The effect of modulated deserpentinization on the deep carbon cycle

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).