Olivine from the Pionerskaya and V. Grib kimberlite pipes, Arkhangelsk diamond province, Russia: Types, composition, and origin

We report the first systematic study of different textural varieties of olivine (olivine from peridotite xenoliths, macrocryst-type Ol -I, and phenocryst-type zoned Ol -II) from two diamondiferous kimberlite pipes of the Arkhangelsk diamond province (V. Grib and Pionerskaya) differing in geologic setting, geochemical and isotopic characteristics, and diamond content. Approximately 550 olivine analyses were obtained by the EPMA technique using the precise method of Sobolev et al. (2007) adapted at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences (Kargin et al., 2014). Olivines from the V. Grib moderate-Ti kimberlites, which are similar to Group I kimberlites in geochemical and Sr-Nd isotopic characteristics and rich in diamond, are dominated by high-Mg# low-Ti Ol -I formed owing to the fractional crystallization of a carbonate-rich protokimberlite melt interacting with orthopyroxene-bearing peridotite material; the fraction of high-Ti Ol -I produced by the metasomatic alteration of peridotite under the influence of silicate aqueous fluids is significantly lower; and xenocrysts weakly affected by metasomatic agents (melts and fluids) occur in minor amounts. Olivines from the low-Ti Pionerskaya kimberlites, which are similar to Group II kimberlites in geochemical and Sr-Nd isotopic characteristics and show a moderate diamond content, are dominated by high-Ti Ol -I, and xenocrysts weakly affected by metasomatic agents are also abundant. In the kimberlites of both pipes, the cores of Ol -II crystals are usually composed of low-Ti olivine similar in composition to Ol -I; both high-Ti and low-Ti olivine cores occur in the Pionerskaya pipe; whereas cores corresponding to high-Ti Ol -I were never found in the V. Grib pipe. The outer zones of olivine and small olivine grains in the groundmass show considerable variations in minor element contents within a narrow Mg# range. It is suggested that the high-Ti rims of Ol -II from the V. Grib and Pionerskaya kimberlites were produced by the late crystallization of kimberlite melt, and the low-Ti rims on the outer zones of Ol -II in the Pionerskaya kimberlites were formed by late-stage equilibration with an aqueous fluid separated from the kimberlite melt and/or possible kinetic effects. Our study revealed the diversity of olivine origin in the kimberlites and showed that there is no single mechanism of olivine formation.