OPHIOLITIC CHROMITITES INDICATE TRANSITION ZONE MANTLE DEPTHS FOR THEIR INITIAL ORIGIN AND A DEEP MANTLE UPWELLING RECORD OF OPHIOLITIC PERIDOTITES

Peridotites in ophiolites have been traditionally interpreted as residues of partial melting products at shallow mantle depths beneath oceanic spreading centers. The occurrence of in-situ diamonds, highly reduced minerals and other unusual UHP minerals in ophiolitic chromitites indicate, however, that peridotites carrying these chromitites and their inclusions may have once resided at Transition Zone depths in the mantle prior to their incorporation into the upper mantle. We show that high-Cr and high-Al chromitites in many Mesozoic Tethyan ophiolites as well as in some Paleozoic ophiolites in various Asian orogenic belts comprise diamonds that may also contain inclusions of coesite, Ni-Mn-Co alloys, and spessarite. Extremely light carbon isotope compositions (Delta-13 Carbon= -28.7 to - 18.3‰) of these diamonds suggest recycled surface carbon for their origin. The occurrence of native elements, Ni-Mn-Co alloys, Fe-Si and Fe-C phases and moissanite in the chromitites also point to the super-reducing conditions of their environment of formation in the mantle. The existence of exsolution lamellae of diopside and coesite in some chromite grains suggests chromite crystallization depths around >380 km, near the mantle transition zone. We think that these UHP minerals, native elements and super-reduced minerals were encapsulated as inclusions in solid chromitite near the top of the transition zone, and that the rising asthenosphere then brought these diamond-carrying chromitites and peridotites to shallow mantle depths. Partial melting processes at these depths redistributed and disseminated them in the oceanic mantle beneath the spreading centers, and subduction-derived fluids in mantle wedges interacted with them to form podiform chromitites. Chromitites in ophiolites thus appear to have undergone multiple melting-crystallization episodes at different mantle depths. High-Cr podiform chromitites only reflect the latest episode of melting–crystallization in the presence of boninitic melts within a mantle wedge of a subduction zone. We need to understand whether: (1) diamonds and UHP minerals occur in ophiolites of all different tectonic settings, and (2) diamond-bearing peridotites are limited only to SSZ ophiolites or common in in-situ, modern oceanic lithosphere [IGCP-649 contribution].