Hydrogen Isotope Fractionation in the System Brucite-Water at 3GPa

Upper mantle rocks are depleted in D by 80‰ relative to the oceans. As water is recycled through the upper mantle-hydrosphere system over geologic time, the offset between mantle and ocean is likely created by fractionation of isotopes during hydration of the oceanic crust and subsequent partial dehydration of subducted material. To understand how δ D of the slab is modified during subduction, we need to know fractionation factors for hydrous mantle minerals at pressures and temperatures relevant to subduction. The aim of this study is to develop experimental and analytical techniques that will allow us to extend measurements to temperatures and pressures attainable in solid media high pressure apparatus. We have made our first measurements in the chemically simple brucite-water system, which has been the subject of previous high pressure experimental (up to 0.8GPa, 600° C) and theoretical work.

Piston cylinder experiments were performed at 3GPa and 500-700° C with starting materials consisting of fine grained (<5μ m) brucite (δ D = -100‰ ) and water (δ D between -190 and +117‰ ). Brucite and water were mixed such that free water constituted 70% of the total H₂O, and were sealed in thick-walled Au capsules. Partial exchange experiments, consisting of three separate piston cylinder runs containing different starting waters, but otherwise identical conditions, were conducted to determine equilibrium fractionation factors. Experimental duration was between 12 and 144 hours. After equilibration, the water was extracted completely from the capsule and divided between several charges, and aliquots of brucite were heated on a vacuum line to liberate water, which was analysed using conventional Zn-reduction methods. Experiments contained enough material for up to 5 water and 2 brucite analyses. In experiments where replicate analyses were made, the average uncertainty (1σ ) was 1.8‰ . Bulk D/H ratios in experiments that used deionised water (δ D = -54‰ ) remained constant within analytical uncertainty, while experiments with waters that were enriched or depleted in D became systematically lighter or heavier by up to 9‰ . This might be due to diffusive exchange with the pressure medium. Results of 48h partial exchange experiments at 500 and 700° C and 12h experiments at 700° C, together with a single 144h experiment at 700° C, indicate that brucite/water fractionation factors are smaller than -10‰ under these conditions. These fractionation factors are significantly smaller than those measured at lower pressure, and so confirm that pressure continues to be an important variable in controlling D/H fractionation at up to 3GPa, even at elevated temperatures. More experiments are required to constrain the fractionation processes that control hydrogen isotopic ratios in the mantle and hydrosphere.