Peridotites, chromitites and diamonds in ophiolites

Ophiolites, which represent segments of oceanic lithosphere obducted onto the continental crust, provide an important window into processes such as continental rifting, mantle melting, asthenospheric upwelling and cooling of oceanic lithosphere. Traditionally, research has focused on crustal sections of ophiolites. However, there is a growing recognition that mantle sections contain important information on ophiolite formation and crustal recycling. In this Review, we outline the importance of chromium spinel and associated mineral inclusions for recording the tectonic setting and past histories of ophiolitic mantle sections, with a focus on the insights they provide into deep crustal recycling. In particular, the presence of ultra-high-pressure mineral inclusions, such as microdiamonds, metal alloys, Mn silicates and coesites, in podiform chromitites and other ophiolitic mantle rocks offer evidence of deep and reduced formation conditions. The composition of ultra-high-pressure minerals, and especially the light carbon isotope composition of ophiolite-hosted diamonds, indicates a contribution of recycled crustal material to the mantle portions of ophiolites. The details of crustal material transport through the mantle remain a subject of debate and ongoing research. A global investigation of high-pressure minerals in ophiolitic peridotites and chromitites should be a target of future research to help clarify understanding of deep crustal recycling.