Fluid-driven metasomatism in the mantle wedge: the hidden role of carbon-saturated COH fluids

COH fluids have a crucial role in a variety of geological processes in subduction zones, where carbon-bearing aqueous fluids released from the down-going slab infiltrate the overlying mantle wedge prompting metasomatic modifications. Yet, the ability of these fluids to mobilize rock components, and hence their metasomatic potential remains poorly constrained. This is mainly due to the scarcity of experimental data on mineral solubility and fluid speciation in COH-bearing systems at high-pressure conditions. Graphite-saturated COH fluids have been suggested as potentially efficient metasomatic agents, due to their high solute concentrations even at relatively low-pressure conditions. Graphite-saturated COH fluids in equilibrium with forsterite and enstatite can contain up to 11 wt.% of total solutes at 1 GPa and 800 °C, an amount significantly higher compared to the solute content for the same phase assemblage in a H₂O-only fluid (i.e. 2.4 wt.%) [1].
Investigating the metasomatic effect of solute-bearing fluids moving away from the source rock and reacting with different lithologies is a complex task. Experimentally, this would require the fluid to be transferred from the initial experimental charge to a different one. However, after quenching most of the dissolved solids originally dissolved in the fluid precipitate. The precipitated solids are both heterogeneous and physically fragile, making their collection and analysis a challenging aspect of the experimental design. Thermodynamic models, such as the Deep Earth Water model [2], allow to tackle the issue from a different perspective and assist in investigating the effect of solute-bearing fluids reacting with different rock assemblages.
Here, we present thermodynamic modeling relative to the interaction between solute-rich COH fluids and mantle wedge rocks (i.e. lherzolite, harzburgite and dunite), to assess their ability to generate orthopyroxene at 1 GPa and temperatures from 700 to 900 °C. Our results show that the fluid generated from the sole dissolution of forsterite and enstatite in graphite-saturated COH fluids at relatively low-pressure conditions can modify the starting ultramafic rock composition, especially at high fluid/rock ratio, through enstatite formation. Fluid-driven metasomatism operated by COH fluids can thus represent an efficient mechanism to produce orthopyroxene-rich levels in the mantle wedge, without the concomitant formation of carbon-bearing phases, which would conceal the trivial role of carbon in the formation of these solute-rich fluids in the exhumed rock record.

[1] Tiraboschi C. et al. (2018) Contributions to Mineralogy and Petrology 173, 1–17.
[2] Sverjensky D.A. et al. (2014) Geochimica et Cosmochimica Acta 129, 125-145.