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Deep mantle roots of the Zarnitsa kimberlite pipe, Siberian craton, Russia: Evidence for multistage polybaric interaction with mantle melts

https://doi.org/10.1016/j.jseaes.2021.104756Get rights and content

Highlights

  • Protokimberlitic magmas used depleted dunites as a feeder for the focused flow.

  • Multistage metasomatism was detected beneath Zarnitsa pipe.

  • Four protokimberlitic chambers traced by Ilm and augites locate at paleoslabs boundaries.

  • Ilmenite and augite megacrysts reveal TRE increase with decreasing pressure.

  • The AFC differentiation accompanied ascend of protokimberlites.

Abstract

Zarnitsa kimberlite pipe in Central Yakutia contains pyrope garnets with Cr2O3 ranging from 9 to 19.3 wt% derived from the asthenospheric mantle. They show mostly S-shaped, inflected rare earth element (REE) patterns for dunitic and harzburgitic, lherzolitic and harzburgitic varieties and all are rich in high field strength elements (HFSE) due to reaction with protokimberlite melts. Lithospheric garnets (<9 wt% Cr2O3) show a similar division into four groups but have more symmetric trace element patterns. Cr-diopsides suggest reactions with hydrous alkaline, protokimberlitic and primary (hydrous) partial melts. Cr-diopsides of metasomatic origin have inclined REE patterns and high LILE, U, Th and Zr concentrations. Four groups in REE of Ti-rich Cr-diopsides, and augites have asymmetric bell-like REE patterns and are HFSE-rich. Mg-ilmenites low in REE were formed within dunite conduits. Ilmenite derived from differentiated melts have inclined REE patterns with LREE ~ 100 × chondrite levels. Thermobarometry for dunites shows a 34 mWm−2 geotherm with a HT branch (>50 mWm−2) at 6–9 GPa, and a stepped HT geotherm with heated pyroxenite lenses at four levels from 6.5 to 3.5 GPa. Parental melts calculated with KDs suggest that augites and high-Cr garnets in the lithosphere base reacted with essentially carbonatitic melts while garnets from lower pressure show subduction peaks in U, Ba and Pb. The roots of the Zarnitsa pipe served to transfer large portions of deep (>9 GPa) protokimberlite melts to the lithosphere. Smaller diamonds were dissolved due to the elevated oxidation state but in peripheral zones large diamonds could grow.

Introduction

Zarnitsa was the first kimberlite pipe found in Siberia in 1954 using pyropes and ilmenites as kimberlite indicator minerals (KIM) (Sarsadskikh and Popugaeva, 1955). It is the largest kimberlite pipe in the Daldyn region in the central part of the Yakutian kimberlite province (YKP) (Fig. 1) (540 × 560 m), (supplementary file 1 (SF1, Fig. 1,2) (Khar’kiv et al., 1998). It is composed of gray eruptive and darker greenish autholithic breccias where rounded debris of kimberlites with different textures are cemented by later kimberlite phases containing abundant debris of various mainly crustal rocks. Their abundance is the main reason for the relatively low diamond grade (~0.3 crt/t). However, the rare occurrence of large-high-quality diamonds allows for its exploration. The largest diamond found in Zarnitsa in 2016 is 207.29 carats and the total capacity of the Zarnitsa quarry is 7,490,000 carats. (SF1, Fig. 2). The main characteristic of Zarnitsa kimberlites is the lack of small octahedral diamonds which are common, for instance, in Udachnaya, and the occurrence of small rounded diamonds with a “mosaic” internal structure (Ragozin et al., 2017, Ragozin et al., 2018) which often demonstrate the intergrowth of small subcrystals.

In Zarnitsa grey (or pale -, greenish -, bluish-, dark-grey) autolithic breccias (ABK) is the commonest rock-type, with abundant autolithic kimberlite xenoliths. ABK contains also xenoliths of black macrocrystal kimberlites (BMK) with fresh olivine, pyroxenes, and other xenocrysts. Several blocks of BMK with abundant mantle xenoliths were found in the Zarnitsa quarry and in the temporary kimberlite stores.

The xenolith chemistry is poorly studied, due to the strong serpentinization of xenoliths (Ashchepkov et al., 2003). Only dark varieties of breccias and rare BMK contain relatively fresh materials. Xenoliths are harzburgites and dunites, with rarer metasomatized and sheared peridotites and pyroxenites. Eclogitic xenoliths are extremely rare (Spetsius and Serenko, 1990, Alifirova et al., 2015). Our finding of 7 eclogites and >100 peridotitic xenoliths in dark breccias from Zarnitsa will be described in another publication (Ashchepkov et al., in preparation).

This study aims to decode the composition and structure of the sub-cratonic lithospheric mantle (SCLM) and the processes of the interaction of the peridotites with the melts and fluids which took place beneath this large pipe and the surrounding small pipes in this group using detailed thermobarometry and trace element geochemistry of kimberlites (Kostrovitsky et al., 2007, Kargin et al., 2011) and xenocrysts (Ashchepkov et al., 2010).

