Origin of spinel-bearing peridotite from oceanic core complexes

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In many oceanic core complexes plagioclase-free, spinel-bearing mantle peridotite occurs directly on the seafloor. Peridotite samples collected on ODP leg 153 are variably serpentinised (50- 100%) and are strongly depleted in light rare earth and other trace elements, indicating that they experienced some 10-20% melt loss. The genesis of this rock type on the ocean floor has remained highly speculative in spite of its enormous significance for our understanding of oceanic tectonic processes. The presence of spinel requires equilibration pressures of at least 8 kbar at 950 °C. Significantly higher pressures in the excess of 20 kbar are however possible. This implies an exhumation of the spinel peridotite from depths between 25 and possibly more than 70 km, which is in strong contrast to the 4 to 7 km of exhumation that have been suggested previously. Geothermobaromeiric techniques (conventional thermobarornetry and phase diagram modelling) will be applied to these rocks to infer their equilibration depth and their pressure-temperature evolution. Ar-Ar age dating will be used in an attempt to put constraints on the exhumation rate and cooling history of these rocks. The timing of the partial melting event that is responsible for the residual geochemical character of the peridotite is debated. It could have occurred during recent uplift or in Proterozoic times. In order to resolve this controversy and to determine the timing of partial melting it is planned to carry out a Sm-Nd, Rb-Sr and Pb isotope study. The overall goal is to distinguish between different lithospheric components and to constrain the age of the source for the protoliths. With such new data and, together with the inferred PT evolution, we aim to assign the source and tectonic evolution of these rocks to Proterozoic and/or recent magmatic and tectonometamorphic events. Ultimately, the expected results will improve our understanding of abyssal peridotite whose origin is inconsistent with current partial melting in the global ridge system, of modern Earth.