Deuterium-Hydrogen (D/H) Fractionation at High Pressure and High Temperature

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Zhang EAR-0106718 This proposal is aimed at understanding the influence of pressure, temperature and composition on deuterium-hydrogen (D/H) isotopic fractionation through high-pressure, high temperature experiments. Isotope fractionation at high pressure is theoretically interesting and affects both the composition of fluids released from subducted slabs, and of the residual water that is entrained in high-pressure phases. Under favorable conditions, these water-containing phases may be sequestered at great depth in the lower mantle. The isotopic signature of such "recycled" water will depend in part upon fractionation processes occurring at mantle conditions. Although the D/H ratio in the present upper mantle is known to be on the whole relatively constant, the primordial D/H ratio for the earth is unknown. Knowledge of high-pressure D/H fractionation factors relevant to degassing of the earth will help constrain the primitive D/H ratio and the proportion of ocean water that originated from the mantle . The D/H ratios of subduction related basalts are relatively high (e.g. Poreda, 1985; Dobson and O'Neil, 1987), indicating that their source has a different D/H ratio to the depleted upper mantle. Although this is attributed in part to the D/H ratio of subducted water, the D/H ratio of the evolved water may be affected by fractionation processes as water is released from hydrous minerals at depth. In the bulk of the mantle, nominally anhydrous minerals are thought to constitute the major reservoir for water (e.g. Bell and Rossmann, 1992; Kohlstedt et al., 1996). It has been suggested that the D/H ratio of water in nominally anhydrous minerals of the upper mantle is fixed by equilibration with hydrous fluids in the mantle wedge (Bell and Ihinger, 2000). Thus, fractionation of D/H between hydrous phases (in the slab), fluid (leaving the slab), hydrous magmas (in the mantle wedge) and nominally anhydrous minerals (the mantle residue) could explain the D/H systematics of the entire upper mantle. D/H fractionation factors at pressures greater than a few kilobars have not yet been measured, however, and cannot be reliably predicted; thus we propose to measure fractionation experimentally under mantle conditions. In the period of this one-year pilot project, we will carry out a preliminary investigation of the effects of pressure and temperature on D/H fractionation between brucite and water at elevated pressure (3-6 GPa) and temperature (500-700 degrees C). This work will determine the magnitude of the pressure effect on D/H fractionation. Working in this simple MgO-H2O system with such a water rich mineral will also allow us to develop and improve our experimental and analytical technique. The results of this systematic study will be fundamental to our understanding of isotopic fractionation at high pressure. In addition, we will begin to investigate the isotopic fractionation factor between a synthetic basalt melt and H2O vapor phase to understand the D/H fractionation during mantle degassing. The effect of dissolved H2O content (hydroxyl to molecular H2O ratio) on the fractionation factor will also be examined.