Mantle wedge peridotites: Fossil reservoirs of deep subduction zone processes

The incompatible element-enriched signature of all subduction minerals in Bardane indicate that previously depleted websterites have been refertilized by COH subduction fluids. Comparison with the Ulten Zone garnet + amphibole ± dolomite peridotites outlines striking similarities in the metasomatic style and in the COH fluid phase involved. Comparable geochemical fingerprints observed in HP and UHP Ulten and Bardane mantle rocks call for concomitant subduction of the continental crust, that would provide the incompatible element-enriched fluids. For Bardane this implies subduction of crustal slabs to 200 km depth, to release the fluid phases necessary for UHP metasomatism and for majoritic garnet crystallization. Mantle refertilization by crust-derived COH subduction fluids thus operates over a large depth range, down to 200 km depth. The textural features of the Bardane and Ulten Zone mantle rocks indicate that rock recrystallization and refertilization is concomitant with fluid infiltration along channelways, outside which the rocks preserve pre-subduction, long-lasting stories and pristine geochemical characteristics. Comparably with uprising magmas, the subducted continental crust appears to be a quite efficient carrier to the surface of ultradeep mantle tectonic 'xenoliths' representing major observatories of diverse metamorphic and geodynamic histories recorded by the Earth's mantle at variable depths.