Full length articleDeep mantle roots of the Zarnitsa kimberlite pipe, Siberian craton, Russia: Evidence for multistage polybaric interaction with mantle melts
Graphical abstract
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.
References (124)
- et al.
Metasomatism in lithospheric mantle roots: constraints from whole-rock and mineral chemical composition of deformed peridotite xenoliths from kimberlite pipe Udachnaya
Lithos
(2013) - et al.
PT conditions and trace element variations of picroilmenites and pyropes from the Arkhangelsk region
J. Asian Earth Sci.
(2013) - et al.
Apatite, SiO2, rutile and orthopyroxene precipitates in minerals of eclogite xenoliths from Yakutian kimberlites, Russia
Lithos
(2015) The continental lithosphere: Reconciling thermal, seismic, and petrologic data
Lithos
(2009)- et al.
Pyroxenites and megacrysts from Vitim picrite-basalts, Russia: Polybaric fractionation of rising melts in the mantle?
J. Asian Earth Sci.
(2011) - et al.
Monomineral universal clinopyroxene and garnet barometers for peridotitic, eclogitic and basaltic systems
Geosci. Front.
(2017) - et al.
Alakit and Daldyn kimberlite fields, Siberia, Russia: Two types of mantle sub-terranes beneath central Yakutia?
Geosci. Front.
(2017) - et al.
Interaction between protokimberlite melts and mantle lithosphere: Evidence from mantle xenoliths from the Dalnyaya kimberlite pipe, Yakutia, Russia
Geosci. Front.
(2017) - et al.
Structure and evolution of the lithospheric mantle beneath Siberian craton, thermobarometric study
Tectonophysics
(2010) - et al.
Composition and thermal structure of the lithospheric mantle beneath kimberlite pipes from the Catoca cluster, Angola
Tectonophysics
(2012)