UHP MINERALS AND CRUST-MANTLE RECYCLING: EVIDENCE FROM OPHIOLITES

Podiform chromitites and peridotites of numerous ophiolites, including the Luobusa and Donqiao bodies in Tibet, the Semail ophiolite of Oman and the Ray-iz ophiolite of Russia, contain various combinations of UHP and other deep mantle minerals, such as diamond, coesite, moissanite, base-metal and PGE alloys and native elements. These are associated with a range of crustal minerals, including zircon, quartz, corundum, kyanite, sillimanite, feldspar, almandine garnet and rutile. Diamonds have been recovered from all ophiolites except Semail and they mostly consist of euhedral grains about 200 µm in size with several different morphologies. Coesite from the Luobusa ophiolite is intergrown with kyanite on the rim of an Fe-Ti alloy grain, and moissanite occurs as euhedral, hexagonal crystals or broken fragments of such crystals in all four ophiolites. The moissanite is characterized by having exceedingly low C isotopic values (mean δ¹³C = -28). Most of the crustal minerals occur as subrounded to subangular grains, about 50-300 microns across, but some are blocky to subangular. The zircons typically have very complex internal structures although a few euhedral to subhedral grains have concentric zoning, suggesting an igneous origin. ²⁰⁶Pb/²⁰⁸U SIMS dates for the Luobusa zircons range from 549±19 to 1657±48 Ma, those from Donqiao from 484±49 to 2515± Ma, all much older than the ophiolites. Most zircons from Oman have similar ages to those from Tibet but 4 grains are essentially the same age as the ophiolite. The association of deep mantle and crustal minerals suggests subduction of crustal material to depths of 150-400 km where it was mixed with mantle material and subsequently incorporated into podiform chromitites and mantle peridotites. Based on the occurrence of these minerals in widely separated ophiolites of variable age, we suggest that diamonds and associated minerals are widely preserved in the upper mantle.