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

Tiraboschi, Carla; Tumiati, Simone; Sverjensky, Dimitri; Pettke, Thomas; Ulmer, Peter; Poli, Stefano (2018). Experimental determination of magnesia and silica solubilities in graphite-saturated and redox-buffered high-pressure COH fluids in equilibrium with forsterite + enstatite and magnesite + enstatite. Contributions to mineralogy and petrology, 173(1) Springer 10.1007/s00410-017-1427-0

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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 °C.
Synthetic forsterite, enstatite, and nearly pure natural magnesite were used as starting materials. Redox conditions were
buffered by Ni–NiO–H2O (ΔFMQ = − 0.21 to − 1.01), employing a double-capsule setting. Fluids, binary H2O–
CO2 mixtures
at the P, T, and fO2 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 (mSiO2 = 1.24 mol/kgH2O
versus mSiO2 = 0.22 mol/kgH2O at P = 1 GPa, T = 800 °C) and magnesia (mMgO = 1.08 mol/kgH2O versus mMgO = 0.28 mol/
kgH2O) 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.

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