Very large intramolecular D-H partitioning in hydrated silicate melts synthesized at upper mantle pressures and temperatures

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 large and controlled 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. Substantial intramolecular D-H partitioning is also observed in a glass with a model hydrous basalt composition. Such large 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 partitioning is likely governed by density/molar volume isotope effects, where deuterium prefers sites with smaller molar volume. Large intramolecular site partitioning in melts could lead to significant D-H partitioning between water saturated melt and exsolved aqueous fluid (where D/H_W,Melt ≠ D/H_W,Fluid) during crystallization of Earth’s magma ocean, potentially controlling the D/H content of the Earth’s oceans.