Path-integral molecular dynamics predictions of H isotope fractionation between brucite and water at elevated temperatures and pressures

Abstract

Hydrogen (H) isotope fractionation during the dehydration of H-bearing minerals is crucial for tracing the subduction processes. Previous studies have shown that H isotope fractionation between brucite and water is highly sensitive to the change of pressure. However, the detailed behaviors of H isotopes at high temperatures and high pressures remain poorly understood. In this study, we investigate the D/H isotope fractionation between brucite and water at elevated temperatures (300–1273 K) and pressures (0–8 GPa) using path-integral molecular dynamics (PIMD) simulations to account for the nuclear quantum effects (NQEs). The deep potential models trained on the first-principles molecular dynamics (FPMD) data were used to accelerate the simulations. The calculated reduced partition function ratios (RPFRs) of water decrease with increasing pressure at all the temperatures studied in this work, while the RPFRs of brucite increase with pressure. Due to the opposing pressure effects on RPFRs of these two phases, the D/H isotope fractionation between brucite and water is highly sensitive to the pressure change, and the fractionations are inversed at high pressures (e.g., at 2 GPa and 200 °C). The reversal of D/H isotope fractionation is attributed to the distinct responses of H bonding environments in brucite and water to the pressure changes.

Using brucite as a proxy for H-bearing minerals in subduction zones, we modeled the δD values of both the water retained in the slab and the water released into the mantle in three different subduction zones. The results demonstrate that the δD values of both the slab water and the released water are closely related to the pressures and temperatures of the slabs. The modeled δD values of the released water mainly range from − 40 ‰ to − 70 ‰, overlapping with the average δD value of the depleted mantle (−60 ± 5 ‰). Due to the reversal of isotope fractionation, the water transported into the deep Earth (>200 km) exhibit relatively high δD values (>−50 ± 10 ‰), suggesting that the extremely low δD signatures (e.g., < −200 ‰) reported in previous works may not be the results of the past deep subductions. The modeled results also indicate that recycled water transported into the deep Earth may have δD values distinct from those of primitive mantle reservoirs.