An experimental study on COH-bearing peridotite up to 3.2 GPa and implications for crust-mantle recycling

Although it is widely accepted that generation of arc magmas is triggered by fluids released from the subducting slab and their interactions with the overlying mantle wedge, it has been generally assumed that these fluids are essentially aqueous. However, the CO2 content of arc magmas and the findings of carbon-bearing phases in mantle-wedge peridotites suggest that during the subduction process carbon species are released from the downgoing slab. We experimentally investigate phlogopitebearing lherzolite interacting with H2O-CO2 fluids in order to model the mantle wedge fluxed by volatiles released from a subucting crustal slab. Experiments have been carried out at 900-1050 C and 1.6-3.2 GPa, at fluid- and graphite-saturated conditions. We used an end-loaded piston cylinder apparatus and a conventional double-capsule technique to constrain the redox state of the experiments, using nickel - nickel oxide oxygen buffer (NNO). At these conditions, our experimental study confirms that amphibole is stable up to 3 GPa in phlogopite-saturated lherzolite fluxed by COH fluids buffered at fO2 in the order of FMQ -1 -0. In addition to hydrates, in the subduction system carbonates should be nearly ubiquitous because the decarbonation occurs only at low-P, high-T conditions, which are not easily attained. Carbonates range from dolomite-only for P below 1.9 GPa at 900 C, to dolomite + magnesite up to 2.4 GPa, to magnesite-only above 2.4 GPa. For P > 2GPa, melting takes place at 1050 C, about 50 C higher than the water-saturated solidus of phlogopite lherzolite. At these conditions, possible in the hottest inner part of the corner flow, melts would display a magnesiocarbonatitic character. Together with COH fluids, phlogopite, amphibole, carbonates and carbonatites are the reservoirs of volatiles in the mantle wedge. Their low density compared to dry host peridotite can promote the upwelling of the metasomatized mantle, possibly producing a cold "COH plume" that could be a major carrier of carbon from the slab to the upper mantle.