Mantle xenoliths of the Nyurbinskaya pipe (Nakyn field, Yakutuya):
relicts of weakly metasomatized lithospheric mantle
Sablukova L.I.* , Sablukov S.M.* , Stegnitsky Yu.B.** , Karpenko M.A.***, Neustroev R.G.****
* “RUSGEO” Limited, Moscow, Russia;** NIGP ALROSA Co. Ltd., Mirny, Russia;*** Nyurbinskaya Mine, ALROSA Co. Ltd., Mirny, Russia;**** ALROSA-Nyurba Co. Ltd., Nyurba, Russia.
Mantle xenoliths abound in rocks related to all intrusion phases in the Nyurbinskaya pipe, varying in size from 2-6 cm to 22 cm (5 kg weight). In all, more than 490 nodules were examined.
The xenoliths are dominated by garnet peridotite and clinopyroxenite and chrome spinel peridotite, with a minor proportions of spinel peridotite and eclogite (Groups A and B). In addition, rare megacrysts of orange Ti-association garnet and macrocrystals of orange and red garnet (eclogite suite) also occur. Mantle xenoliths have a granulose structure. Cataclastic peridotite is almost completely lacing, while banded (layered) rocks and rocks with unbalanced mineral suites are quite abundant. Approximately 10% of garnet peridotite and clinopyroxenite xenoliths contain picroilmenite.
Chrome spinel from garnet-containing and chrome spinel peridotite is characterized by a high Cr2O3 content (45-62% wt. %), lower than average FeO and TiO2 contents, and a characteristic Cr3+ ó Al3+ isomorphism.
Spinel peridotite xenoliths form an isolated group, with chrome spinel corresponding in composition to the low-pressure dept facies (less than 30 wt. % Cr2O3).
Garnet from garnet peridotite and clinopyroxenite varies in composition, primarily in Cr2O3 content (0.1-9 wt. %), with medium-Cr and low-Fe lherzolite suite garnet varieties predominating.
Clinopyroxene from petrologically different nodules in the Nyurbinskaya pipe has a quite constant composition, with a high Ca index, higher than average Na and Al contents, lower than average Fe and Mg contents, and a varying Cr content, which is generally atypical of mantle nodules occurring in Yakutian and Archangelsk kimberlites.
Picroilmenite from different mantle xenoliths is characterized by higher than average TiO2 and MgO contents, therewith being almost free of hematite component, thus sharply differing in composition from typical kimberlitic picroilmenite.
Garnet peridotite and clinopyroxenite xenoliths are characterized by a narrow range of TP forming parameters: 768-1002o Ñ and 29.8-40.5 kbar (according to the geothermobarometer proposed by Nikitina and Simakov, 1994 for the clinopyroxene + garnet paragenesis; 31 samples were analyzed). The behaviour of nodule forming parameters fits well to the 40 mW/m2 conductive geotherm.
Metasomatism manifests itself in the examined nodules quite weakly, just as the development of phlogopite which could proceed almost concurrently with the formation of high-K kimberlitic melts.
The abundance and characteristic size of mantle xenoliths and the sharp predominance of garnet peridotite makes the Nyurbinskaya pipe kimberlites similar to kimberlite rocks from other Ykutian kimberlite fields. Along with this, kimberlites of the Nyurbinskaya pipe differ drastically from other Yakutian kimberlites by their high chrome spinel peridotite content, and by the presence of picroilmenite and clinopyroxene with peculiar mineral chemistry in mantle xenoliths, and absence of characteristic picroilmenite megacrysts.
Kimberlite pipes of the Nakyn field appear to be older than other Yakutian diamondiferous kimberlite pipes. Their peculiar geochemical and mineralogical features might reflect the early, relict condition of the lithospheric mantle, somewhat primitive, yet unaffected by a Fe-Ti mantle metasomatism from a Late Devonian asthenospheric diapir (mantle plume) so intense as was the case with mantle substrate of younger kimberlites in the Malo-Botuobinsky, Daldyn-Alakitsky and Verkhne-Munsky districts. The post-plume, metasomatized upper mantle condition is characteristic of the majority of Yakutian kimberlite fields, while evidence of probably pre-plume (“relict”) condition of geochemically depleted upper mantle has yet only been found in some Nakyn field kimberlite rocks. This is why the direct examination of mantle rock fragments from kimberlites of the Nakyn field is undoubtedly of interest as an investigation of relict rocks characterizing the zones that previously existed in the upper mantle but than vanished as a result of complete transformation in later geological events. This direct examination of mantle rock fragments is no less (and, maybe, even more) important and interesting than the indirect study of mantle rocks through the medium of geochemical and isotopic characteristics of their host kimberlites.
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