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1. 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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2. 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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3. Anomalous Behavior of Viscosity and Electrical Conductivity of MgSiO ₃ Melt at Mantle Conditions

   https://doi.org/10.1029/2021GL093573  

   Luo, Haiyang; Karki, Bijaya B.; Ghosh, Dipta B.; Bao, Huiming (July 2021, Geophysical Research Letters)

   Abstract Silicate melts have served as transport agents in the chemical and thermal evolution of Earth. Molecular dynamics simulations based on a deep neural network potential trained byab initiodata show that the viscosity of MgSiO₃melt decreases with increasing pressure at low pressures (up to ∼6 GPa) before it starts to increase with further compression. The melt electrical conductivity also behaves anomalously; first increasing and then decreasing with pressure. The melt accumulation implied by the viscosity turnover at ∼23 GPa along mantle liquidus offers an explanation for the low‐velocity zone at the 660‐km discontinuity. The increase in electrical conductivity up to ∼50 GPa may contribute to the steep rise of Earth's electrical conductivity profiles derived from magnetotelluric observations. Our results also suggest that small fraction of melts could give rise to detectable bulk conductivity in deeper parts of the mantle. 

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4. Anomalous elastic behavior of phase egg, AlSiO3(OH), at high pressures

   Mookherjee, Mainak; Panero, Wendy R.; Wunder, Bernd; Jahn, Sandro (January 2019, The American mineralogist)

   Phase egg, [AlSiO3(OH)], is an aluminosilicate hydrous mineral that is thermodynamically stable in lithological compositions represented by Al2O3-SiO2-H2O (ASH) ternary, i.e., a simplified ternary for the mineralogy of subducted sediments and continental crustal rocks. High-pressure and high-temperature experiments on lithological compositions resembling hydrated sedimentary layers in subducting slabs show that phase egg is stable up to pressures of 20–30 GPa, which translates to the transition zone to lower mantle depths. Thus, phase egg is a potential candidate for transporting water into the Earth’s mantle transition zone. In this study, we use first-principles simulations based on density functional theory to explore the pressure dependence of crystal structure and how it influences energetics and elasticity. Our results indicate that phase egg exhibits anomalous behavior of the pressure dependence of the elasticity at mantle transition zone depths (~15 GPa). Such anomalous behavior in the elasticity is related to changes in the hydrogen bonding O-H···O configurations, which we delineate as a transition from a low-pressure to a high-pressure structure of phase egg. Full elastic constant tensors indicate that phase egg is very anisotropic resulting in a maximum anisotropy of compressional wave velocity, AvP ≈ 30% and of shear wave velocity, AvS ≈ 17% at zero pressures. Our results also indicate that the phase egg has one of the fastest bulk sound velocities (vP and vS) compared to other hydrous aluminous phases in the ASH ternary, which include topaz-OH, phase Pi, and d-AlOOH. However, the bulk sound velocity of phase egg is slower than that of stishovite. At depths corresponding to the base of mantle transition zone, phase egg decomposes to a mixture of d-AlOOH and stishovite. The changes in compressional DvP and shear DvS velocity associated with the decomposition is ~0.42% and –1.23%, respectively. Although phase egg may be limited to subducted sediments, it could hold several weight percentages of water along a normal mantle geotherm. 

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5. Nonlinear effects of hydration on high-pressure sound velocities of rhyolitic glasses

   https://doi.org/10.2138/am-2021-7597  

   Gu, Jesse T.; Fu, Suyu; Gardner, James E.; Yamashita, Shigeru; Okuchi, Takuo; Lin, Jung-Fu (July 2021, American Mineralogist)

   Abstract Acoustic compressional and shear wave velocities (VP, VS) of anhydrous (AHRG) and hydrous rhyolitic glasses (HRG) containing 3.28 wt% (HRG-3) and 5.90 wt% (HRG-6) total water concentration (H2Ot) have been measured using Brillouin light scattering (BLS) spectroscopy up to 3 GPa in a diamond-anvil cell at ambient temperature. In addition, Fourier-transform infrared (FTIR) spectroscopy was used to measure the speciation of H2O in the glasses up to 3 GPa. At ambient pressure, HRG-3 contains 1.58 (6) wt% hydroxyl groups (OH–) and 1.70 (7) wt% molecular water (H2Om) while HRG-6 contains 1.67 (10) wt% OH– and 4.23 (17) wt% H2Om where the numbers in parentheses are ±1σ. With increasing pressure, very little H2Om, if any, converts to OH– within uncertainties in hydrous rhyolitic glasses such that HRG-6 contains much more H2Om than HRG-3 at all experimental pressures. We observe a nonlinear relationship between high-pressure sound velocities and H2Ot, which is attributed to the distinct effects of each water species on acoustic velocities and elastic moduli of hydrous glasses. Near ambient pressure, depolymerization due to OH– reduces VS and G more than VP and KS. VP and KS in both anhydrous and hydrous glasses decrease with increasing pressure up to ~1–2 GPa before increasing with pressure. Above ~1–2 GPa, VP and KS in both hydrous glasses converge with those in AHRG. In particular, VP in HRG-6 crosses over and becomes higher than VP in AHRG. HRG-6 displays lower VS and G than HRG-3 near ambient pressure, but VS and G in these glasses converge above ~2 GPa. Our results show that hydrous rhyolitic glasses with ~2–4 wt% H2Om can be as incompressible as their anhydrous counterpart above ~1.5 GPa. The nonlinear effects of hydration on high-pressure acoustic velocities and elastic moduli of rhyolitic glasses observed here may provide some insight into the behavior of hydrous silicate melts in felsic magma chambers at depth. 

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