An Experimental Study on COH-bearing Peridotite up to 3•2 GPa and Implications for Crust-Mantle Recycling

We experimentally investigated phlogopite- and C-O-H-bearing lherzolite to model the mantle wedge fluxed by volatiles released from a subducting crustal slab. Experiments have been carried out at 900-1050 degrees C and 1 center dot 6-3 center dot 2 GPa, at fluid- and carbon-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 the nickel-nickel oxide oxygen buffer (NNO). Following thermodynamic calculations, we expect inner fO(2) values to be systematically below NNO, with fluids that are mixtures of CO2 and H2O. Estimated fO(2) in the runs are between delta FMQ approximate to -0 center dot 7 at 3 GPa and delta FMQ approximate to -1 center dot 1 at 1 center dot 8 GPa, values that have been reported for natural mantle-wedge xenoliths. At the conditions investigated, the hydrous phases are phlogopite and pargasitic amphibole. Whereas phlogopite is ubiquitous, amphibole disappears at 3 center dot 1 GPa at 900 degrees C and 2 center dot 7 GPa at 1050 degrees C, where the solidus is encountered. The amphibole-out reaction also consumes orthopyroxene and liberates water. From low to high P, we observed first carbonate-free, amphibole-bearing assemblages, then carbonate + amphibole-bearing assemblages, and finally amphibole-free, carbonate-bearing assemblages. Carbonate-free assemblages melt to produce trachyandesite at T > 1050 degrees C, whereas dolomitic carbonatites have been found beyond the solidus of carbonate-bearing assemblages. Carbonates occur as dolomite at < 1 center dot 9 GPa, 900 degrees C and at < 2 center dot 1 GPa, 1050 degrees C; magnesite at > 2 center dot 4 GPa, 900 degrees C and > 2 center dot 7, 1050 degrees C; between these limits, a magnesite + dolomite-bearing assemblage constitutes a two-carbonate field. P-T pseudosections fail to reproduce the experimental results concerning amphibole breakdown and reaction positions involving carbonates. The amount of COH fluid is thought to have a major role, even in fluid-saturated peridotites. Clinopyroxene and olivine are not expected at fluid-oversaturated conditions, for which dolomite or magnesite are stable respectively. The presented results are useful for unravelling the exhumation history of orogenic lherzolites bearing COH phases and to suggest a way to transfer carbon species to the mantle wedge. We suggest that once carbon-bearing fluids react with mantle-wedge peridotites, a sort of buoyant 'cold plume' will form containing low-density phases such as amphibole, carbonates and carbonatitic melt. This plume could represent an important source of CO2 and H2O, and it is one of a series of processes that ultimately lead to arc magmatism.