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

Friday, 19 December 2014
Celia Dalou, University of Texas at Austin, Austin, TX, United States, Charles Le Losq, Carnegie Inst Washington, Geophysical Laboratory, Washington, DC, United States and Bjorn O Mysen, Carnegie Inst Washington, Washington, DC, United States
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 ΔHfluid -1.2 ±0.5 kJ/mol. In contrast, the D/H ratios of coexisting melts display strong temperature dependence. The ΔHmelt 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.