Metasomatic origin of garnet xenocrysts from the V. Grib kimberlite pipe, Arkhangelsk region, NW Russia

AbstractThis paper presents new major and trace element data from 150 garnet xenocrysts from the V. Grib kimberlite pipe located in the central part of the Arkhangelsk diamondiferous province (ADP). Based on the concentrations of Cr2O3, CaO, TiO2 and rare earth elements (REE) the garnets were divided into seven groups: (1) lherzolitic “depleted” garnets (“Lz 1”), (2) lherzolitic garnets with normal REE patterns (“Lz 2”), (3) lherzolitic garnets with weakly sinusoidal REE patterns (“Lz 3”), (4) lherzolitic garnets with strongly sinusoidal REE patterns (“Lz 4”), (5) harzburgitic garnets with sinusoidal REE patterns (“Hz”), (6) wehrlitic garnets with weakly sinusoidal REE patterns (“W”), (7) garnets of megacryst paragenesis with normal REE patterns (“Meg”). Detailed mineralogical and geochemical garnet studies and modeling results suggest several stages of mantle metasomatism influenced by carbonatite and silicate melts. Carbonatitic metasomatism at the first stage resulted in refertilization of the lithospheric mantle, which is evidenced by a nearly vertical CaO-Cr2O3 trend from harzburgitic (“Hz”) to lherzolitic (“Lz 4”) garnet composition. Harzburgitic garnets (“Hz”) have probably been formed by interactions between carbonatite melts and exsolved garnets in high-degree melt extraction residues. At the second stage of metasomatism, garnets with weakly sinusoidal REE patterns (“Lz 3”, “W”) were affected by a silicate melt possessing a REE composition similar to that of ADP alkaline mica-poor picrites. At the last stage, the garnets interacted with basaltic melts, which resulted in the decrease CaO-Cr2O3 trend of “Lz 2” garnet composition. Cr-poor garnets of megacryst paragenesis (“Meg”) could crystallize directly from the silicate melt which has a REE composition close to that of ADP alkaline mica-poor picrites. P-T estimates of the garnet xenocrysts indicate that the interval of ∼60–110 km of the lithospheric mantle beneath the V. Grib pipe was predominantly affected by the silicate melts, whereas the lithospheric mantle deeper than 150 km was influenced by the carbonatite melts

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

Interaction between protokimberlite melts and mantle lithosphere: evidence from mantle xenoliths from the Dalnyaya kimberlite pipe, Yakutia (Russia)

The Dalnyaya kimberlite pipe (Yakutia, Russia) contains mantle peridotite xenoliths (mostly lherzolites and harzburgites) that show both sheared porphyroclastic (deformed) and coarse granular textures, together with ilmenite and clinopyroxene megacrysts. Deformed peridotites contain high-temperature Fe-rich clinopyroxenes, sometimes associated with picroilmenites, which are products of interaction of the lithospheric mantle with protokimberlite related melts. The orthopyroxene-derived geotherm for the lithospheric mantle beneath Dalnyaya is stepped similar to that beneath the Udachnaya pipe. Coarse granular xenoliths fall on a geotherm of 35 mWm-2 whereas deformed varieties yield a 45 mWm-2 geotherm in the 2–7.5 GPa pressure interval. The chemistry of the constituent minerals including garnet, olivine and clinopyroxene shows trends of increasing Fe# (= Fe/(Fe+Mg) with decreasing pressure. This may suggest that the interaction with fractionating protokimberlite melts occurred at different levels. Two major mantle lithologies are distinguished by the trace element patterns of their constituent minerals, determined by LA-ICP-MS. Orthopyroxenes, some clinopyroxenes and rare garnets are depleted in Ba, Sr, HFSE and MREE and represent relic lithospheric mantle. Re-fertilized garnet and clinopyroxene are more enriched. The distribution of trace elements between garnet and clinopyroxene shows that the garnets dissolved primary orthopyroxene and clinopyroxene. Later high temperature clinopyroxenes related to the protokimberlite melts partially dissolved these garnets. Olivines show decreases in Ni and increases in Al, Ca and Ti from Mg-rich varieties to the more Fe-rich, deformed and refertilized ones. Minerals showing higher Fe# (0.11–0.15) are found within intergrowths of low-Cr ilmenite-clinopyroxene-garnet related to the crystallization of protokimberlite melts in feeder channels. In P-f(O2) diagrams, garnets and Cr-rich clinopyroxenes indicate reduced conditions at the base of the lithosphere at -5 log units below a FMQ buffer. However, Cr-poor clinopyroxenes, together with ilmenite and some Fe-Ca-rich garnets, demonstrate a more oxidized trend in the lower part of lithosphere at -2 to 0 log units relative to FMQ. Clinopyroxenes from xenoliths in most cases show conditions transitional between those determined for garnets and megacrystalline Cr-poor suite. The relatively low diamond grade of Dalnyaya kimberlites is explained by a high degree of interaction with the oxidized protokimberlite melts, which is greater at the base of the lithosphere

