A. M. Agashev

Geochemical evolution of rocks at the base of the lithospheric mantle: Evidence from study of xenoliths of deformed peridotites from kimberlite of the Udachnaya pipe

This paper presents the results of study of the geochemical and mineralogical compositions of deformed peridotite xenoliths from the Eastern Udachnaya kimberlite pipe. The performed investiga� tions were mainly aimed at obtaining information on the composition and evolution of the lower layers of the lithospheric mantle in the central part of the Sibe� rian Craton. Xenoliths of deformed peridotites are present in kimberlite pipes of all old cratons and are the deepest of all mantle nodules as is evident from the PTcondi� tions of equilibrium. These rocks provide the main information on the composition and evolution of the lower layers of the lithospheric mantle; because of this, their study is of significant interest (1-7). Because of the small sizes and serpentinization (partial or com� plete) of the deformed peridotites, most of the previ� ously performed investigations concerned only the composition of individual rockforming minerals of xenoliths. There is a lack of data on the bulk composi� tion of these rocks at present, and they were obtained for partially serpentinized xenoliths (7-9). Representative collection of xenoliths of deformed peridotites from the eastern body of the Late Devo� nian Udachnaya pipe related to the Daldyn kimberlite field of the Yakutian kimberlite province, which is the largest diamond deposit in Russia, was selected for geochemical study. This body consists of two com� bined kimberlite bodies: Western Udachnaya and Eastern Udachnaya; the eastern body is unique in rela� tion to the amount deepseated rock xenoliths, their diversity, and preservation. Xenoliths are presented by a large set of facial groups and varieties, among which deformed peridotites occupy a significant place (2, 3). Samples of deepseated rocks selected for the study are uniquely fresh. These xenoliths contain no signs of secondary alterations, and the L.O.I. value in the stud� ied rocks is close to zero. The weight of the samples varies from 500 g to several kilograms. Only the central parts of xenoliths, without surfaces in the contact with kimberlite, were taken for analysis of the bulk compo� sition. Most of the xenoliths from this collection are typi� cal lherzolites composed of olivine, orthopyroxene, clinopyroxene, and garnet in various proportions. According to the content of clinopyroxene (2-4 wt %), five samples may be formally attributed to garnet harzburgites. In all samples, large (up to 1 cm) grains of rockforming minerals and the matrix composed of finegranular olivine form porphyroclastic or mosaic� porphyroclastic textures. Olivine in the studied xeno� liths is the prevailing mineral; its content varies from 60 to 85%. The concentration of the forsterite mole� cule in olivines from nodules ranges from 86.4 to 91.3 mol %. Orthopyroxene, as well as olivine, is pre� sented by relatively large grains (porphyroclasts) with a size from 2 to 10 mm and small grains (neoblasts) with a size of <0.5 mm. The amount of orthopyroxene in the studied samples varies from 5 to 18%. The study of the orthopyroxene composition demonstrates that it relates to highmagnesium varieties (En = 88.2- 92.9 mol %), however being more ironrich in compar� ison with orthopyroxenes from granular garnet peridot� ites. Orthopyroxenes contain admixtures of Al 2O3 (0.42- 0.68 wt %), Cr2O3 (0.11-0.52 wt %), and TiO2 (0.21- 0.6 wt %). The clinopyroxene content varies from 2 to 14% of rock volume. This mineral is characterized by a low concentration of the diopside component (Ca/(Ca + Mg) = 35.9-43.3 mol %) that results from high temperatures of the equilibrium of characterized associations. The concentration of garnet in rocks var� ies from 4 to 15%. Garnet grains are practically not deformed, have an isometric shape, and often form linearly elongated chains. The chemical composition of garnets corresponds to the lherzolite paragenesis

Thermal perturbations in the lithospheric mantle as evidenced from P-T equilibrium conditions of xenoliths from the Udachnaya kimberlite pipe

84 The Udachnaya kimberlite pipe (Yakutia) is a unique geological object. The rocks of this pipe con tain a large number of unaltered xenoliths of the cra tonic lithospheric mantle (hereinafter called CLM) from a depth of about 230 km to the upper crustal lev els [1–3, 6–8]. This work is based on an integrated multiyear study of 250 peridotite xenoliths that were transported upward by kimberlites of the Udachnaya Vostochnaya pipe. The mineral paragenesises and tex tural–structural peculiarities of samples of mantle rocks were studied under a microscope. The chemical compositions of minerals were determined using elec tron microprobe analysis. On the basis of modern methods of thermobarometry [9, 13], the P–T equi librium parameters for rocks of the sequence of the cratonic lithospheric mantle were revealed and the P–T curve (geotherm) was plotted. According to this, assumptions about the origin of rocks occupying dif ferent fields on this geothermal curve were made.

Equigranular eclogites from the V. Grib kimberlite pipe: Evidence for Paleoproterozoic subduction on the territory of the Arkhangelsk diamondiferous province

nological Institute, Russian Academy of Sciences. The U–Pb isotope age was determined for 19 zircon grains. The chemical composition of eclogites was reconstructed on the basis of mineral densities, their chemical composition, and the modal concentration in the rock. The modal proportion of 40% garnet + 60% clinopyroxene was used for reconstruction of the REE concentrations. The compositions of rutile (0.3– 0.4%) and ilmenite (1%) were taken into consider�

Sr and Nd isotope composition of deformed peridotite xenoliths from Udachnaya kimberlite pipe

New results of Rb–Sr and Sm–Nd isotope analyses have been obtained on samples of deformed peridotite xenoliths collected from the Udachnaya kimberlite pipe (Yakutia). The data obtained imply two main stages of metasomatic alteration of the lithospheric mantle base matter in the central part of the Siberian Craton. Elevated ratios of Sr isotopes may be considered as evidence of an ancient stage of metasomatic enrichment by a carbonatite melt. The acquired Nd isotope composition together with the geochemistry of the deformed peridotite xenoliths suggests that the second stage of metasomatic alteration took place shortly before formation of the kimberlite melt. The metasomatic agent of this stage had a silicate character and arrived from an asthenosphere source, common for the normal OIB type (PREMA) and the Group-I kimberlite.

PGE distribution in deformed lherzolites of the Udachnaya kimberlite pipe (Yakutia)

The results of the first study of the PGE distribution in deformed lherzolites of the Udachnaya kimberlite pipe (Yakutia) are presented here. The complex character of evolution of the PGE composition in the Deformed lherzolites is assumed to be the result of silicate metasomatism. At the first stage, growth in the amount of clinopyroxene and garnet in the rock is accompanied by a decrease in the concentration of the compatible PGE (Os, Ir). During the final stage, the rock is enriched with incompatible PGE (Pt, Pd) and Re possible due to precipitation of submicron-sized particles of sulfides in the interstitial space of these mantle rocks.