The hidden role of Mg–Si–COH fluids on mantle wedge metasomatism

Abstract

COH fluids have long been thought to play a key role in subduction zone processes as their release from the subducting slab drives metasomatic alteration in the overlying mantle wedge. Among these, graphite-saturated COH fluids derived from a model peridotitic assemblage exhibit high solute concentrations of Mg and Si, compared to COH- and H₂O-only fluids: up to 17 wt% at 1 GPa and 900 °C, making them potentially efficient metasomatic agents. Probing experimentally the metasomatic effect of these fluids is however limited by the inability to recover the fluid phase with its solute load upon quenching, to further test the interaction with relevant graphite-free mantle wedge lithologies. To overcome this challenge, we employ thermodynamic modeling to both reproduce the solute load observed in experiments and subsequently simulate fluid–rock interactions at controlled conditions.

Here we use EQ3/6 coupled with the Deep Earth Water model to investigate the metasomatic effects of COH fluids generated by the dissolution of forsterite and enstatite in graphite-saturated COH fluids, interacting with graphite-free lherzolite, harzburgite, and dunite at 1 GPa and 700–900 °C, over a range of fluid/rock ratios. For comparison, we also simulate metasomatism by a H₂O-only fluid at identical conditions.

Our results confirm that Mg–Si–COH fluids drive significant compositional changes in the host rocks. While H₂O-only fluids primarily stabilize clinochlore up to 800 °C, graphite-saturated COH fluids promote orthopyroxene formation, doubling its mineral proportions for high fluid/rock ratios. These results highlight the enhanced metasomatic potential of carbon-bearing fluids, which, in the model, can generate orthopyroxene-rich assemblages and silica-enriched mantle domains comparable to those observed in natural subduction settings. They also expose a paradox: carbon is essential to produce the solute-rich fluid, yet no carbon-bearing phases remain in the final rock assemblage. This implies that carbon-bearing fluids may have been more influential in subduction zone metasomatism than previously recognized, despite leaving no direct mineralogical evidence in the exhumed rock record.