Zonation in garnets from the Udachnaya pipe: heating and melt infiltration in the lithospheric mantle of the Siberian craton

Xenoliths in kimberlites and other volcanic rocks are our best window into the subcranotic lithospheric mantle. Chemical overprinting associated with melt-rock interactions is almost ubiquitous in these mantle xenoliths [1]. Such local changes in chemistry may be recorded by the formation of compositional zoning in minerals. Studies of major and trace element zoning provide important information about the nature and time scales of metasomatic processes and thermal events in the upper mantle.

Usually, garnets from peridotite xenoliths have pronounced zoning, whereas olivine and pyroxenes are homogeneous. Currently, only zoning in garnets of sheared and coarse peridotite xenoliths from kimberlites of the Kaapvaal craton (southern Africa) and the minette neck The Thumb (North American craton) has been studied in detail (e.g., [2,3]). There is no detailed study on major-, minor- and trace-element zoning in garnets of peridotite xenoliths from kimberlites of the Siberian craton.

In our study, we provide a detailed description of complex major- and trace-element zoning patterns in garnets of two unique fresh sheared peridotites from the Udachnaya kimberlite pipe (Siberian craton). The mantle residence pressure and temperature of the peridotites UV-3/05 (lherzolite) and UV-33/04 (harzburgite) are 6.4 GPa and 1350°C [4] and 6.0 GPa and 1320°C [5], respectively.

The profiles of minor and major elements are complex and symmetric. The profiles change their slope signs (positive/negative) several times. It should be noted that the Ni content increases from the cores to the rims. The chondrite-normalized REE patterns show a continuous change from the cores to the rims. The cores display sinusoidal patterns (LREE enrichment peaking at Sm), whereas patterns of the rims are ‘normal’ (with HREE enriched by 15–19× chondrite abundances for Gd through Lu).

The profiles are consistent with the formation of garnet overgrowths and increasing temperature, followed by diffusive equilibration between the rims and cores over hundreds or thousands of years. Using melt-garnet distribution coefficients of trace elements, we showed that the metasomatic melt, which caused the formation of the garnet overgrowths, had a genetic link to the kimberlite magmatism that formed the Udachnaya pipe. The profile lengths of Zr, Ce, Sm, Eu, Gd, and Hf are longer than the profile lengths of Tb, Dy, Ho, Er, Tm, Yb, and Lu. This indicates that the composition of the melt changed (from composition in equilibrium with upper mantle peridotite to kimberlitic composition) during its percolation through the mantle, as predicted by the theory proposed by Navon and Stolper (1987).

This study was supported by the Russian Science Foundation (grant No 18-77-10062).

References: [1] Pearson, D.G. and Wittig, N., 2014, Treatise on Geochemistry, 255-292. [2] Griffin et al., 1989, Geochim. Cosmochim. Acta, 53(2), 561-567. [3] Smith et al., 1991, Contrib. Mineral. Petrol., 107(1), 60-79. [4] Golovin et al., 2018, Chem. Geol., 261-274. [5] Agashev et al., 2013, Lithos, 160, 201-215. [6] Navon, O. and Stolper, E., 1987, J. Geol., 95(3), 285-307.