Fluid-mediated carbon release by infiltration of serpentinite dehydration fluids during subduction: insights from thermodynamic models of serpentinite-hosted carbonate rocks

Serpentinites can significantly modulate the carbon fluxes in subduction zones because they occasionally host substantial concentrations of carbonate formed during the oceanic stage of subducting oceanic lithosphere (ophicalcite; [1]) or during metasomatic reaction with CO₂-bearing fluids at the subduction plate interface (e.g. hybrid carbonate–talc rocks; [2]). At subarc depth, fluid-mediated carbon release from lithologies like serpentinite-hosted carbonates is critical to understand the global carbon balance and magnitude of carbon fluxes from the subducting plate into the deep mantle. However, the solubility of carbon and the open-system metasomatic reactions during fluid-rock interactions in carbonated serpentinites at high P are not fully understood. In line with previous studies of prograde devolatilization [3], newer models show that the carbon release during prograde devolatilization reactions of serpentinite-hosted carbonate rocks is limited even if accounting for the higher carbon solubility of electrolytic fluids compared to molecular fluid models [4]. Therefore, devolatilization reactions driven by infiltration of Atg-serpentinite dehydration fluids into serpentinite-hosted meta-carbonate rocks determines how much carbon in the mantle lithosphere subducts deep into the mantle. Here we present the results of thermodynamic modelling – using the implementation of the DEW aqueous database in Perple_X [5] – to explore subduction fluid compositions and metasomatism of serpentinite-hosted carbonate rocks during prograde and infiltration-driven devolatilization reactions. The chemical system of serpentinite + carbonate is ideal to understand the interplay of changes in fluid composition, pH, bulk chemical modification and mineral assemblage during open-system fluid infiltration metamorphism. Our models provide new insights into the interaction of carbon-bearing subduction fluids with the cold hydrated mantle wedge, and the carbon release from serpentinite-hosted carbonates related to infiltration of serpentinite dehydration fluids at subarc depths. Our results further show that even though high fluid fluxes from serpentinite dehydration will transform meta-ophicalcites and talc-carbonate rocks into carbonate-garnet-clinopyroxene-olivine rocks and carbon-bearing orthopyroxenites, these rocks can subduct carbon beyond subarc depths into the deeper mantle where they may be related to the formation of deep diamonds, carbonatites and kimberlites.

REFERENCES

[1] Menzel et al., 2019, JMG 37, 681– 715.

[2] Spandler et al., 2008, CMP 155, 181-198.

[3] Kerrick & Connolly, 1998, Geology 26, 375-378.

[4] Menzel et al., 2020, EPSL 531.

[5] Connolly & Galvez, 2018, EPSL 501, 90-102.