Diamonds from the V. Grib pipe, Arkhangelsk kimberlite province, Russia

Abstract

A large collection (717 samples) of diamonds from the V. Grib deposit, discovered in 1996 in the Verhotinskoe field of the Arkhangelsk kimberlite province, was studied. The diamond crystals are characterized by high transparency and preservation. The collection consists of complete crystals (71%), chipped and damaged crystals (preservation > 50%; 14%), and fragments (preservation < 50%; 15%). Resorption is generally moderate resulting in a dominance of octahedral and mixed octahedral–dodecahedral shapes. Moderate resorption points to rapid ascent of the transporting kimberlite magma. A characteristic feature of V. Grib is a large number of green diamonds. This may relate to the close proximity of radioactive deposits. Microscopic green surface spots have no cathodoluminescence. The internal diamond morphology was studied by UV- and cathodoluminescence. The main typomorphic feature of diamond crystals from the V. Grib pipe is a high percentage of crystals without UV-luminescence. Presence of sectorial growth was also identified by luminescence. Analysis of mineral inclusions, carbon isotopic composition, nitrogen content and nitrogen aggregation state provided important genetic information. Peridotitic inclusions (olivine, chromian spinel and pyrope) predominate, sulfides are almost completely absent. The carbon isotopic composition of the host diamonds is typical for peridotitic diamonds worldwide. IR-spectroscopy suggests the presence of two diamond populations with low and high nitrogen concentrations. Three sub-populations may be identified based on a combination of morphology, nitrogen and hydrogen defects. Residence temperatures ( T Nitrogen), based on a mantle residence time of 3 Ga, fall between 1050 and 1170 °C. Diamond crystallization in V. Grib occurred in multiple stages. This is documented through luminescence patterns, data on nitrogen concentration and aggregation state, and the presence of “diamond-in-diamond” inclusions.