Carbon isotopic composition and origin of SiC from kimberlites of Yakutia, Russia
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Mineral redox buffer
Xenolith
Isotope Geochemistry
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Mineral redox buffer
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The study of mantle xenoliths from kimberlite pipes allows to establish the composition, evolution processes and thermal condition of the lithospheric mantle under ancient cratons. The Mirny kimberlite field belongs to the diamond-bearing kimberlite fields in the center of the Siberian craton. The collection of mantle xenoliths from the Mir pipe (57 samples) was investigated by authors. Four main petrographic groups were identified: peridotites (Grt lherzolites), Grt websterites, Grt clinopyroxenites and eclogites. The pyroxenite xenoliths attract the special attention.   Garnets from lherzolites and websterites are also characterized by a relatively high Mg# content (75–83) and low TiO2 contents (up to 0.2 wt %). Eclogites are characterized by high-calcium (3.78 - 9.46 wt.%) and high-iron (7.77 - 17.20 wt.%) composition of garnet getting into the ​​wehrlite paragenesis area. Thus, the lithospheric mantle under the Mirny kimberlite field differs from the lithospheric mantle under other diamondiferous fields (for example, Udachnaya kimberlite pipe). The Mirny mantle xenoliths are characterized by the pyroxenites widespread development (up to 50%), the low-Ti composition and deformed lherzolites absence.In addition, websterites and lherzolites show a wide range of crystallization parameters (600 - 1200°C; 2 - 6 GPa) probably due to their gradual cooling after magmatic crystallization and the exsolution structures formation. Clinopyroxenites are characterized by narrow variations in the P-T crystallization parameters (812 - 960°C; 3-4 GPa) indicated their later crystallization from asthenospheric melts. Eclogites are characterized by relatively low calculated temperature parameters (720–840°C; 2.2–3.7 GPa) confirming their origin in subduction zones at shallow depths. The sporadic calculated values for websterites and clinopyroxenites are locating within the diamond stability area. The use of the Opx - thermobarometer (in samples founding Opx) revealed 2 trends in the crystallization of orthopyroxene. Crystallization of individual Opx grains in websterites occurred earlier than Cpx with higher P-T parameters - higher by ~100С and ~0.5 Ha. The second trend (pressure reduction with a slight decrease in temperature) notes the formation of Opx decay structures in an initially homogeneous crystal of monoclinic pyroxene. Minerals from pyroxenites demonstrate a wide development of melting processes in the lithospheric mantle in the south of the Siberian craton Craton and the formation of megacrystalline pyroxene cumulates. The origin of eclogites is assumed from subducted oceanic crust marking the subduction component in the process of formation of the lithospheric mantle.The research was supported by Russian Science Foundation grant № 22-77-10073.
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Abstract Silicon and oxygen are potential light elements in the Earth's core and may be involved in metal‐silicate reactions at the present day core‐mantle boundary. We have performed multianvil experiments at 25 GPa and 2770–3080 K to understand the simultaneous partitioning of these elements between liquid iron–rich metal and silicate melt. The presence of O in liquid Fe at high temperatures influences the partitioning of Si, causing more Si to partition into the metal than would be expected based on lower temperature measurements. Although Si and O are mutually exclusive in Fe metal at <3000 K, the level at which both element concentrations are similar in the liquid metal rises above 1 wt % at >3000 K. We have developed a thermodynamic model based on these experiments that accounts for the interaction between O and Si in the liquid metal. Comparison between this model and the previous results of diamond‐anvil cell experiments up to 71 GPa indicates very little pressure dependence but a strong temperature dependence for O and Si partitioning. Our model predicts that subequal concentrations of Si and O, sufficient to explain the outer core density deficit, would have partitioned into core‐forming metal if equilibration occurred between the metal and a magma ocean with a bulk silicate Earth composition at an average depth of ~1200 km (~50 GPa and ~3300 K). An O‐ and Si‐enriched buoyant layer may have developed at the top of the outer core as a result of subsequent equilibration with the overlying mantle.
