Evidence of fluid inclusions in metamorphic microdiamonds from the Kokchetav massif, northern Kazakhstan

Microdiamonds from garnet clinopyroxenites of the Kokchetav massif (norther n Kazakhstan) and associated alluvial diamonds have been investigated using infrared spectroscopy. All diamonds have high nitrogen contents (from 747 up to 4488 +/- 20% at, ppm) and are of type Ib-IaA. Nitrogen data are consi stent with a diamond formation over a narrow temperature range, Microdiamon ds from garnet clinopyroxenites are characterized by water and carbonate in clusions, similar to those ocurring in fibrous coated diamonds from kimberl ite, which suggest that diamonds grew from a C-H-O fluid. In contrast, allu vial microdiamonds do not contain any carbonate inclusions and H2O is absen t or present in only minor amounts. Alluvial diamonds are considerably more N-rich but have a similar range of nitrogen aggregation states. It is sugg ested that alluvial diamonds belong to a separate diamond population compar ed with the microdiamonds from garnet clinopyroxenites. The Kokchetav microdiamonds an distinct from kimberlitic fibrous diamonds i n that they contain abundant Ib centres, have high nitrogen contents and sh ow different populations depending on host rock type. These characteristics support a metamorphic origin for the microdiamonds from the UHPM rocks of the Kokchetav massif. Copyright (C) 1998 Elsevier Science Ltd.