CONTAMINATION OF DIAMONDS IN UHP METAMORPHIC ROCKS - AN EXAMPLE FROM THE DABIE TERRANE, CHINA -

The problem of contamination by industrial diamonds (cutting saw, polishing paste) during sample preparation has been serious as exemplified by a recent paper (Dobrzhinetskaya et al., 2014). Reported diamond inclusions in detrital zircon grains (~4.2 to ~3.0 Ga) from Archean Jack Hills conglomerate of Western Australia proved to be contaminated by diamond polishing paste. They suggested that advanced analytical tools including FIB–TEM techniques are crucial to prove the contact relationship between diamond inclusions and host minerals. During our study of lamellae and inclusions in Cpx of one Grt clinopyroxenite polished section from Raobazhai, North Dabie, we found several microdiamond grains in three holes of one Cpx crystal. These diamonds possess strong Raman diamond bands at 1332 cm^-1. The positions and contact relations of the diamond grains with the host Cpx were examined using reflected light microscope and the multi-layered 2D mappings of the Raman band. These microdiamond grains apparently occurred in the “holes” of Cpx and do not have direct contact with the host Cpx. Several additional polished thin sections of the same rock were subsequently prepared in Japan; no microdiamond inclusion was found. This study concluded that these microdiamond grains in original section are contaminated material during polishing.

Search for diamond occurrences in Dabie-Sulu UHP rocks has been a national priority in the 1990s since the first report of diamonds by Xu et al. (1992) and Okay (1993); both studies have been subsequently proved to be mis-identification. Numerous efforts of Raman identification of fine-grained mineral inclusions in zircon, garnet, omphacite and other UHP phases from Dabie-Sulu UHP rocks fail to find diamond. Instead spatial distribution of quartz and coesite are conclusive in thousands of zircon separates from all UHP rocks including eclogite, gneiss, schist, quartzite and marble (e.g., Liu and Liou, 2011). P–T estimates from conventional and pseudosection calculations lie within the P–T forbidden zone and yield the peak UHP metamorphism within the diamond stability field. The rarity of diamond is due to the oxidized protoliths evidently produced through intense meteoric water–rock interactions during snowball Earth conditions.