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金伯利岩中的锆石按照颗粒大小可以分为细粒锆石（一般小于200μm）和巨晶锆石（一般大于500μm）。前人的研究结果显示在金伯利岩中粒径较大的巨晶锆石的U-Pb体系在高温的地幔中一直保持着开放状态，直到寄主金伯利岩浆的喷发才使地幔锆石的U-Pb体系封闭，因此这些巨晶锆石是确定金伯利岩年龄的重要矿物之一。然而，近年来的研究表明，金伯利岩中还存在一些时代远老于金伯利岩年龄的锆石，也具有较大的粒径（以下称古老锆石巨晶）。它们的存在无疑影响了利用锆石U-Pb方法确定金伯利岩年龄的准确性。本文以西伯利亚雅库特（Yakutia）金伯利岩省中的四颗古老锆石巨晶为研究对象，通过形态学、年代学、微量元素和Hf同位素组成，讨论古老锆石巨晶的来源。同时，我们统计和对比了全球多个金伯利岩中能够确定金伯利岩年龄的锆石和古老锆石巨晶的形态学、U、Th含量和微量元素组成、Hf-O同位素等特征。研究结果显示，金伯利岩中的古老锆石巨晶的tDM年龄和O同位素组成与可以用来确定金伯利岩年龄的锆石巨晶具有明显的差别。这些手段在未来的研究中可以用来区分可确定金伯利岩年龄的锆石巨晶和古老的锆石巨晶。;Zircon from kimberlite can be divided into fine-grained zircon (<200μm) and megacrystal zircon (>500μm) according to its grain size. Previous studies have shown that the U-Pb system of mantle zircon is an open state in the high temperature mantle until the eruption of host kimberlite. Therefore, mantle zircon is one of the important minerals for dating the age of kimberlite. However, recent studies have shown that some zircon megacrysts from kimberlite with ages much older than kimberlite also have large grain sizes. Their presence undoubtedly influences the quasi-dating of kimberlite ages. In this study, four ancient zircon megacrysts from the Siberian kimberlite were studied,and the origin of the ancient zircon megacrysts was discussed through morphology, chronology, trace elements and Hf isotopic compositions. Moreover, the morphology, U and Th contents, trace element composition, and Hf-O isotopes of mantle zircon and old zircon from cratonic kimberlite rocks in the world are collected and compared. The results show obvious differences in tDM ages and O isotopic compositions between the large old zircon and mantle zircons. These methods can be used to distinguish old zircon from mantle zircon megacrysts in kimberlite in future studies.
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Pyrope
Metasomatism
Peridotite
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Primitive mantle
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This paper reports the results on the composition of lithosphere mantle under the Mirny kimberlite field. The authors investigated 57 samples of the mantle xenoliths collected from the Mir pipe. The samples were represented by peridotites (Grt lherzolites) and pyroxenites (Grt websterite, Grt clinopyroxenite and eclogite). The composition of minerals (garnet, clinopyroxene) and various rocks in the lithosphere mantle under the Mirny kimberlite field were analyzed based on petrographic features and chemical data. Besides, PT conditions of rock crystallization were calculated using different geothermobarometers. Garnets from peridotites and websterites show relatively high Mg# (75–83) and low TiO 2 contents (up to 0.2 wt. %). Since the eclogite has high-Ca (3.78–9.46 wt. %) and high-Fe (7.77–17.20 wt. %) garnet composition, it lies in the area of wehrlite paragenesis. In general, garnets from the lithosphere mantle under the Mirny kimberlite field have low-Ti garnet composition (up to 0.7 wt. %). Thus, the lithosphere mantle under the Mirny kimberlite field differs from the lithosphere mantle under other diamondiferous fields in a widespread development of eclogite and pyroxenite (up to 50 %), low-Ti composition of rocks, as well as virtual absence of deformed lherzolites. These signs probably indicate minor alteration of silicate metasomatism in the lithospheric mantle under the Mirny field (in contrast to the center of the Siberian craton).
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Carbonatite
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Abstract We present the first oxygen fugacity ( f O 2 ) profile through the cratonic lithospheric mantle under the Panda kimberlite (Ekati Diamond Mine) in the Lac de Gras kimberlite field, central Slave Craton, northern Canada. Combining this data with new and existing data from garnet peridotite xenoliths from an almost coeval kimberlite (A154-N) at the nearby Diavik Diamond Mine demonstrates that the oxygen fugacity of the Slave cratonic mantle varies by several orders of magnitude as a function of depth and over short lateral distances. The lower part of the diamond-bearing Slave lithosphere (>120–130 km deep) has been oxidized by up to 4 log units in f O 2 , and this is clearly linked to metasomatic enrichment. Such coupled enrichment and oxidation was likely caused by infiltrating carbonate-bearing, hydrous, silicate melts in the presence of diamond, a process proposed to be critical for “pre-conditioning” deep lithospheric mantle and rendering it suitable for later generation of kimberlites and other SiO 2 -undersaturated magmas.
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