Experimental evidence of pressure effects on spinel dissolution and peridotite serpentinization kinetics under shallow hydrothermal conditions

Abstract

Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs, as well as fluid and element transfer between the Earth’s mantle and crust. However, fluid-rock element exchange and serpentinization kinetics under shallow hydrothermal conditions is still largely unconstrained. Here we present two constant temperature (230 °C) time-series of natural peridotite (77.5% olivine; 13.7% enstatite; 6.8% diopside; 2% spinel) serpentinization experiments: at 13.4 MPa; and 20.7 MPa. Al-enriched lizardite was the main secondary mineral in all runs after olivine (olv) and orthopyroxene (opx) serpentinization (without any detectable brucite, talc or magnetite), while primary spinel and diopside partially dissolved during the experiments. Initial serpentinization stages comprises intrinsically coupled reactions between olivine and enstatite, as Al and Si are progressively transferred from orthopyroxene-derived to olivine-derived serpentine, while the opposite is true for Mg and Fe, with homogenization of serpentines compositions after 40 days. The Ni/Cr ratios of serpentines, however, remain diagnostic of the respective primary mineral. Estimated average serpentine content indicates fast serpentinization rates of 0.55 wt.%·day−1 (0.26 mmol·day−1) and 0.26 wt.%·day−1 (0.13 mmol·day−1) at 13.4 and 20.7 MPa, respectively. Approximately 2x faster serpentinization kinetics at lower pressure is likely linked to enhanced spinel dissolution leading to one order of magnitude higher available Al, which accelerates olivine serpentinization while delays orthopyroxene dissolution. Additionally, time-dependent increase in solid products masses suggests rock volume expands linearly 0.37% ± 0.01% per serpentine wt.% independently of pressure. Mass balance constrains suggests olv:opx react at ∼5:2 and ∼3:2 M ratios, resulting in Si-deficient and Si-saturated serpentines at the end of the low-pressure series (13.4 MPa) and high-pressure series (20.7 MPa), respectively. Elevated starting peridotite olv:opx ratio (7.94:1) therefore indicates orthopyroxene serpentinization is ∼3.3x and ∼5.4x faster than olivine at 13.4 MPa and 20.7 MPa, respectively. This contradicts previous assumptions that olivine should dissolve faster than orthopyroxene at experimental conditions. Finally, serpentinization-derived fluids develop pH > 10 and become enriched in H2, CH4, Ca2+ and Si within 6 weeks. Aqueous silica concentrations are highest after 5 days (265.75 and 194.79 µmol/kg) and progressively decrease, reaching 13.84 and 91.54 µmol/kg at 13.4 and 20.7 MPa after 40 days, respectively. These concentrations are very similar to the low-silica (M6) and high-silica (Beehive) endmembers of the Lost City Hydrothermal Field (LCHF). Beyond fluid characteristics, serpentinization products and conditions analogous to the LCHF suggest similar mechanisms between our experiments and natural processes. Our results demonstrate constant temperature serpentinization of a common protolith leads to distinct serpentine and fluid compositions at different pressures. Although additional data is necessary, recent studies and our experiments suggest peridotite serpentinization rates at 230 °C rapidly decrease with increasing pressures at least up to 35 MPa. Whether pressure directly influences olivine and orthopyroxene serpentinization kinetics or indirectly controls reaction rates due to spinel dissolution under hydrothermal conditions deserves further investigation.