The effect of modulated deserpentinization on the deep carbon cycle

Abstract

Interaction between fluids and different rock types at the subducting slab interface directly influences deep volatile recycling processes. Recent studies [1] suggest that an influx of reduced fluids from graphite-bearing metapelites can promote deserpentinization at lower temperatures and modify the intrinsic redox capacity of the released fluid. Here we report a newly discovered high-pressure deserpentinization front ­– Cerro Pingano (Betic Cordillera, Spain), located 20 km north of the type locality of Cerro del Almirez [1] – where additional observations involving carbonate-bearing chlorite-harzburgites further support the influx of C-bearing fluids concomitantly to deserpentinization. This small body (<0.5 km2) is hosted within a metasedimentary sequence of graphite-bearing mica schists and marbles which together underwent an Alpine high-pressure metamorphism. Antigorite (Atg-)serpentinite (without carbonate) is separated from Chl-harzburgite (enriched in magnesite) through a sharp boundary of chlorite (Chl-)serpentinite that can be traced across a ~50 cm wide reaction front. Atg-serpentinite shows an S-C fabric with a strong crystallographic preferred orientation (CPO) characterized by [001]Atg perpendicular to the foliation. Although the transformation to Chl-harzburgite is associated with fabric weakening, the low-strain olivine, orthopyroxene, and tremolite show a remarkable distribution of the poles to their (010) perpendicular to the compositional layering. Magnesite [0001] axes are distributed perpendicular to the layering, and petrographic observations indicate textural equilibrium between Ol + Opx + Chl + Tr + Mag. The transformation of Atg-serpentinite to Chl-harzburgite is also associated with a decrease of Fe3+/ΣFe and a measurable increase of C, Na, and K content in bulk chemistry, suggesting an interaction with externally derived fluids. We tested this hypothesis with thermodynamic modelling of the fluid-rock interaction between the host-rock derived fluids and Atg-serpentinites. Thermodynamic models of graphite-bearing mica schist predict dehydration due to chloritoid and chlorite breakdown, and release of highly reducing CH4 + CO2-bearing fluids. Open-system infiltration models show that host-rock derived fluids could effectively reduce the serpentinite bulk composition, consistently with the observed decrease of Fe3+/ΣFe between Atg-serpentinites and Chl-harzburgites. The infiltration-induced reduction shifts the stability of Chl-harzburgite mineral assemblage to temperatures ~50°C lower than predicted by closed-system models without external fluids input. While the microstructural record suggests that Chl-harzburgite fabric was inherited from the Atg-serpentinite precursor, the changes in bulk chemistry and redox conditions result from interaction with highly reductive aqueous fluids derived from host graphite-bearing mica schists. Hence, we infer that the record of specific prograde metamorphic reactions may reflect changes in redox conditions, not necessarily associated uniquely with P-T changes. The redox contrast between the reduced fluids and prograde dehydrating serpentinites thus represents a new way to precipitate carbonates that can be further subducted and may devolatilized beyond subarc depths.   [1] Padrón-Navarta, J.A. et al. (2023) Nat. Geosci. Research funded by RUSTED project PID2022-136471N-B-C21 & C22 funded by MICIN/ AEI/10.13039/501100011033 and FEDER program, and “Juan de la Cierva” Fellowship JFJC2021-047505-I by MCIN/AEI/10.13039/501100011033 and CSIC (M. Bukała).