Section snippets

Geological setting

The Zarnitsa pipe together with 17 surrounding kimberlite pipes form the cluster, located within the Archean West Daldyn granulite –orthogneiss terrane of Siberian craton (Gladkochub et al. 2006). The age of the mafic volcanic rocks is 3.2 Ga, metamorphism in the crust took place 2.8–2.65 Ga and collision is dated as 1.9 Ga according to Rosen et al (2006). Zarnitsa kimberlite cluster is controlled by the Daldyn-Olenek zone of deep-seated faults. This cluster is the densest in Yakutia and

Samples

For this study, we collected ~ 1240 garnets, ~340 Cr-diopsides, ~128 low-Cr clinopyroxenes, 175 ilmenites, 84 phlogopites, 32 amphiboles and 40 chromites from mineral concentrates from prospecting pits #148-151 and crushed BMK samples as well from the mantle xenoliths. Garnets were selected from the 0.25–0.5 mm fraction for analyses.

Mantle xenoliths in the BMK are garnet dunites including giant grained types (Pokhilenko et al., 1991) and harzburgites (dominant among the xenolith population)

Electron microprobe analyses (EMPA)

The collected material was analyzed in the Analytic Center of Sobolev V.S. Institute of Geology and Mineralogy SB RAS (IGM SB RAS.) Electron Probe Micro-Analyzers (EPMA) Camebax Micro and Jeol JXA8320 were used for the analysis of studied minerals: garnets (Gar), clinopyroxenes (Cpx) (Cr-diopsides, augites), phlogopites (Phl), ilmenites (Ilm), and chromites (Cpx) according to the common procedure (Lavrentiev & Usova, 1994). The accelerating voltage was 15 kV and the beam current was 15nA with

Mineralogy

Garnets on the Cr2O3-CaO diagram (Cr2O3 vs. FeO, MgO, Na2O) for mantle garnets with divisions for the Cr-bearing garnets after Sobolev et al. (1973) and (Dawson and Stephens, 1975) (Fig. 3A) contain up to 19.2 wt% Cr2O3, plotting mainly within the lherzolitic field (G5 and G9) (Grütter et al., 2004). The highest amount of both sub-Ca (G10) and pyroxenitic (G1, G12) varieties are found within the 7–18 wt% Cr2O3 interval with the extremes of variations at 9 wt% Cr2O3 (Fig. 3). The values of the

Thermobarometry and reconstruction of mantle section

We provide combined PTXfO2 diagrams based on mineral grains from concentrates of Zarnitsa kimberlites (Fig. 9). The data set for thermobarometry was greatly extended in comparison to the previous study (Ashchepkov et al., 2010, Ashchepkov et al., 2017b). We added the high-Cr pyrope garnets and eclogitic pyrope-almandines and Cr-diopsides and minerals from mantle xenoliths. In this new version, the PTXfO2 diagram is constructed using numerous analyses of pyrope garnets (1148), including sub-Ca

Garnets.

We made LA-ICP-MS analyses for different types of garnets (Fig. 11, Fig. 12, Fig. 13). The separate diagram for high Cr-garnets (19.5–9 wt% Cr2O3) from concentrates which are mostly Ti-enriched relates to the pressures higher than 6.5. They reveal quite different REE patterns but rather similar incompatible element enrichment (Th, U, Nb, Ta) except for the large ion lithophile element (LILE) group. The shape of their REE patterns depends on the CaO content. The Srn is 1–1.5 lower than LREEn

Location of magmatic bodies in mantle sections

Thermobarometry for garnets (Ashchepkov, 2006; Ashchepkov et al., 2010; Ashchepkov et al., 2014a; Ashchepkov et al., 2017a) reveals very high temperatures for the peridotitic and eclogitic garnets and slightly lower for the ortho- and clinopyroxenes, and the deepest mantle root in Daldyn field beneath the Zarnitsa pipe (Fig. 9). In addition, ilmenites demonstrate splitting of PT arrays to the cold geotherm typical for metasomatic associations and to high-T branches (Boyd, 1984; Boyd et al., 1997

Conclusions

1. The Devonian mantle structure beneath the Zarnitsa pipe was layered.

2. Depleted mantle column beneath Zarnitsa pipe served as the melt feeder

3. Protokimberlite melts, traced by the ilmenites and Ti-augites, traced boundary of subducted paleo slabs

3. The metasomatism: essentially differ in trace elements 1) hydrous ancient LREE and LILE; 2) protokimberlite HFSE rich.

4. Protokimberlites differentiated by AFC process show increased of TRE during ascend.

5. Hydrous metasomatism dissolved small

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Supported by RFBR grants 19-05-00788, 16-05-00860 and joint research projects IGM SB RAS and ALROSA Stock company 77-2, 65-03, 02-05. Work was done on state assignment of IGM SB RAS. The study was performed by the governmental assignment in terms of Project IX.129.1.4.

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