Aragonite in the Interstitial Space of a Mantle Xenolith from the Udachnaya Kimberlite Pipe (Siberian Craton): Direct Evidence for the Presence of Carbonatite Melts in the Deep Lithospheric Mantle.

This paper reports the first discovery of zoned aragonite, a high-pressure CaCO3 polymorph, in a mantle xenolith from kimberlite. Aragonite is the most common epigenetic mineral in the studied xenolith and is located in the interstitial space, where it can occupy up to 80 vol %. According to the P‒T parameters of the last equilibrium (6.9 GPa and 1350°C), the studied sheared lherzolite belongs to the deepest mantle xenoliths from kimberlites. Based on the stability of aragonite, it could have formed in sheared lherzolite both in situ in the lithospheric mantle and during ascent of the kimberlite magma to a depth of ~80 km. The studied aragonite has high contents of SrO (from ~0.7 to 8.3 wt %) and Na2O (up to ~1.1 wt %). Diffusion modeling at mantle temperatures of 1100–1350°C shows that the aragonite grains lose zonation in SrO over ~1–7 years. Thus, the zonation pattern indicates a link between the formation of aragonite in the sheared lherzolite and magmatism, which formed the eastern body of the Udachnaya pipe. In general, such high contents of aragonite in the interstitial space of the studied xenolith are direct evidence of the existence of carbonatite melts in the lithospheric mantle of ancient cratons at depths below ~230 km shortly before the formation of kimberlites on the surface. The results in the present study again raise the question regarding the compositions of primitive kimberlite melts and true concentrations of alkalis (in particular, Na2O) in them. [ABSTRACT FROM AUTHOR]