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

Hydrated (with D (sub 2) O and H (sub 2) O) sodium tetrasilicate glasses, quenched from melts at 1400 degrees C and 1.5 GPa, are studied using (super 1) H, (super 2) H, and (super 29) Si solid-state nuclear magnetic resonance (NMR) spectroscopy. Whereas D (sub 2) O and H (sub 2) O 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/H (sub W,Melt) not = D/H (sub W,Fluid) ) during crystallization of Earth's magma ocean, potentially controlling the D/H content of the Earth's oceans.