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

Deep-seated xenoliths and xenocrysts from Sytykanskaya pipe: evidence for the evolution of the mantle beneath Alakit, Yakutia, Russia

Heavy mineral concentrate and xenoliths from late autolithic breccia and porphyritic kimberlite of the Sytykanskaya pipe (Alakit field, Yakutia, Russia) were studied by EPMA and LA-ICPMS methods to obtain PTXfO2 diagrams. Trends in P- Fe# - CaO - fO2 for minerals from the porphyritic kimberlite show greater discontinuities than xenocrysts from the breccia. Xenoliths show the widest variation at all pressures. Protokimberlite systems are marked by ilmenite PT points that range from the lithosphere base (7.5 GPa) to a pyroxenite lens situated at intermediate depths (5 - 3.5 GPa) with increasing Cr because of AFC that formed two metasomatic groups with differing Fe#Ol (∼ 10-12 and 13-15). The first Opx-Gar-based mantle geotherm for the Alakit field based on 10 mineral associations is close to the 35 mW/m2 geotherm at 6.5 GPa and 600°C, i.e. near to the Moho. The oxidation state for the megacrystalline ilmenites in the lithosphere base is higher than for other kimberlites in Yakutia. Calculated parental melts for clinopyroxene and garnet by xenocrysts from the breccia show highly inclined linear REE patterns with deep HFSE troughs similar to differentiated protokimberlite magmas. Melts calculated for metasomatic xenoliths have less inclined slopes without troughs in spider diagrams. Garnets reveal S-shaped REE patterns. Calculated melts for garnets from graphite-bearing Cr-websterites located mainly in middle part of the mantle column show slightly inclined convex REE patterns and Ba-Sr troughs with variable enrichment in Nb-Ta-U. The calculated parental melts for clinopyroxenes have inclined REE spectra with a depression in HFSE. Metasomatic clinopyroxenes have enriched patterns with Ba, Zr peaks. 40Ar-39Ar analyses of dispersed phlogopites from the Alakit mantle xenoliths yield a Proterozoic (1154 Ma) age, corresponding to continental arc metasomatism. Alkaline and Ti-rich veins with alkali amphiboles close to richterite formed at ∼1015 Ma and mark a plume event in Rodinia mantle. The∼600-550 Ma stage relates to Rodinia break-up. The last metasomatic event near 385 Ma is related to the protokimberlite

Diamond-Bearing Root Beneath the Northern East European Platform (Arkhangelsk Region, Russia): Evidence from Cr-Pyrope Trace-Element Geochemistry

In this study, we reconstruct the composition and metasomatic evolution of the lithospheric mantle beneath the poorly-studied southern Arkhangelsk region, based on the geochemistry of 145 Cr-pyrope grains recovered from samples of modern rivers and stream sediments, to evaluate the diamond exploration potential of these territories. Based on the concentrations of Cr2O3, CaO, TiO2, and rare earth elements (REEs), the garnets are divided into four groups: (1) low-chromium lherzolitic pyropes with fractionated heavy REE patterns; (2) low- to medium-chromium pyropes of lherzolitic and megacryst associations with flat heavy REE patterns; (3) high-chromium lherzolitic pyropes with “humped” REE patterns; and (4) high-chromium and low-chromium lherzolitic and harzburgitic pyropes with sinusoidal REE patterns. The pyrope geochemistry suggests a multi-stage model for the evolution of the lithospheric mantle, including partial melting to different degrees and further metasomatic overprints by silicate and carbonatite melts. The results confirm that the lithospheric mantle beneath the study area is suitable for the formation and preservation of diamonds. The significant percentage of diamond-associated pyropes (15%) emphasizes the likelihood of high diamond contents in kimberlites to be discovered within the study area

