LITHOSPHERIC MANTLE COMPOSITION AND STRUCTURE VARIATIONS UNDER THE SIBERIAN PLATFORM KIMBERLITE FIELDS OF DIFFERENT AGES

We discuss herein the differences both in composition and structure of the lithospheric mantle of the Siberian Platform under kimberlite fields of different ages. Having analyzed the pyrope garnets from kimberlites of different ages this study aimed to identify the genesis, extent and probable causes for the evolution of the composition and structure of the lithospheric mantle within various SP blocks. Acquired results revealed noticeable differences in the lithosphere thickness of different blocks for Middle Paleozoic and Mesozoic, as well as within the same time. It was found, that the greatest differences in the thickness of the lithosphere individual blocks are typical for the Middle Paleozoic. The results obtained for pyrope garnets from the Paleozoic sedimentary secondary collectors was used to get direct evidence on the existence of thick lithosphere blocks (230–240 km) under the southern margin of the Olenek uplift, north and south of the Kyutyungde trough, as well as under the western and eastern margins of the Udzhinsky uplift. Significant variations in the intensity and extent of silicate-type metasomatism were identified by investigating of pyropes from peridotites of the lithospheric mantle, sampled by kimberlites of both Middle Paleozoic and Triassic fields. Direct evidence indicates high prospects for discovering new fields of diamondiferous high-grade kimberlites of the Middle Paleozoic age in some areas of the Siberian Platform, primarily in its Arctic region

ВАРИАЦИИ СОСТАВА И СТРОЕНИЯ ЛИТОСФЕРНОЙ МАНТИИ ПОД РАЗНОВОЗРАСТНЫМИ КИМБЕРЛИТОВЫМИ ПОЛЯМИ СИБИРСКОЙ ПЛАТФОРМЫ

We discuss herein the differences both in composition and structure of the lithospheric mantle of the Siberian Platform under kimberlite fields of different ages. Having analyzed the pyrope garnets from kimberlites of different ages this study aimed to identify the genesis, extent and probable causes for the evolution of the composition and structure of the lithospheric mantle within various SP blocks. Acquired results revealed noticeable differences in the lithosphere thickness of different blocks for Middle Paleozoic and Mesozoic, as well as within the same time. It was found, that the greatest differences in the thickness of the lithosphere individual blocks are typical for the Middle Paleozoic. The results obtained for pyrope garnets from the Paleozoic sedimentary secondary collectors was used to get direct evidence on the existence of thick lithosphere blocks (230–240 km) under the southern margin of the Olenek uplift, north and south of the Kyutyungde trough, as well as under the western and eastern margins of the Udzhinsky uplift. Significant variations in the intensity and extent of silicate-type metasomatism were identified by investigating of pyropes from peridotites of the lithospheric mantle, sampled by kimberlites of both Middle Paleozoic and Triassic fields. Direct evidence indicates high prospects for discovering new fields of diamondiferous high-grade kimberlites of the Middle Paleozoic age in some areas of the Siberian Platform, primarily in its Arctic region.Обсуждается проблема различий состава и строения литосферной мантии Сибирской платформы под разновозрастными кимберлитовыми полями. Целью работы являлось выяснение характера, масштабов и вероятных причин эволюции состава и строения литосферной мантии различных блоков платформы на основании изучения пироповых гранатов из разновозрастных кимберлитов различных полей. Анализ полученных результатов показал наличие существенных различий в мощности литосферы различных блоков не только на разное время (среднепалеозойское и мезозойское), но и для одного времени, причем наибольшие различия мощности отдельных блоков литосферы характерны для среднепалеозойского времени. На основании изучения пироповых гранатов из осадочных коллекторов палеозойского возраста получены прямые признаки существования на донижнекарбоновое время блоков мощной литосферы (до 230–240 км) под южным обрамлением Оленекского поднятия (участки к северу и югу от Кютюнгдинского прогиба), а также под западным и восточным обрамлением Уджинского поднятия. Значительные вариации интенсивности и масштабов метасоматоза силикатного типа установлены на основании изучения пиропов из перидотитов разрезов литосферной мантии, «опробованных» кимберлитами полей как среднепалеозойского, так и триасового возраста. Полученные результаты прямо указывают на высокие перспективы выявления новых полей высокоалмазоносных кимберлитов среднепалеозойского возраста в ряде районов Сибирской платформы, в первую очередь ее арктических территорий

