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1. Viscosity of Dry and Hydrous Diopside Melts at High Pressures: Implications for Upper Mantle Magma Dynamics

   https://doi.org/10.1029/2025JB032842  

   Chen, Qi; Zhao, Bin; Yu, Tony; Ashley, Aaron Wolfgang; Wang, Yanbin (December 2025, Journal of Geophysical Research: Solid Earth)

   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. 

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2. Mobility of Magmas Within the Earth: Insights From the Elasticity and Transport Properties of Hydrous Albitic Melts

   https://doi.org/10.1029/2024GC011510  

   Ashley, Aaron Wolfgang; Bajgain, Suraj; Mookherjee, Mainak (June 2024, Geochemistry, Geophysics, Geosystems)

   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. 

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3. Behavior and properties of water in silicate melts under deep mantle conditions

   https://doi.org/https://doi.org/10.1038/s41598-021-90124-7  

   Karki, Bijaya B; Ghosh, Dipta B; Karato, Shun-ichiro (May 2021, Scientific reports)

   null (Ed.)

   Water (H2O) as one of the most abundant fluids present in Earth plays crucial role in the generation and transport of magmas in the interior. Though hydrous silicate melts have been studied extensively, the experimental data are confined to relatively low pressures and the computational results are still rare. Moreover, these studies imply large differences in the way water influences the physical properties of silicate magmas, such as density and electrical conductivity. Here, we investigate the equation of state, speciation, and transport properties of water dissolved in Mg1-xFexSiO3 and Mg2(1-x)Fe2xSiO4 melts (for x = 0 and 0.25) as well as in its bulk (pure) fluid state over the entire mantle pressure regime at 2000 to 4000 K using first-principles molecular dynamics. The simulation results allow us to constrain the partial molar volume of the water component in melts along with the molar volume of pure water. The predicted volume of silicate melt+water solution is negative at low pressures and becomes zero above 15 GPa. Consequently, the hydrous component tends to lower the melt density to similar extent over much of the mantle pressure regime irrespective of composition. Our results also show that hydrogen diffuses fast in silicate melts and enhances the melt electrical conductivity in a way that differs from electrical conduction in the bulk water. The speciation of the water component varies considerably from the bulk water structure as well. Water is dissolved in melts mostly as hydroxyls at low pressure and as -O-H-O-, -O-H-O-H- and other extended species with increasing pressure. On the other hand, the pure water behaves as a molecular fluid below 15 GPa, gradually becoming a dissociated fluid with further compression. On the basis of modeled density and conductivity results, we suggest that partial melts containing a few percent of water may be gravitationally trapped both above and below the upper mantle-transition region. Moreover, such hydrous melts can give rise to detectable electrical conductance by means of electromagnetic sounding observations. 

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4. Effects of H2O–CO2 Fluids, Temperature, and Peridotite Fertility on Partial Melting in Mantle Wedges and Generation of Primary Arc Basalts

   https://doi.org/10.1093/petrology/egad047  

   Lara, Michael; Dasgupta, Rajdeep (July 2023, Journal of Petrology)

   Abstract Many lines of evidence from high P–T experiments, thermodynamic models, and natural observations suggest that slab-derived aqueous fluids, which flux mantle wedges contain variable amounts of dissolved carbon. However, constraints on the effects of H2O–CO2 fluids on mantle melting, particularly at mantle wedge P–T conditions, are limited. Here, we present new piston cylinder experiments on fertile and depleted peridotite compositions with 3.5 wt.% H2O and XCO2 [= molar CO2 / (CO2 + H2O)] of 0.04–0.17. Experiments were performed at 2–3 GPa and 1350°C to assess how temperature, peridotite fertility, and XCO2 of slab-derived fluid affects partial melting in mantle wedges. All experiments produce olivine + orthopyroxene +7 to 41 wt.% partial melt. Our new data, along with previous lower temperature data, show that as mantle wedge temperature increases, primary melts become richer in SiO2, FeO*, and MgO and poorer CaO, Al2O3, and alkalis when influenced by H2O–CO2 fluids. At constant P–T and bulk H2O content, the extent of melting in the mantle wedge is largely controlled by peridotite fertility and XCO2 of slab-fluid. High XCO2 depleted compositions generate ~7 wt.% melt, whereas, at identical P–T, low XCO2 fertile compositions generate ~30 to 40 wt.% melt. Additionally, peridotite fertility and XCO2 have significant effects on peridotite partial melt compositions. At a constant P–T–XCO2, fertile peridotites generate melts richer in CaO and Al2O3 and poorer in SiO2, MgO + FeO, and alkalis. Similar to previous experimental studies, at a constant P–T fertility condition, as XCO2 increases, SiO2 and CaO of melts systematically decrease and increase, respectively. Such distinctive effects of oxidized form of dissolved carbon on peridotite partial melt compositions are not observed if the carbon-bearing fluid is reduced, such as CH4-bearing. Considering the large effect of XCO2 on melt SiO2 and CaO concentrations and the relatively oxidized nature of arc magmas, we compare the SiO2/CaO of our experimental melts and melts from previous peridotite + H2O ± CO2 studies to the SiO2/CaO systematics of primitive arc basalts and ultra-calcic, silica-undersaturated arc melt inclusions. From this comparison, we demonstrate that across most P–T–fertility conditions predicted for mantle wedges, partial melts from bulk compositions with XCO2 ≥ 0.11 have lower SiO2/CaO than all primitive arc melts found globally, even when correcting for olivine fractionation, whereas partial melts from bulk compositions with XCO2 = 0.04 overlap the lower end of the SiO2/CaO field defined by natural data. These results suggest that the upper XCO2 limit of slab-fluids influencing primary arc magma formation is 0.04 < XCO2 < 0.11, and this upper limit is likely to apply globally. Lastly, we show that the anomalous SiO2/CaO and CaO/Al2O3 signatures observed in ultra-calcic arc melt inclusions can be reproduced by partial melting of either CO2-bearing hydrous fertile and depleted peridotites with 0 < XCO2 < 0.11 at 2–3 GPa, or from nominally CO2-free hydrous fertile peridotites at P > 3 GPa. 

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5. Insights into the chemical evolution of sub-arc magmas from the high-pressure electrical conductivity of basaltic and andesitic magmas

   https://doi.org/10.1130/G52545.1  

   Ashley, Aaron Wolfgang; Manthilake, Geeth; Schiavi, Federica; Mookherjee, Mainak (March 2025, Geology)

   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. 

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