Infiltration of fluids in subduction zones and in the sub continental mantle lithosphere are responsible for metasomatism and melting of the mantle. The ratio of fluid to rock changes in different settings and processes, but its effect on the mineralogy and solidus location has never been experimentally examined. As the main component in such fluids is H2O, this study aims at determining the peridotite solidus temperature as well as the subsolidus mineralogy at different H₂O contents.
High pressure high temperature experiments were conducted at a pressure of 4 GPa and temperatures of 850-1000°C in the peridotite+H2O system with 5 and 10 wt.% H₂O contents. Capsules contained a diamond trap to allow the analysis of the fluid or melt phase using LA-ICP-MS. The mineral phases were analyzed by the Electron Probe Microanalyser. The oxygen fugacity was calculated based on the amount of Fe in small Ir grains doped into the starting material. Additionally, ThermoCalc software was used to model phase equilibria and predict the wet solidus position under varying bulk H₂O concentrations.
The solidus for a peridotite containing 10 wt.% H₂O, was observed between 900-950°C, compared to a solidus between 900 and 1000°C for a similar peridotite containing 15 wt.%. The solidus for a peridotite containing 5 wt.% is still unclear. The log⁡ fO2 increases with increasing temperature, regardless of the initial H₂O content in the system. ThermoCalc modeling results suggests that the solidus temperature decreases with initial introduction of H2O into a peridotite and the stabilization of amphibole. Further increase in H2O content results in the disappearance of amphibole and the increase in solidus temperature.
These findings contribute to a more accurate understanding of the effect of fluid infiltration into a peridotite in various environments in the mantle.