Metasomatism in lithospheric mantle roots: Constraints from whole-rock and mineral chemical composition of deformed peridotite xenoliths from kimberlite pipe Udachnaya

International audienceWe report new data on the geochemical evolution, metasomatic and deformation processes in the lower layers of cratonic lithospheric mantle based on a detailed study of extraordinarily fresh and large deformed peridotite xenoliths from the Udachnaya kimberlite pipe, Siberia. Based on our T-P estimates, the deformed peridotites are localized in a depth range of 170 to 220 km near the base of cratonic mantle. The degree of deformation is not correlated with the depth and equilibration temperatures of the xenoliths. The deformed peridotites are depleted in magmaphile major elements indicating their residual nature, but are enriched in incompatible trace elements, Fe and K. The deformed peridotites experienced a complex evolution, i.e., they were formed initially as high-degree melt extraction residues and later were subjected to three main stages of metasomatic modification. (1) An old, mostly cryptic metasomatism by melt/fluid of carbonatitic composition, which formed garnets with sinusoidal REE patterns (Smn/Ern > 1). (2) Silicate metasomatism, which led to the most significant changes in mineralogical and chemical composition of the deformed peridotites. (3) Fe and Ti metasomatism just before the entrainment of the deformed rocks into kimberlite magma. Enrichment of the peridotites in large-ion lithophile elements (K, Rb, Ba) is related to the formation of kelyphitic rims around garnet. The nature of the metasomatic agent of silicate metasomatism was evaluated from mass-balance of measured and calculated whole-rock compositions, ratios of highly incompatible elements (D < 0.1) and fractional crystallization modeling. All this evidence indicates that the agent of silicate metasomatism had a composition intermediate between that of kimberlites and HIMU OIB. The metasomatic processes responsible for the formation of deformed peridotites and precipitation of megacryst suites cannot be widespread at the base of the cratonic mantle. Rather, they are local features only existing in the cratonic mantle below kimberlite fields and localized along the metasomatic vein system

Types of Xenogenic Olivine from Siberian Kimberlites

This work is devoted to the systematization of the composition of xenogenic olivine from kimberlites as the main mineral composing the lithospheric mantle. Based on data on the composition of olivines from xenoliths and megacrysts from kimberlites, a general division into four types is proposed: olivines of ultrahigh-temperature (HTP-1), high-temperature (HTP-2) and low-temperature (LTP) peridotites, as well as olivines of low-chromium megacryst association (MCA). The separation scheme uses the CaO content as an indicator of the formation temperature and the Mg/(Mg + Fe) ratio as an indicator of the degree of enrichment in olivines. In contrast to Al, the Ca content in olivines from cratonic peridotites is high enough to use only EPMA when applying the proposed scheme. According to this scheme the study of more than 1500 individual olivine xenocrysts from a number of kimberlite bodies of the Siberian platform was made. It revealed three characteristic distributions of olivine types: without high-temperature differences (Obnazhennaya pipe), with significant development of HTP-2 (Olivinovaya and Vtorogodnitsa pipes), and with a significant development of HTP-1 (Dianga pipe). Only the latter type of distribution is characterized by the presence of a noticeable amount of megacryst association olivines. The study of other minor elements (TiO2 and NiO) in olivines allowed us to propose a model for the formation of high-temperature olivines of two different types due to the interaction of megacryst melt of various fractionation stages on depleted rocks of the lithospheric mantle. HTP-2 olivines arose upon exposure to a fractionated melt of the late stages of crystallization, and HTP-1 olivines appeared upon exposure to unfractionated (less enriched with incompatible components) megacryst melt at higher temperatures of the initial stage of crystallization