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1. Properties of Hydrous Aluminosilicate Melts at High Pressures

   https://doi.org/https://pubs.acs.org/doi/10.1021/acsearthspacechem.8b00157  

   Bajgain, Suraj K.; Peng, Ye; Mookherjee, Mainak; Jing, Zhicheng; Solomon, Matthew. (January 2019, ACS earth and space chemistry)

   In this study, we use f irst-principles molecular dynamics simulations to explore the behavior of anhydrous aluminosilicate melt with a stoichiometry of NaAlSi2O6 up to pressures of ∼30 GPa and temperatures between 2500 and 4000 K. We also examine the effect of water (∼4 wt % H2O) on the equation of state and transport properties of the aluminosilicate melt and relate them to atomistic scale changes in the melt structure. Our results show that water reduces the density and bulk modulus of the anhydrous melt. However, the pressure derivative of the bulk modulus of the hydrous melt is larger than that of the anhydrous melt. The pressure dependence of the transport property exhibits an anomalous behavior. At a pressure of ∼12 GPa, anhydrous aluminosilicate melts exhibit maxima in diffusion and minima in viscosity. Dissolved water in melts also affects both diffusion and viscosity. In hydrous aluminosilicate melts, the maxima in diffusion and the minima in viscosity occur at ∼14 GPa. The anomalous behavior of transport properties is related to the pressure-induced changes in the melt structure. At shallower depths, i.e., up to 100 km, relevant for subduction zone settings, the lower density compounded by the lower viscosity of hydrous aluminosilicate melts is likely to provide buoyancy for upward migration. At greater depths of ∼180−200 km, greater compressibility of the hydrous aluminosilicate melts together with the minimum viscosity could hinder magma migration and may explain the presence of a partial melt layer at the lithosphere−asthenosphere boundary. 

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2. The Viscosity and Atomic Structure of Volatile-Bearing Melilititic Melts at High Pressure and Temperature and the Transport of Deep Carbon

   https://doi.org/10.3390/min10030267  

   Stagno, Vincenzo; Stopponi, Veronica; Kono, Yoshio; D’Arco, Annalisa; Lupi, Stefano; Romano, Claudia; Poe, Brent T.; Foustoukos, Dionysis I.; Scarlato, Piergiorgio; Manning, Craig E. (March 2020, Minerals)

   Understanding the viscosity of mantle-derived magmas is needed to model their migration mechanisms and ascent rate from the source rock to the surface. High pressure–temperature experimental data are now available on the viscosity of synthetic melts, pure carbonatitic to carbonate–silicate compositions, anhydrous basalts, dacites and rhyolites. However, the viscosity of volatile-bearing melilititic melts, among the most plausible carriers of deep carbon, has not been investigated. In this study, we experimentally determined the viscosity of synthetic liquids with ~31 and ~39 wt% SiO2, 1.60 and 1.42 wt% CO2 and 5.7 and 1 wt% H2O, respectively, at pressures from 1 to 4.7 GPa and temperatures between 1265 and 1755 °C, using the falling-sphere technique combined with in situ X-ray radiography. Our results show viscosities between 0.1044 and 2.1221 Pa·s, with a clear dependence on temperature and SiO2 content. The atomic structure of both melt compositions was also determined at high pressure and temperature, using in situ multi-angle energy-dispersive X-ray diffraction supported by ex situ microFTIR and microRaman spectroscopic measurements. Our results yield evidence that the T–T and T–O (T = Si,Al) interatomic distances of ultrabasic melts are higher than those for basaltic melts known from similar recent studies. Based on our experimental data, melilititic melts are expected to migrate at a rate ~from 2 to 57 km·yr−1 in the present-day or the Archaean mantle, respectively. 

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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. Explosive Activity on Kīlauea's Lower East Rift Zone Fueled by a Volatile‐Rich, Dacitic Melt

   https://doi.org/10.1029/2021GC010046  

   Wieser, Penny E.; Edmonds, Marie; Gansecki, Cheryl; Maclennan, John; Jenner, Frances E.; Kunz, Barbara; Antoshechkina, Paula; Trusdell, Frank; Lee, R. L.; EIMF (February 2022, Geochemistry, Geophysics, Geosystems)

   Abstract Magmas with matrix glass compositions ranging from basalt to dacite erupted from a series of 24 fissures in the first 2 weeks of the 2018 Lower East Rift Zone (LERZ) eruption of Kīlauea Volcano. Eruption styles ranged from low spattering and fountaining to strombolian activity. Major element trajectories in matrix glasses and melt inclusions hosted by olivine, pyroxene and plagioclase are consistent with variable amounts of fractional crystallization, with incompatible elements (e.g., Cl, F, and H₂O) becoming enriched by 4–5 times as melt MgO contents evolve from 6 to 0.5 wt%. The high viscosity and high H₂O contents (∼2 wt%) of the dacitic melts erupting at Fissure 17 account for the explosive Strombolian behavior exhibited by this fissure, in contrast to the low fountaining and spattering observed at fissures erupting basaltic to basaltic‐andesite melts. Saturation pressures calculated from melt inclusion CO₂‐H₂O contents indicate that the magma reservoir(s) supplying these fissures was located at ∼2–3 km depth, which is in agreement with the depth of a dacitic magma body intercepted during drilling in 2005 (∼2.5 km) and a seismically imaged lowVp/Vsanomaly (∼2 km depth). Nb/Y ratios in erupted products are similar to lavas erupted between 1955 and 1960, indicating that melts were stored and underwent variable amounts of crystallization in the LERZ for >60 years before being remobilized by a dike intrusion in 2018. We demonstrate that extensive fractional crystallization generates viscous and volatile‐rich magma with potential for hazardous explosive eruptions, which may be lurking undetected at many ocean island volcanoes. 

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5. 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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