Peridotite xenoliths from the Udachnaya-East kimberlite: windows onto the evolution of the Siberian sub-cratonic lithospheric mantle

Peridotite xenoliths from kimberlites are useful tools for exploring the architecture and composition of the thick sub-cratonic lithosphere, and thus understanding the long-term evolution of the Earth’s mantle. However, the continuous infiltration of kimberlite-related melts and fluids prior to - and during - the transport of mantle-derived fragments towards the surface makes it difficult to extract information about the original texture and chemistry of the mantle rocks and the deep-seated metasomatic processes.

In this study, fresh spinel- to garnet-bearing peridotite xenoliths from Udachnaya-East were studied to unveil the nature and composition of the lithospheric mantle beneath the Siberian craton. The studied samples have mostly harzburgitic to dunitic composition, even though lherzolites and rare wehrlites are also found. Occasionally, harzburgites are orthopyroxene-rich (up to 40 vol.%) or garnet-rich (up to 30 vol.%). The texture of the peridotites is extremely variable, ranging from protogranular to highly recrystallized and/or sheared. In spinel-bearing rocks, primary olivine is Mg- and Ni-rich (Fo_90-93; NiO = 0.34-0.46 wt%), orthopyroxene has Mg# of 92-94 and Al₂O₃ in the range of 0.3-3.0 wt%, while clinopyroxene is Mg-rich (Mg# 94-96), with Al₂O₃ comprised between 1.0 and 3.5 wt%. In garnet-bearing peridotites, olivine ranges from Mg- and Ni-rich (Fo₉₂; NiO = 0.45 wt%) to Fe-rich and Ni-poor (Fo₈₇; NiO = 0.25 wt%), while pyroxenes have Mg# from 93 to 87-88 and comparatively low Al₂O₃ contents (orthopyroxene: 0.5-1.1 wt%; clinopyroxene: 0.8-2.2 wt%). High-precision electron microprobe analyses complemented by thermo- and oxy-barometric models were used to reconstruct the thermo-chemical log of the Siberian sub-cratonic mantle, in comparison to what proposed by Liu et al. (2022). Textural-compositional studies of the reaction zones enabled to discriminate the secondary-formed minerals with composition ascribable to the liquid line of descent of kimberlite-related melts at Udachnaya (Casetta et al. 2023) from those formed during melt/fluid-rock reactions taking place in the mantle before xenoliths’ entrainment by the host kimberlites. Altogether, our results enable to trace the P-T-X evolution experienced by the Siberian mantle, opening a window onto the comprehension of the interactions between kimberlitic-related fluid/melts and the sub-cratonic lithosphere.

 

Casetta, F., Asenbaum, R., Ashchepkov, I., Abart, R., & Ntaflos, T. (2023). Mantle-Derived Cargo vs Liquid Line of Descent: Reconstructing the P–T–fO₂–X Path of the Udachnaya–East Kimberlite Melts during Ascent in the Siberian Sub-Cratonic Lithosphere. Journal of Petrology, 64(1), egac122.

 Liu, Z., Ionov, D. A., Nimis, P., Xu, Y., He, P., & Golovin, A. V. (2022). Thermal and compositional anomalies in a detailed xenolith-based lithospheric mantle profile of the Siberian craton and the origin of seismic midlithosphere discontinuities. Geology.