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1. First-principles computation of diffusional Mg isotope fractionation in silicate melts

   https://doi.org/https://doi.org/10.1016/j.gca.2020.08.028  

   Luo, Haiyang ; Karki, Bijaya B ; Ghosh, Dipta B ; Bao, Huiming ( October 2020 , Geochimica et cosmochimica acta)

   null (Ed.)

   Diffusional isotope fractionation occurs in geochemical processes (such as magma mixing, bubble growth, and crystal growth), even at magmatic temperatures. Isotopic mass dependence of diffusion is commonly expressed as Di Dj ¼ mj mi
    b , where Di and Dj are diffusion coefficients of two isotopes whose masses are mi and mj. How the dimensionless empirical parameter b depends on temperature, pressure, and composition remains poorly constrained. Here, we conducted a series of first-principles molecular dynamics simulations to evaluate the b factor of Mg isotopes in MgSiO3 and Mg2SiO4 melts using pseudo-isotope method. In particular, we considered interactions between Mg isotopes by simultaneously putting pseudo-mass and normalmass Mg atoms in a simulation supercell. The calculated b for Mg isotopes decreases linearly with decreasing temperature at zero pressure, from 0:158  0:004 at 4000 K to 0:121  0:017 at 2200 K for MgSiO3 melt and from 0:150  0:004 at 4000 K to 0:101  0:012 at 2200 K for Mg2SiO4 melt. Moreover, our simulations of compressed Mg2SiO4 melt along the 3000 K isotherm show that the b value decreases linearly from 0:130  0:006 at 0 GPa to 0:060  0:011 at 17 GPa. Based on our diffusivity results, the empirically established positive correlation between b and solvent-normalized diffusivity (Di/DSi) seems to be applicable only at constant temperatures or in narrow temperature ranges. Analysis of atomistic mechanisms suggests that the calculated b values are inversely correlated with force constants of Mg at a given temperature or pressure. Good agreement between our first principles results with available experimental data suggests that interactions between isotopes of major elements must be considered in calculating b for major elements in silicate melts. Also, we discuss diffusion-controlled crystal growth by considering our calculated b values. 

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2. Constraints on deep, CO2-rich degassing at arc volcanoes from solubility experiments on hydrous basaltic andesite of Pavlof Volcano, Alaska Peninsula, at 300 to 1200 MPa

   https://doi.org/10.2138/am-2021-7531  

   Mangan, Margaret T. ; Sisson, Thomas W. ; Hankins, W. Ben ; Shimizu, Nobumichi ; Vennemann, Torsten ( May 2021 , American Mineralogist)

   Abstract The solubility of CO2 in hydrous basaltic andesite was examined in fO2-controlled experiments at a temperature of 1125 °C and pressures between 310–1200 MPa. Concentrations of dissolved H2O and CO2 in experimental glasses were determined by ion microprobe calibrated on a subset of run glasses analyzed by high-temperature vacuum manometry. Assuming that the solubility of H2O in mafic melt is relatively well known, estimates of XH2Ofluid and PH2Ofluid in the saturating fluid were modeled, and by difference, values for XCO2fluid and PCO2fluid were obtained (XCO2 ~0.5–0.9); fCO2 could be then calculated from the fluid composition, temperature, and pressure. Dissolved H2O over a range of 2.3–5.5 wt% had no unequivocal influence on the dissolution of CO2 at the pressures and fluid compositions examined. For these H2O concentrations, dissolved CO2 increases with fCO2 following an empirical power-law relation: dissolved CO2 (ppmw) = 14.9−3.5+4.5[fCO2 (MPa)]0.7±0.03. The highest-pressure results plot farthest from this equation but are within its 1 standard-error uncertainty envelope. We compare our experimental data with three recent CO2-H2O solubility models: Papale et al. (2006); Iacono-Marziano et al. (2012); and Ghiorso and Gualda (2015). The Papale et al. (2006) and Iacono-Marizano et al. (2012) models give similar results, both over-predicting the solubility of CO2 in a melt of the Pavlof basaltic andesite composition across the fCO2 range, whereas the Ghiorso and Gualda (2015) model under-predicts CO2 solubility. All three solubility models would indicate a strong enhancement of CO2 solubility with increasing dissolved H2O not apparent in our results. We also examine our results in the context of previous high-pressure CO2 solubility experiments on basaltic melts. Dissolved CO2 correlates positively with mole fraction (Na+K+Ca)/Al across a compositional spectrum of trachybasalt-alkali basalt-tholeiite-icelandite-basaltic andesite. Shortcomings of current solubility models for a widespread arc magma type indicate that our understanding of degassing in the deep crust and uppermost mantle remains semi-quantitative. Experimental studies systematically varying concentrations of melt components (Mg, Ca, Na, K, Al, Si) may be necessary to identify solubility reactions, quantify their equilibrium constants, and thereby build an accurate and generally applicable solubility model. 

