L. V. Solov’eva

Kimberlites and megacrystic suite: Isotope-geochemical studies

Major and rare-earth element data on Cr-poor megacrystic suite from Yakutian kimberlites were generalized. Sr-Nd isotopes were studied in garnet, clinopyroxene, and phlogopite megacrysts as well as in garnet and clinopyroxene from deformed xenoliths. It was shown that Sr-Nd composition of these minerals is similar to that in the least altered diamondiferous kimberlites. The crystallization age of megacrystic minerals was determined by Rb-Sr isochron and Ar-Ar (for phlogopite megacrysts) methods. Obtained data indicate that crystallization of Cr-poor megacrystic suite began at the prekimberlitic stage and continued to the pipe emplacement. It was established that garnets from coarse-porphyric deformed lherzolites and megacrysts are similar in major and rare-earth element compositions and were derived from a common asthenospheric source. However, the distribution of incompatible elements and P-T estimates of crystallization cannot be explained by hypothesis of fractional crystallization of garnet megacrysts. It is suggested that megacrystic assemblage crystallized directly in asthenospheric melt. En route to the surface, this melt caused a metasomatic reworking of lithospheric mantle, on the one hand, and was enriched in Mg and Cr owing to the contamination by lithospheric material, on the other hand.

Origin of Garnet Megacrysts from Kimberlites

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.

Petrological Features of Olivine-Phlogopite Lamproites of the Sayan Region: Evidence from Sr-Nd Isotope and ICP-MS Trace-Element Data

ISSN 0016-7029, Geochemistry International, 2006, Vol. 44, No. 7, pp. 729–735.

Zoning of garnets in deformed peridotites from the Udachnaya kimberlite pipe

997 The evolution of Middle Paleozoic kimberlite mag matism, the most productive for diamonds on the Siberian Craton, is related to the rising of the Yakutian thermochemical plume, the existence of which is con firmed by a number of geological, petrological, and geochemical evidences [1–3]. The sources of basic melts are formed beneath the lithosphere of the Sibe rian Craton at that time. The filtration of these melts though the material of the uppermost asthenospheric layer and lowermost lithospheric layer results in the formation of low chromium megacrysts and deformed peridotites [3]. The data on rare element geochemistry and the Sr–Nd isotope systematics of these formations provides evidence for deep matter of the plume and the matter of the ancient lithosphere as a source for their material [2, 3].

Phlogopite and phlogopite–amphibole parageneses in the lithospheric mantle of the Birekte terrane (Siberian craton)

Mantle xenoliths containing phlogopite and phlogopite–amphibole mineralization from kimberlites of the Kuoika field have been studied. Such xenoliths were found in two series of rocks: magnesian (Mg) pyroxenite–peridotite and Fe-type phlogopite–ilmenite hyperbasite. The 40Ar/39Ar phlogopite age (1600–1800 Ma) and Re–Os and oxygen isotope data in rocks and minerals of the first series of rocks allow us to suggest that Phl–Amph metasomatism of the lithospheric mantle under the Birekte block and its accretion to the Siberian craton occurred in the subduction zone. The second series of rocks is comagmatic to potassium ultramafites and mafites, finding in the Siberian Platform. The phlogopite ages (870–850 Ma) from Phl–Ilm ultramafites corresponds to the beginning of the breakup of the supercontinent Rodinia and is close to ancient age datings of the alkaline ultramafic-carbonatite Tomtor massif. Phlogopite from xenoliths with garnet is much younger in age (500–600 Ma).

Rare-element composition of pyropes and lamproites and ancient dispersion haloes of the southwestern Siberian platform

The problem of heterogeneity of the mantle lithosphere of the southwestern portion of the Siberian Platform has been considered, and the diamond content in potential mother lodes within this area has been estimated based on original geochemical data on the rare-element composition of pyropes from diamondiferous lamproites of the Ingashin field within the Prisayan region and ancient dispersion haloes of minerals accompanying diamonds in the area between the Angara and Uda rivers. Pyropes from lamproites are characterized by low concentrations of Zr (0.18–9.05 ppm), Hf (0.03–0.37 ppm), and rare earth elements (Sm 0.04–0.49, Eu 0.02–0.16, and Dy 0.05–0.96 ppm). Pyropes from the Lower Carboniferous Baeron Formation within the Tangui-Chuksha area are significantly different from pyropes of the Ingashin lamproites in high contents of Zr (30.36–139.23 ppm) and Hf (0.4–2.22 ppm). These pyropes are characterized by elevated concentrations of rare earth elements (Sm 1.34–3.68, Eu 0.53–1.17, and Dy 1.0–2.05 ppm). The distribution patterns of rare incompatible elements in pyropes of the Lower Carboniferous Mura massif within the Mura area manifest even stronger differences with pyropes of the Ingashin lamproites and in many respects with pyropes from Lower Carboniferous sediments of the Baeron Formation within the Tangui-Chuksha area. The results obtained indicate that there is no large-scale regional spreading of pyropes from Mid-Riphean lamproite bodies in the course of washout of these bodies and that the mantle lithosphere in the southwestern portion of the Siberian Platform is laterally heterogeneous in mineralogical-geochemical terms. The chemical composition and the peculiar distribution pattern of rare elements in pyropes from lamproites of the Prisayan region indicate a depleted, primarily lherzolite composition of the upper mantle that was transformed through low-temperature potassium metasomatosis. In terms of the chemical and rare-element compositions, pyropes from Lower Carboniferous sediments of the Tangui-Chuksha and Mura areas belong to a wider range of mantle rocks: depleted peridotites, metasomatic peridotites under low (900–1000°C) and high (>1000°C) temperature conditions, and megacrysts. This suggests that the composition of the lithospheric mantle in this area of the southern portion of the Siberian Platform is characterized by a considerably differentiated stratification of mantle rocks, some of which were credibly formed in the diamond stability field.