Experimental investigation of the fluid driven carbonation of serpentinites and spinel-peridotites: implications for the carbon and trace element cycle in the forearc region of the mantle wedge

Abstract

Significant amounts of carbon (C) are released from the subducting slab into fluids, but less carbon is outgassed in arc volcanoes implying the storage of C in the mantle wedge. This experimental study investigates the fate of C bearing aqueous fluids when they travel through the (partially) hydrated forearc mantle targeting the forearc mantle as a potentially relevant carbon reservoir. Thermodynamic modelling and piston cylinder experiments are utilized to gain a comprehensive understanding on the reaction of slab-derived COH fluids with serpentinites and peridotites between 1-2.5 GPa and 375-750 °C. Powdered, natural serpentinite was used to establish the phase relations of the antigorite-CO2-H2O-system. Cylindrical cores of either natural serpentinite or peridotites are used as starting material investigating the reaction mechanisms, timescales and magnitudes of the CO2-H2O-H2-fluid to rock reaction under realistic grain sizes, natural porosity and given texture. Partitioning coefficients for fluid mobile elements between carbonates and COH-fluids are established experimentally. The results are applied to carbonated high pressure rocks from the Western European Alps. The volatile composition of experimentally derived fluids is quantified in an optimized analytical set up and measurement procedure using gas chromatography. Solids are examined by XRD, Raman spectroscopy, SEM, FE-EPMA, EPMA and Laser-Ablation-ICP-MS. 3D-µ-CT (three dimensional high resolution micro X-ray computed tomography) of recovered rock cores visualizes textures and porosity and determines phase abundances. This study demonstrates that carbonation of the (partially) hydrated forearc mantle efficiently sequesters CO2 from the fluid into newly formed carbonates. With decreasing CO2 activity in the fluid, magnesite is formed with quartz, talc and antigorite at 1-2.5 GPa and 375-650 °C. Above antigorite stability, magnesite is stable with talc, talc+enstatite and enstatite±forsterite. Excess aluminium derived from the consumed serpentine or pyroxenes leads to the formation of chlorite and kyanite. With progressing carbonation and decreasing CO2 activity in the fluid, metasomatic reaction zones of distinct mineral assemblages are formed in core experiments. Reaction zones are fluid permeable because interconnected porosity is created during carbonation of serpentinite and peridotite which likely occurs via an interface coupled dissolution precipitation mechanisms. Carbonation occurs as soon as the COH-fluid is in contact with primary silicates. Carbonation is fast and completed within <1 h of the experiments using powdered serpentinite and within <96 h using serpentinite cores. The carbonation rate of peridotite cores approximates the rate of serpentinite cores. The extent of C sequestering is enhanced when reducing fluids cause additionally graphite precipitation. Fluid mobile elements (e.g. Sr, Ba, Pb) are compatible in carbonates and suitable for monitoring the carbonation process. Carbonation of the partially hydrated forearc mantle is relevant to the cycle of C and trace elements in subduction zones. Over time, the forearc mantle may become a significant reservoir for C contributing to explain the imbalance between carbon released from the slab and carbon output in arc magmatism. Thus, carbonation within the forearc mantle should be considered when estimating carbon fluxes within the Earth’s deep C cycle. Further, a carbonated forearc mantle may host a crustal trace element and isotope signature imposed by metasomatizing fluids liberated from the subducting oceanic crust.