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Olivine in ultramafic lamprophyres: chemistry, crystallisation, and melt sources of Siberian Pre- and post-trap aillikites

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Abstract

We studied olivines from the Devonian pre-trap (the Ilbokich occurrence) and the Triassic post-trap (the Chadobets occurrence) carbonate-rich ultramafic lamprophyres (UMLs) in the southwestern portion of the Siberian craton. On the basis of detailed investigations of major, minor, and trace-element distributions, we have reconstructed the main processes that control the origins of these olivines. These include fractional crystallisation from melt, assimilation, and fractional crystallisation processes with orthopyroxene assimilation, melt-reaction diffusive re-equilibration, alkali enrichment, and CO2 degassing of the melt. Furthermore, we inferred the composition of the sources of the primary UML melt and their possible correlations with proto-kimberlitic melts, as well as the influence of the Triassic Siberian plume on the composition of the lithospheric mantle. The main differences between olivines from the Ilbokich and the Chadobets aillikites were that the olivines from the former had more magnesium-rich cores (Mg# = 89.2 ± 0.2), had Mg- and Cr-rich transition zones (Mg# = 89.7 ± 0.2 and 300–500 ppm Cr), had lower Ni (up to 3100 ppm) and Li (1.4–1.5 ppm), and had higher B (0.8–2.6 ppm) contents, all at higher Fo values (90–86), relative to the olivines from the latter (Mg# = 88–75; 200–300 ppm Cr; up to 3400 ppm Ni; 1.4–2.4 ppm Li; 0.4–2.2 ppm B). The Siberian aillikite sources contained a significant amount of metasomatic material. Phlogopite-rich MARID-type veins provided the likely metasomatic component in the pre-trap Devonian Ilbokich aillikite source, whereas the Triassic Chadobets aillikitic post-trap melts were derived from a source with a significant carbonate component. A comparison of UML olivines with olivines from the pre-trap and post-trap Siberian kimberlites shows a striking similarity. This suggests that the carbonate component in the aillikitic source could have been produced by evolved kimberlite melts. The differences in the lithospheric metasomatic component that contributed to pre-trap and post-trap aillikitic melts can be interpreted as reflections of the thermal impact of the Siberian Traps, which reduced phlogopite-bearing metasomes within the southwestern Siberian sub-continental lithospheric mantle.

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Acknowledgements

We are grateful to A.V. Postnikov for permission and assistance in the Ilbokich UMLs sampling; V.S. Kamenetsky for useful discussion. S.G. Simakin and E.V. Potapov (Yaroslavl Branch of the Institute of Physics and Technology, Institution of Russian Academy of Sciences) are thanked for SIMS study of olivines. We thank Sonja Aulbach and Geoffrey H. Howarth for their detailed and helpful comments and Hans Keppler for his additional remarks and editorial handling. The part of this research was conducted at the Laboratory of Analytical Techniques of High Spatial Resolution, Department of Petrology, Moscow State University. The purchase of the microprobe was financially supported by the Program for Development MSU. This study was supported by the Russian Foundation for Basic Research, Project Nos. 18-05-00644 A.

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Correspondence to A. A. Nosova.

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Communicated by Hans Keppler.

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Nosova, A.A., Sazonova, L.V., Kargin, A.V. et al. Olivine in ultramafic lamprophyres: chemistry, crystallisation, and melt sources of Siberian Pre- and post-trap aillikites. Contrib Mineral Petrol 173, 55 (2018). https://doi.org/10.1007/s00410-018-1480-3

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  • DOI: https://doi.org/10.1007/s00410-018-1480-3

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