Thermoelasticity of Water in Silicate Melts: Implications for Melt Buoyancy in Earth's Mantle
Thermoelasticity of Water in Silicate Melts: Implications for Melt Buoyancy in Earth's Mantle
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DOI：
10.1029/2021jb022359
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发表时间：
2021-08
期刊：
影响因子：
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通讯作者：
G. Zeff;Q. Williams
中科院分区：
文献类型：
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作者：
G. Zeff;Q. Williams
对硅酸盐熔体中水的偏摩尔体积的实验数据和理论模拟的综合分析表明，有限应变理论成功地描述了在高压和高温下溶解在硅酸盐熔体中的H2O组分的压缩。然而，由于水组分的高压缩性，需要四阶状态方程拟合，以准确地模拟在上地幔深部，过渡带和下地幔压力的硅酸盐熔体中水的体积的实验结果。从以前的冲击压缩实验中的含水矿物的熔化发生沿着Hugoniot的数据被用来提供在深部地幔温度和压力的硅酸盐熔体中的水的偏摩尔体积的实验约束。水成分的状态方程表明，根据弹性平均技术，410 km地震间断面上方中性或负浮力的镁铁质/超镁铁质熔体中可能存在的水量上限为5.6 wt%：小于先前推断的值，并且与熔体局限于410 km间断面上方的狭窄深度范围一致。如果熔体主要分布在低纵横比薄膜中的沿着晶界，则低至2%的熔融程度可以产生观察到的地震速度降低。最低地幔包含负浮力含水液体的能力取决于铁含量和水合作用之间的权衡：在这些深度，部分熔体中可能存在更高程度的水合作用。
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.