Diamond in oceanic peridotites-chromitites and recycled in deep mantle

Microdiamonds have been recovered from mantle rocks and associated podiform chromitites in many ophiolites across the world and, in particular, in-situ diamonds were found in ophiolitic chromitites in Southern Tibet and Northern Ural. Microdiamonds in ophiolites present a new type occurrence of diamonds on Earth, different from those diamonds occurring in kimberlites and ultrahigh pressure metamorphic belts. The discoveries of pressure-sensitive minerals such as coesites with stishovite pseudomorph, high-pressure facies chromitites and Qingsongites (BN) indicate that ophiolitic chromitite may form at depths of >150~300 km or even deeper in the mantle. The very light C isotope composition (δ¹³C -18‰ to -28‰) of these ophiolitic diamonds, Mn-bearing mineral inclusions observed in these diamonds and coesite occurring in chromite all indicate the recycling of ancient continental or oceanic materials into the deep mantle (>300 km) or down to the mantle transition zone via subduction. These new observations and data strongly suggest that microdiamonds and their host podiform chromitite may have formed near the transition zone in the deep mantle, and that they were then transported upward into shallow mantle depths by convection processes. Thus, a new model has been proposed for deep subduction and recycling of oceanic crust in deep mantle. The discovery of diamonds and other UHP minerals from peridotites and chromitities in ophiolites doubts the current shallow genesis of ophiolitic chromitites and raises a serious of new scientific questions which leads to a new research direction.