Deep-seated xenoliths and xenocrysts from Sytykanskaya pipe: evidence for the evolution of the mantle beneath Alakit, Yakutia, Russia

• Published in: Geoscience Frontiers
• Main Authors: Ashchepkov, I.V., Logvinova, A.M., Ntaflos, T., Reimers, L.F., Spetsius, Z.V., Vladykin, N.V., Yudin, D.S., Travin, A.V., Downes, Hilary, Palesskiy, V.S.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2014
• Subjects: Earth and Planetary Sciences; Alakit; Gar’; Yakutia
• Online Access: https://eprints.bbk.ac.uk/id/eprint/11067/; https://eprints.bbk.ac.uk/id/eprint/11067/1/11067.pdf; https://doi.org/10.1016/j.gsf.2014.08.005

Heavy mineral concentrate and xenoliths from late autolithic breccia and porphyritic kimberlite of the Sytykanskaya pipe (Alakit field, Yakutia, Russia) were studied by EPMA and LA-ICPMS methods to obtain PTXfO2 diagrams. Trends in P- Fe# - CaO - fO2 for minerals from the porphyritic kimberlite show greater discontinuities than xenocrysts from the breccia. Xenoliths show the widest variation at all pressures. Protokimberlite systems are marked by ilmenite PT points that range from the lithosphere base (7.5 GPa) to a pyroxenite lens situated at intermediate depths (5 - 3.5 GPa) with increasing Cr because of AFC that formed two metasomatic groups with differing Fe#Ol (∼ 10-12 and 13-15). The first Opx-Gar-based mantle geotherm for the Alakit field based on 10 mineral associations is close to the 35 mW/m2 geotherm at 6.5 GPa and 600°C, i.e. near to the Moho. The oxidation state for the megacrystalline ilmenites in the lithosphere base is higher than for other kimberlites in Yakutia. Calculated parental melts for clinopyroxene and garnet by xenocrysts from the breccia show highly inclined linear REE patterns with deep HFSE troughs similar to differentiated protokimberlite magmas. Melts calculated for metasomatic xenoliths have less inclined slopes without troughs in spider diagrams. Garnets reveal S-shaped REE patterns. Calculated melts for garnets from graphite-bearing Cr-websterites located mainly in middle part of the mantle column show slightly inclined convex REE patterns and Ba-Sr troughs with variable enrichment in Nb-Ta-U. The calculated parental melts for clinopyroxenes have inclined REE spectra with a depression in HFSE. Metasomatic clinopyroxenes have enriched patterns with Ba, Zr peaks. 40Ar-39Ar analyses of dispersed phlogopites from the Alakit mantle xenoliths yield a Proterozoic (1154 Ma) age, corresponding to continental arc metasomatism. Alkaline and Ti-rich veins with alkali amphiboles close to richterite formed at ∼1015 Ma and mark a plume event in Rodinia mantle. The∼600-550 Ma stage relates to Rodinia break-up. The last metasomatic event near 385 Ma is related to the protokimberlite.