NITROGEN IN CHROMITES AND OLIVINES COEXISTING WITH DIAMOND

Abstract

The practically constant presence of nitrogen in natural diamond, at concentrations of up to 0.55 percent by weight, indicates that it is involved in generation of this mineral in the upper mantle. The authors made systematic attempts to determine the possible concentration of nitrogen in chromespinelids and olivines coexisting with diamond in inclusions and xenoliths of natural peridotite. Nitrogen was detected with a JXA-5A microprobe at an accelerating voltage of 8 keV. The differences in the nitrogen content of the chromite and olivine, and particularly in chromite extracted from a single diamond crystal, were comparable with the variations in the distribution of nitrogen in individual diamond crystals. The nitrogen content of most diamond ranges from negligible to 0.25 percent by weight, and it probably correlates with the N2 content of the chromite and olivine inclusions present in this diamond. The detection of impurity nitrogen in chromite and olivine, which typically coexist with diamond, indicate that nitrogen plays a definite role in the formation of rocks of the lower lithosphere.