Origin of garnet megacrysts from kimberlites

Abstract

The asthenospheric source is commonly accepted for the genetic interpretation of minerals in megacryst assemblages from kimberlites and deformed lherzolites. Both assemblages are regarded as highest temperature and high-pressure formations in the mantle [1‐5]. According to [1, 2, 6], minerals of the megacryst assemblage are products of fractional crystallization of the asthenospheric melt. At the same time, the problem of close association of the formation of megacrysts and deformed lherzolites is discussed in [6], in which the lithospheric origin of deformed peridotites is suggested and evidence of refertilization of the lower lithosphere by asthenospheric melts is presented. All these problems remain to be solved and require further investigations. In this communication, we discuss data on the major oxide and REE contents in garnet megacrysts from kimberlites of the Mir pipe (Malobotuobinsk field); the Udachnaya, Dal’nyaya, and Zarnitsa pipes (Daldyn field); the Zapolyarnaya, Novinka, Komsomol’skaya‐ Magnitnaya pipes (Verkhnemunsk field) in the Yakutian province; and the Grib pipe in the Arkhangel’sk province; and garnets from xenoliths of deformed peridotites in the Udachnaya-Vostochnaya pipe. Our aim was to characterize the REE patterns in garnets from the deformed lherzolites and megacrysts, to establish the geochemical evolution of melts in equilibrium with these minerals, and to discuss their possible origin. The xenoliths and megacrysts were examined in detail under an optic microscope. The cores and marginal zones of garnet grains were analyzed for major oxides on a JXA-33 (Jeol) microprobe at the Institute of Geochemistry, Irkutsk. The trace element contents were determined with the SIMS method on a Cameca IMS ion probe at the Institute of Microelectronics and Informatics, Russian Academy of Sciences (Yaroslavl), using the technique described in [7]. The latter method provided measurements accurate to 10‐15% for concentrations of >0.1 ppm and 40‐50% at concentrations of <0.1 ppm.