Age, provenance and Precambrian evolution of the Anabar shield from U-Pb and Lu-Hf isotope data on detrital zircons, and the history of the northern and central Siberian craton

International audienceThe Anabar shield in northern Siberia is one of the world’s least studied Precambrian areas, and provides a ‘window’ into the crustal basement of the central and northern Siberian craton. We report U-Pb and Hf isotope data for detrital zircons sampled in a profile across its major structural units. They define a U-Pb age range from 1.8 to 3.4 Ga with three main periods: 1.8–2.0 Ga, 2.4–2.8 Ga and 3.0–3.4 Ga. The oldest zircons yield super-chondritic εHf(t) implying that the parental magmas of their source rocks were juvenile, i.e. formed from depleted mantle (DM). Thus, the crustal basement of the Anabar shield, and probably the whole central and northern Siberian craton, started to form in the mid-Paleoarchean, and included no recycled crust. Zircons with 2.5–2.7 Ga ages define two εHf(t) intervals. One is super-chondritic (+2 to +7) implying juvenile sources, the other is sub-chondritic (−3 to −12) indicative of recycled crust, probably formed at 3.2–3.4 Ga, in magma sources. Nearly all 1.8–2.0 Ga zircons have sub-chondritic εHf(t) (−2 to −29) implying derivation from sources dominated by recycled crust formed at ∼2.6 Ga and ∼3.4 Ga and little or no juvenile addition. These events accompanied amalgamation of the entire craton by welding of Archean domains.The Bekelekh unit of the Daldyn series has the highest proportion of ∼2.6 Ga zircons and may be the oldest ‘nucleus’ of the Anabar shield, whereas the Kilegur unit of the same series is essentially Proterozoic (1.95 Ga). The largest amount of 3.1–3.4 Ga zircons, as well as common 2.6–2.7 Ga zircons, occur in the Ambardakh unit of the Upper Anabar series. Our data suggest alternation of areas with dominant ages of 1.95 Ga and ∼2.6 Ga, with the younger zircons coming from granites and granulites, and the older ones from gneisses. They show no evidence for significant ages differences for the Anabar and Olenek provinces. The final amalgamation of the entire Siberian craton by welding of Archean blocks, may have taken place at around 1954 ± 6 Ma

The Geochemical features of the garnets from peridotites of Udachnaya pipe (Yakutia)

Study of mantle xenoliths from the kimberlite pipes allows to get an important information about the origin and transformations of the lithosphere substance. The most deep-seated rocks of the lithospheric mantle are deformed peridotites, having the features of recrystallization and structures of the flow. These rocks undergo significant transformations and enrichment with different components under the effect of asthenospheric melts penetrating by the cracks and faults. The most depleted rocks which are not affected by the enrichment processes are supposed to be the megacrystalline dunites. The garnets from 5 deformed lherzolites (mineral composition: olivine, orthopyroxene, clinopyroxene, garnet), and 20 megacrystalline harzburgite-dunites (mineral composition: olivine, garnet, sometimes chromite, orthopyroxene) from the kimberlite pipe Udachnaya (Yakutia) were analyzed by the LAM-ICPMS. The chondrite normalized REE patterns of garnets from deformed lherzolites are mostly characterized by smooth increasing of LREE values with gradation in the field of HREE. The distribution curves of REE in garnets of megacrystalline dunites have highly sinusoidal character. The very big difference in REE values is seen in the width of dunite field. It shows the different degree and time of their enrichment. The most of distribution curves for garnets of megacrystalline dunites have the peaks at the LREE and MREE showing increased Ce (7 samples), Pr (4 samples), Nd (7 samples), Sm (2 samples) concentrations proving early enrichment. The sharp sinusoidality of REE distribution curves, early enrichment with the LREE prove that some garnets of studied dunites were not in the equilibrium with the melt. In general these garnets could be formed as a result of transformation of earlier chromites, which is confirmed by the increased Cr-concentration in them (for example, garnet UV167/09 contains 11.84 wt.% Cr₂O₃), either as a result of exsolution of primary Al, Cr-containing orthopyroxene, presenting in garzburgites.1 page(s