Very large differences in intramolecular D-H partitioning in hydrated silicate melts synthesized at upper mantle pressures and temperatures
Very large differences in intramolecular D-H partitioning in hydrated silicate melts synthesized at upper mantle pressures and temperatures
复制标题
上地幔压力和温度下合成的水合硅酸盐熔体的分子内 D-H 分配存在很大差异
DOI：
10.2138/am-2015-4940
复制
发表时间：
2015
影响因子：
3.1
通讯作者：
G. Cody
中科院分区：
文献类型：
--
作者：
Ying Wang;S. X. Cody;D. Foustoukos;B. Mysen;G. Cody
用1H、2H和29Si固体核磁共振谱研究了在1400℃和1.5 Gpa的压力下从熔体中急冷的水合(用D2O和H2O)的四硅酸钠玻璃。虽然D2O和H2O对硅酸盐熔体的解聚程度相似，但在玻璃内部不同分子位置之间的分子内分配是非常不同的，表现为氘与O-D···O距离较短的位置的强烈优先缔合。这一偏好与总含水率和D/H比无关。在模型含水玄武岩组成的玻璃中也观察到了本质上不同的分子内D-H分配。由于合成温度较高，传统的平衡分馏不可能产生如此大的同位素分配差异。通过缓慢的猝灭实验排除了潜在的动力学同位素效应。表观分馏可能受密度/摩尔体积同位素效应的影响，其中氚偏爱摩尔体积较小的位置。在地球岩浆海洋的结晶过程中，熔体中分子内位置分配的巨大差异可能导致水饱和熔体和出溶的水流体(其中D/HW，Melt≠D/HW，流体)之间的D-H分配的显著差异，从而潜在地控制地球海洋的D/H含量。
Abstract Hydrated (with D2O and H2O) sodium tetrasilicate glasses, quenched from melts at 1400 °C and 1.5 GPa, are studied using 1H, 2H, and 29Si solid-state nuclear magnetic resonance (NMR) spectroscopy. Whereas D2O and H2O depolymerize the silicate melt to similar degrees, protium and deuterium intramolecular partitioning between different molecular sites within the glasses is very different and exemplified by a strong preferential association of deuterons to sites with short O-D···O distances. This preference is independent of total water content and D/H ratio. Substantially different intramolecular D-H partitioning is also observed in a glass with a model hydrous basalt composition. Such large differences in isotope partitioning cannot result from classic equilibrium fractionation because of the high synthesis temperature. Potential kinetic isotope effects are excluded via a slow quench experiment. The apparent fractionation is likely governed by density/molar volume isotope effects, where deuterium prefers sites with smaller molar volume. Large differences in intramolecular site partitioning in melts could lead to significant differences in D-H partitioning between water-saturated melt and exsolved aqueous fluid (where D/HW,Melt ≠ D/HW,Fluid) during crystallization of Earth’s magma ocean, potentially controlling the D/H content of the Earth’s oceans.