Diamond in the Oceanic Lithosphere. Volcanic Diamonds and Diamonds in Ophiolites

Diamonds were lately identified in chromitites from ophiolites and in volcanic rocks. Although the tectonic settings of diamonds found in these rocks are different, the diamonds are identical in small size, cuboctahedral habit, sets of minor admixture elements, and isotopic characteristics. A model is suggested for their formation during various stages of a single evolutionary cycle of the oceanic lithosphere, in relation to the geochemical and dynamic features of an ascending flow of mantle material, which produces the oceanic lithosphere at mid-oceanic ridges. In contrast to the continental lithosphere, in which mantle diamonds are usually related to kimberlite and lamproite magmatism in the presence of abundant CO2-rich fluid, diamonds in the oceanic lithosphere crystallize in environments poor in fluid and containing carbon mostly in its reduced forms. In the asthenospheric part of the ascending flow, carbon may occur in the form of nanometer-sized diamonds. In the upper parts of the oceanic lithosphere, the diamonds are overgrown and become microdiamonds (0.2–0.7 mm) within chromitites. After basaltic magma is derived from pyrolite, the residual harzburgites with lenses of diamondiferous chromitites are brought (at spreading) to the convergent boundaries of oceanic lithospheric plates, where the following two processes can proceed. If the oceanic lithosphere collides with a continental plate, the obducted material of the oceanic lithosphere is transferred to the surface of the continental margin and forms ophiolite massifs hosting diamond-bearing chromitites. If the oceanic lithosphere is subducted, the residual peridotite already enriched in volatiles is remelted. The arc magmas thus derived host diamond microcrystals, which have been formed in the chromitites and are sometimes found in volcanic lavas and ashes.