RECYCLED CRUSTAL MINERALS FROM THE UPPER MANTLE

Numerous grains of crustal silicate minerals, including zircon, corundum, quartz, and feldspar have been recovered from podiform chromitites of the Donqiao and Luobusa ophiolites of Tibet and the Semail ophiolite of Oman. The Tibetan ophiolites also contain ultrahigh pressure minerals such as diamond and coesite. Although several dozen grains of crustal silicates have been recovered from each of the ophiolites examined, they are exceedingly rare in bulk samples and have not yet been observed in situ. However, the fact that the same group of minerals has been discovered in samples from 3 widely separated ophiolites, which were processed in different laboratories, argues against natural or anthropogenic contamination. The zircon grains range from 20 to 300 µm across, and are mostly well rounded with complex internal structures. A few grains are euhedral to subhedral and have concentric zoning suggesting an igneous origin. Zircon grains from the Luobusa ophiolite contain low-pressure inclusions of quartz, rutile, orthoclase, mica, ilmenite and apatite. ²⁰⁶Pb/²³⁸U SIMS dates for the Luobusa zircons reveal a mixed population, with ages from 1657 to 549 Ma, far older than the ophiolite (~125 Ma). Zircons from the Semail ophiolite also yielded a wide range of ages, from 84 to 1396 Ma, but several grains of euhedral to subhedral morphology have essentially the same age as the ophiolite (~90-95 Ma), suggesting that they grew in situ. The older zircons in both ophiolites are interpreted as xenocrysts, probably introduced into the mantle by subduction. Grains of quartz, corundum and feldspar range up to about 0.5 mm in size and are moderately to well rounded. Smaller, angular fragments of such grains are also present. The morphology of these grains suggests derivation from reworked sedimentary material, probably from the same sources as the xenocrystic zircon. The preservation of the crustal silicates in the mantle, particularly quartz, implies an effective isolation from the enclosing rock, presumably by inclusion in stable refractory phases.