An experimental study on the possible stability of whewellite CaC2O4·H2O under subduction zone conditions

Abstract

Whewellite was reported as mineral in fluid inclusions in high-pressure metamorphic rocks, lending support to theoretical predictions that organic species may be stable and abundant in high-pressure subduction zone fluids. In this study, we have experimentally studied the stability of whewellite at typical subduction zone pressure and temperature conditions, both by in-situ observations using an externally-heated diamond cell and by piston-cylinder experiments with long run durations. Heating whewellite in a diamond cell to 250 °C and 1.5 GPa for a few hours and subsequent cooling essentially just caused re-crystallization. However, upon heating to higher temperatures already within a few hours, decomposition to graphite, aragonite, and CO2 is obvious. Moreover, piston cylinder experiments with run durations of 7 to 26 days and conditions ranging from 300°C at 2 GPa to 700 °C at 4.5 GPa all show complete decomposition of whewellite to graphite and aragonite according to 2 CaC2O4·H2O = 2 CaCO3 + C + CO2 + 2 H2O. Overall, our results suggest that whewellite in fluid inclusions from high-pressure metamorphic rocks is likely a secondary phase formed upon exhumation at rather low temperatures. The observation of whewellite as such does not provide any evidence for the stability of organic species in high-pressure subduction zone fluids. However, combined with the data from natural fluid inclusions, our data suggest that the abiotic synthesis of carboxylic acids and their salts is feasible at rather low temperature and pressures in near-surface environments of Earth’s crust and possibly, also on other planets.