MINERAL INCLUSIONS IN DIAMONDS FROM OPHIOLITIC PERIDOTITE AND CHROMITITES

Diamonds and a group of other unusual minerals have been identified in ophiolitic peridotites and chromitites along the Yarlung Zangbo suture zone, south Tibet. Preliminary results show that the minerals in the harzburgites are similar to those in the chromitites, suggesting a genetic relationship between them. Mineral inclusions in diamonds were investigated using Focus Ion Bean (FIB) and TEM at the GFZ in Potsdam, Germany. In general, the diamonds show extremely low dislocation density, and occasionally stacking faults are observed. Ni-Mn-Co-alloys are very common in the diamonds, and they are micrometre-sized crystals with very high dislocation density. These alloys are quite uniform in composition with Ni = 69.1 - 70.3 at.%, Mn = 24.2 – 26.0 at.%, and Co = 5.0 - 5.6 at.%. Graphite single crystals and patches of amorphous carbon occur between the Ni-Mn-Co-alloys and host diamond. The graphite single crystals are thought to have formed from a C-rich fluid during diamond crystallization and entrapment of the Ni-Mn-Co-alloy. A mineral assemblage of galaxies (Al-Mn-spinel), tephroite (a type of Mn-Ol, Mn2SiO4 (atom%, Si 31.76, Ca 5.53, Mn 62.69); spessartine (a type of Mn-Grt, atom% Al 12.13~ 25.16, Si 25.06~37.12, Mn 27.96~54.18; Ca 1.35~10.28; minor Ni and Fe ); Mn-oxide (MnO) and native Mn, suggests formation from previously subducted crust materials. The oxide mineral was confirmed by EELS, and it is most likely a spinel phase. Diffraction data fits that of magnetite or similar spinel phases very well. NaCl and quartz are confirmed by electron diffraction data. Inclusions in the Tibetan diamonds are distinctly different from those in kimberlite diamonds and ultrahigh pressure (UHP) metamorphic diamonds. We suggest that they represent a new type of a diamond that originated from a previously subducted crust.