An experimental investigation of C-O-H fluid-driven carbonation of serpentinites under forearc conditions

Abstract

The carbonation of serpentinites in the forearc region of the mantle wedge in subduction zones and of serpentinites within the subducting slab by fluids derived from prograde dehydration and decarbonation has important implications for the deep Earth carbon cycle. This study shows that the carbonation of serpentinites under the forearc can establish, over time, a significant reservoir for carbon within a partially hydrated mantle wedge and that carbonation of (ultra-) mafic rocks within the subducting slab contributes to C-transfer to greater depths and might supply carbon for arc volcanism or the deep mantle. We report a new high pressure experimental investigation of the interactions between oxidised C–O–H fluids and serpentinite and model the reaction progress with time series experiments. The CO2, H2O and alkane (CnH2n+2, n=1–6; e.g. methane, ethane) contents in the fluid phase from quenched experimental run products have been analysed by gas chromatography and the results are compared with thermodynamic calculations. With progressive carbonation, the formation of magnesite + chlorite together with quartz, quartz + talc or talc at 1–2 GPa and 500–650 °C was observed. At temperatures above antigorite stability (T≳700 °C and 2 GPa) magnesite + chlorite is stable together with talc, talc + enstatite, enstatite or enstatite + forsterite for decreasing CO2-content in the fluid. Carbonation of serpentinite is a rapid process where magnesite forms within the first hour of the experiments, filtering CO2 from the fluid effectively and equilibrium is approached within 48 h. The CO2-sequestration and magnesite production are less pronounced at higher temperatures. Therefore, C–O–H fluids released under subarc conditions might migrate through the mantle with only minor changes in their carbon budget whereas significant carbonation likely occurs under the forearc. Furthermore, partition coefficients for Ca, Ba, Sr and Pb between magnesite and a C-poor aqueous fluid have been established to demonstrate the potential of newly formed magnesite to sequester fluid mobile elements. Show more