Titanium-rich metasomatism in the lithospheric mantle beneath the Arkhangelsk Diamond Province, Russia – Insights from ilmenite-bearing xenoliths and HP-HT reaction experiments

Kargin A, Bussweiler Y, Nosova A, Sazonova L, Berndt J, Klemme S

Abstract

To provide new insights into the interaction of ultramafic alkaline melts with the subcontinental lithospheric mantle, we present results of a petrographical-mineralogical study of ilmenite-bearing mantle xenoliths from the Grib kimberlite, Archangelsk, Russia along with results from reaction experiments between harzburgite and Fe-Ti bearing carbonate-silicate melts similar to aillikite. The compositions of orthopyroxene, ilmenite and garnet from our mantle xenoliths are similar to compositions of minerals of the low-Cr megacryst suite from different kimberlite occurrences worldwide including the Grib kimberlite as well as minerals from sheared lherzolite xenoliths captured by the Grib kimberlite. This suggests that ilmenite-bearing xenoliths, megacrysts, and sheared lherzolite xenoliths could have a common origin and / or formed under similar conditions. The reaction experiments were performed at 4 GPa and 1200 °C with varying proportions of aillikite melt (0, 10, and 50 wt. %) that reacted with harzburgite. The experimental runs with 10 % and 50 % aillikite resulted in two layers within the capsule, with an ilmenite-bearing reaction zone at the contact between aillikite and harzburgite, and an ilmenite-free zone characterized by higher garnet and clinopyroxene abundances. An increase of aillikite melt is directly correlated with increasing TiO2 and decreasing Cr2O3 contents and Mg# values in the mineral phases, most significant for pyroxenes. Overall, the experiments produce a chemical gradation of minerals from Cr-rich (Fe-Ti-poor) to Cr-poor (Fe-Ti-rich) which is strikingly similar to the chemical gradation observed in minerals from natural mantle-derived xenoliths from kimberlites. In summary, a comparison of our experimental data and natural samples indicates links between the generation of megacrysts and Ti-rich metasomatism of the lithospheric mantle by ultramafic alkaline (aillikite-related) melts and their possible evolution towards kimberlites. Also obtained results illustrate the importance of melt-rock ratios in generating the mineralogical and chemical diversity in mantle xenolith suites.

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