sphere experiments. We limited undesirable drag effects by using small sphere‐to‐capsule diameter ratios (d/D) such thatd/D ≤ 0.12, and evaluated uncertainties due to such drag using a Monte Carlo approach. Our results show that meltηfirst decreases withP(i.e., ∂η/∂P < 0) and then increases with continued compression (∂η/∂P > 0) with a well‐definedηminimum (η_min) at ∼6 GPa along a ∼2,000 K isotherm. We find that the viscosity of the melt can be described by an Arrhenius formalism with an activation volume that varies withPandT. The results indicate thatηof aluminosilicate magmas decrease with depth and temperature in the crust, thereby mobilizing the magmas to promote rapid volcanic eruptions. The results also suggest that TTG magmas relevant for the early Earth could pond during ascent due to the anomalousP‐effect onη.