Subduction metamorphism of serpentinite-hosted carbonates beyond antigorite-serpentinite dehydration (Nevado-Filábride Complex, Spain)

Abstract

At sub-arc depths, the release of carbon from subducting slab lithologies is mostly controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atg-) serpentinite to prograde peridotite. Here we investigate carbonate–silicate rocks hosted in Atg-serpentinite and prograde chlorite (Chl-) harzburgite in the Milagrosa and Almirez ultramafic massifs of the palaeo-subducted Nevado-Filábride Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at P–T conditions before and after the dehydration of Atg-serpentinite in a warm subduction setting. In the Milagrosa massif, carbonate–silicate rocks occur as lenses of Ti-clinohumite–diopside–calcite marbles, diopside–dolomite marbles and antigorite–diopside–dolomite rocks hosted in clinopyroxene-bearing Atg-serpentinite. In Almirez, carbonate–silicate rocks are hosted in Chl-harzburgite and show a high-grade assemblage composed of olivine, Ti-clinohumite, diopside, chlorite, dolomite, calcite, Cr-bearing magnetite, pentlandite and rare aragonite inclusions. These NFC carbonate–silicate rocks have variable CaO and CO2 contents at nearly constant Mg/Si ratio and high Ni and Cr contents, indicating that their protoliths were variable mixtures of serpentine and Ca-carbonate (i.e., ophicarbonates). Thermodynamic modelling shows that the carbonate–silicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550–600°C and 1.0–1.4 GPa) and Chl-harzburgite (Almirez massif; 1.7–1.9 GPa and 680°C). Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonate–silicate bulk rock compositions formed before prograde metamorphism, likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaO–MgO–SiO2 ternary, these processes resulted in a compositional variability of NFC serpentinite-hosted carbonate–silicate rocks along the serpentine-calcite mixing trend, similar to that observed in serpentinite-hosted carbonate-rocks in other palaeo-subducted metamorphic terranes. Thermodynamic modelling using classical models of binary H2O–CO2 fluids shows that the compositional variability along this binary determines the temperature of the main devolatilization reactions, the fluid composition and the mineral assemblages of reaction products during prograde subduction metamorphism. Thermodynamic modelling considering electrolytic fluids reveals that H2O and molecular CO2 are the main fluid species and charged carbon-bearing species occur only in minor amounts in equilibrium with carbonate–silicate rocks in warm subduction settings. Consequently, accounting for electrolytic fluids at these conditions slightly increases the solubility of carbon in the fluids compared with predictions by classical binary H2O–CO2 fluids, but does not affect the topology of phase relations in serpentinite-hosted carbonate-rocks. Phase relations, mineral composition and assemblages of Milagrosa and Almirez (meta)-serpentinite-hosted carbonate–silicate rocks are consistent with local equilibrium between an infiltrating fluid and the bulk rock composition and indicate a limited role of infiltration-driven decarbonation. Our study shows natural evidence for the preservation of carbonates in serpentinite-hosted carbonate–silicate rocks beyond the Atg-serpentinite breakdown at sub-arc depths, demonstrating that carbon can be recycled into the deep mantle.