OPHIOLITE-TYPE DIAMOND

Ophiolite-type diamonds are the diamonds that occur within ophiolitic peridotites or chromitites, which are different from the kimberlitic diamonds occurred in continental cratons, which are also different from the ultrahigh pressure metamorphic diamonds formed in the subduction zone, and the diamonds formed in impact craters.

The ophiolite-type diamonds are small, mostly about 200-500 um in size. They are euhedral and cubic-octahedral ctrystals with sharp edges but some with rounding on edges as the result of resorption. Some diamonds are covered by metal film or metal patch, similar to coated diamond. CL images show simple patterns of symmetry, no zoning structure as commonly seen in kimberlite diamond. FTIR analyses show ophiolitic diamonds have Ib type, IaA type and mixing Ib-IaA type. The C isotope are very consistent, and the δ¹³C_VPDB vary in the range of -18.3 ‰ and -28.7‰,averaging -24.6 ‰. The mineral inclusions in ophiolite diamond are also very typical, containing Mn-bearing minerals, such as Mn-olivine, Mn-garnet, Mn-spinel, MnO and etc, as well as coesite and graphite, REE and fluid inclusions, much different from those in kimberlite, which are Mg-olivine, Mg-garnet, chromite and etc. Trace elements of the ophiolite diamond both from Tibetand Ural show a wide range of variation, and all higher than those in kimberlites.

As a conclusion, the ophiolite diamond form in an extremely high reduced environment.These occurrences indicate a completely new environment for diamond formation, which can be regarded as ophiolite-type diamond.