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3. Eu speciation in apatite at 1 bar: An experimental study of valence-state partitioning by XANES, lattice strain, and Eu/Eu* in basaltic systems

   https://doi.org/10.2138/am-2022-8388  

   Tailby, Nicholas D. ; Trail, Dustin ; Watson, Bruce ; Lanzirotti, Antonio ; Newville, Matthew ; Wang, Yanling ( May 2023 , American Mineralogist: Journal of Earth and Planetary Materials)

   Abstract Partition coefficients for rare earth elements (REEs) between apatite and basaltic melt were determined as a function of oxygen fugacity (fO2; iron-wüstite to hematite-magnetite buffers) at 1 bar and between 1110 and 1175 °C. Apatite-melt partitioning data for REE3+ (La, Sm, Gd, Lu) show near constant values at all experimental conditions, while bulk Eu becomes more incompatible (with an increasing negative anomaly) with decreasing fO2. Experiments define three apatite calibrations that can theoretically be used as redox sensors. The first, a XANES calibration that directly measures Eu valence in apatite, requires saturation at similar temperature-composition conditions to experiments and is defined by: ( E u 3 + ∑ E u ) Apatite  = 1 1 + 10 - 0.10 ± 0.01 × l o g ⁡ ( f o 2 ) - 1.63 ± 0.16 . The second technique involves analysis of Sm, Eu, and Gd in both apatite and coexisting basaltic melt (glass), and is defined by: ( Eu E u * ) D Sm × Gd = 1 1 + 10 - 0.15 ± 0.03 × l o g ⁡ ( f o 2 ) - 2.46 ± 0.41 . The third technique is based on the lattice strain model and also requires analysis of REE in both apatite and basalt. This calibration is defined by ( Eu E u * ) D lattice strain = 1 1 + 10 - 0.20 ± 0.03 × l o g ⁡ ( f o 2 ) - 3.03 ± 0.42 . The Eu valence-state partitioning techniques based on (Sm×Gd) and lattice strain are virtually indistinguishable, such that either methodology is valid. Application of any of these calibrations is best carried out in systems where both apatite and coexisting glass are present and in direct contact with one another. In holocrystalline rocks, whole rock analyses can be used as a guide to melt composition, but considerations and corrections must be made to either the lattice strain or Sm×Gd techniques to ensure that the effect of plagioclase crystallization either prior to or during apatite growth can be removed. Similarly, if the melt source has an inherited either a positive or negative Eu anomaly, appropriate corrections must also be made to lattice strain or Sm×Gd techniques that are based on whole rock analyses. This being the case, if apatite is primary and saturates from the parent melt early during the crystallization sequence, these corrections may be minimal. The partition coefficients for the REE between apatite and melt range from a maximum DEu3+ = 1.67 ± 0.25 (as determined by lattice strain) to DLu3+ = 0.69 ± 0.10. The REE partition coefficient pattern, as observed in the Onuma diagram, is in a fortuitous situation where the most compatible REE (Eu3+) is also the polyvalent element used to monitor fO2. These experiments provide a quantitative means of assessing Eu anomalies in apatite and how they be used to constrain the oxygen fugacity of silicate melts. 

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4. 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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5. Thermoelasticity of Water in Silicate Melts: Implications for Melt Buoyancy in Earth's Mantle

   https://doi.org/10.1029/2021JB022359  

   Zeff, Garrett ; Williams, Quentin ( September 2021 , Journal of Geophysical Research: Solid Earth)

   A comprehensive analysis of experimental data and theoretical simulations on the partial molar volume of water in silicate melt indicates that finite strain theory successfully describes the compression of the H₂O component dissolved in silicate melt at high pressures and temperatures. However, because of the high compressibility of the water component, a fourth order equation of state fit is required to accurately simulate experimental results on water's volume in silicate melts at a deep upper mantle, transition zone, and lower mantle pressures. Data from previous shock compression experiments on hydrous minerals in which melting occurs along the Hugoniot are used to provide an experimental constraint on the partial molar volume of water in silicate melt at deep mantle temperatures and pressures. The equation of state of the water component indicates that, depending on elastic averaging technique, the amount of water that could be present in neutrally or negatively buoyant mafic/ultramafic melts above the 410 km seismic discontinuity is upper‐bounded at 5.6 wt%: smaller than previously inferred, and consistent with melt being confined to a narrow depth range above the 410 km discontinuity. If melt is predominantly distributed along grain boundaries in low aspect ratio films, extents of melting as low as 2% could produce observed seismic velocity reductions. The ability of the lowermost mantle to contain negatively buoyant hydrous liquids hinges on the trade‐off between iron content and hydration: at these depths, substantially higher degrees of hydration could be present within partial melts.

    

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