In situ study of the fractionation of hydrogen isotopes between aluminosilicate melts and coexisting aqueous fluids at high pressure and high temperature - Implications for the δD in magmatic processes

The hydrogen isotopic composition of melt inclusions trapped in phenocrysts during their crystallization and growth in a magma may contribute to a better understanding of the water cycle between the atmosphere, the hydrosphere and the lithosphere. Such understanding relies on the knowledge of the hydrogen isotopic fractionation factors between aqueous fluids, silicate melts, and minerals at temperature and pressure conditions relevant to the Earth's interior. Significant D/H fractionation between silicate melts and aqueous fluids was reported at hundreds of MPa and °C by using in situ measurements in hydrothermal diamond anvil cell (HDAC) experiments (Mysen, 2013a, 2013b, Am. Mineral. 98, 376-386 and 1754-1764). However, the available dataset is focused on fluids and melts with D/H ratios close to unity. The relevance of such data for natural processes that involve per mil variations of δD-values may not always be clear. To address such concerns, the effect of the bulk D/H ratio on hydrogen isotope partitioning between water-saturated silicate melts and coexisting silicate-saturated aqueous fluids has been determined in the Na₂O-Al₂O₃-SiO₂-H₂O-D₂O system. To this end, in situ Raman spectroscopic measurements were performed on fluids and melts with bulk D/H ratios from 0.05 to 2.67 by using an externally-heated diamond anvil cell in the 300-800 °C and 200-1500 MPa temperature and pressure range, respectively.In these pressure/temperature ranges, the D/H ratios of fluids in equilibrium with melt barely change with temperature (in average δH_fluid=0.47±1.15kJ/mol). In contrast, the D/H ratios of coexisting melts display strong dependence on temperature (average δH_melt=7.18±1.27kJ/mol). The temperature-dependence of the D/H fractionation factor between melt and fluid (α_fluid-melt=D/H_fluid/D/H_melt) is comparable in all the experiments and can be written: 1000ln (α_fluid-melt)=263(±26)T^-2-126(±48). Therefore, the α_fluid-melt is independent of the bulk D/H ratio of the melt + fluid system.Experimentally determined α_fluid-melt using D-enriched fluids, therefore, can be applied to natural systems. It follows that for water-saturated magma strong isotopic fractionation of D and H between water dissolved in magmas and deep aqueous fluids may occur. The δD-values in melt inclusions in phenocrysts in such water-saturated magma will reflect such fluid/melt fractionation effects. A likely result is underestimation of the δD isotopic composition of slab fluids based on δD-values in melt inclusions. The temperature-dependent hydrogen isotope fractionation must be taken into account in the modeling of slab fluid-magma interaction in the mantle wedge.