In-situ measurements of D/H fractionation between melt and coexisting aqueous fluids in the Na2O-Al2O3-SiO2-H2O system

Abstract:

Hydrogen isotope partitioning (as H₂O and D₂O) between water-saturated silicate melts and coexisting silicate-saturated aqueous fluids with several different initial D/H ratios in the Na₂O–Al₂O₃–SiO₂–H₂O system has been determined. In-situ measurements in a hydrothermal diamond anvil cell (HDAC) with the fluid and melt at the desired temperatures (≤800˚C) and pressures (≤1115 MPa) were carried out with microRaman and FTIR spectroscopy techniques.

For bulk D/H ratios were used: 0.05 ±0.02, 0.13 ±0.05, 0.53 ±0.01 and 2.35 ±0.04. Three experimental series (D/H: 0.05, 0.13, 0.53) with coexisting fluid and melt have comparable pressure/temperature trajectories (350-650 ºC/322-626, 313-741 and MPa; 248-648 MPa, respectively), whereas the experimental series with D/H=2.35 had a lower pressure/temperature trajectory (400-680 ºC/192-496 MPa). In these pressure/temperature ranges, the D/H ratios of fluids barely change with temperature, with an average small negative ΔH_fluid -1.2 ±0.5 kJ/mol. In contrast, the D/H ratios of coexisting melts display strong temperature dependence. The ΔH_melt decreases from 14.6 ±2.2 to -3.7 ±1.1 kJ/mol with the D/H ratio increasing from 0.05 ±0.02 to 2.35 ±0.04. Consequently, the (D,H) exchange equilibrium between melt and fluid is temperature dependent, and varies so that its ΔH increases from -15.9 ±2.7 to 0.3 ±0.4 kJ/mol with increasing D/H ratios.

Hydrogen isotope fractionation between silicate melts and low density phases (aqueous fluids or gases) may affect the δD values during, for example degassing of mantle derived-magmatic liquids. Moreover, D/H fractionation between silicate minerals and melts in the Earth’s interior can be affected by the significant temperature and composition-dependent D/H fractionation in silicate melts at high temperatures and pressures.