OPHIOLITES: A NEW SCIENTIFIC WINDOW FOR TRACING DEEPLY SUBDUCTED AND RECYCLED OCEANIC-CONTINENTAL CRUST

Ultrahigh-pressure and super-reducing minerals, including diamond, coesite, pseudomorphic stishovite, qingsongite (BN) and Ca-Si perovskite, and moissanite (SiC), Ni-Mn-Co alloys, Fe-Si, and Fe-C phases, have been discovered in peridotites and/or chromitites of ophiolites around the world. These mineral groups collectively suggest extremely high­ pressures (perhaps >300 km) and super-reducing conditions in the mantle. All of the analysed diamonds have unusually light carbon isotope compositions (δ¹³C = -28.7 to -18.3‰) that distinguish them from kimberlitic–UHP metamorphic varieties. The presence of diopside–coesite exsolution lamellae in chromite grains suggests chromite crystallization depths >380 km, near the mantle transition zone.

Carbon isotopes and inclusions in the diamonds indicate formation from previously subducted surface material. Formation in the mantle proceeds in three stages: 1–Carbon-bearing fluids/melts originate in the MTZ, where they may be mixed with fluids/melts from the upper part of the lower mantle. 2-Some of the material transported to the MTZ becomes mixed with highly reduced material derived from zones with extremely low fO₂, as required for the formation of moissanite and native elements. 3. As the melts/fluids of the transition zone rise into the upper mantle, diamond, stishovite, qingsongite and possibly Ca-perovskite are precipitated. Ascent above the MTZ transports peridotites with chromite and UHP minerals to shallow mantle depths, where they undergo decompressional melting and oceanic lithosphere formation. Ophiolite-hosted diamonds and associated minerals are likely to be common in the in-situ oceanic mantle, where they present a scientific window into the life cycle of carbon and deeply subducted oceanic–continental crust.