Experimental determination of magnesia and silica solubilities in graphite-saturated and redox-buffered high-pressure COH fluids in equilibrium with forsterite plus enstatite and magnesite plus enstatite

摘要

We experimentally investigated the dissolution of forsterite, enstatite and magnesite in graphite-saturated COH fluids, synthesized using a rocking piston cylinder apparatus at pressures from 1.0 to 2.1 GPa and temperatures from 700 to 1200 degrees C. Synthetic forsterite, enstatite, and nearly pure natural magnesite were used as starting materials. Redox conditions were buffered by Ni-NiO-H2O (Delta FMQ = -0.21 to -1.01), employing a double-capsule setting. Fluids, binary H2O-CO2 mixtures at the P, T, and fO(2) conditions investigated, were generated from graphite, oxalic acid anhydrous (H2C2O4) and water. Their dissolved solute loads were analyzed through an improved version of the cryogenic technique, which takes into account the complexities associated with the presence of CO2-bearing fluids. The experimental data show that forsterite + enstatite solubility in H2O-CO2 fluids is higher compared to pure water, both in terms of dissolved silica (mSiO(2) = 1.24 mol/kg(H2O) versus mSiO(2) = 0.22 mol/kg(H2O) at P = 1 GPa, T = 800 degrees C) and magnesia (mMgO = 1.08 mol/kg(H2O) versus mMgO = 0.28 mol/kg(H2O)) probably due to the formation of organic C-Mg-Si complexes. Our experimental results show that at low temperature conditions, a graphite-saturated H2O-CO2 fluid interacting with a simplified model mantle composition, characterized by low MgO/SiO2 ratios, would lead to the formation of significant amounts of enstatite if solute concentrations are equal, while at higher temperatures these fluid, characterized by MgO/SiO2 ratios comparable with that of olivine, would be less effective in metasomatizing the surrounding rocks. However, the molality of COH fluids increases with pressure and temperature, and quintuplicates with respect to the carbon-free aqueous fluids. Therefore, the amount of fluid required to metasomatize the mantle decreases in the presence of carbon at high P-T conditions. COH fluids are thus effective carriers of C, Mg and Si in the mantle wedge up to the shallowest level of the upper mantle.