Lithospheric mantle heterogeneity beneath the Siberian craton: Evidence from garnet xenocryst database with implications for kimberlite compositions

Abstract

This contribution reports some 16,000 major and minor element analyses of garnet xenocrysts derived from 18 (out of the 21 known) kimberlite fields of the Yakutian Kimberlite Province (YaKP) on the Siberian craton in Russia. Using TiO₂–in–garnet as an indicator of heterogeneity within the subcontinental lithospheric mantle (SCLM), as well as garnet mg# (mg#=Mg²⁺/(Mg²⁺+Fe²⁺)*100), we distinguish three subpopulations of garnet: 1) high content of TiO₂ (0.26–0.50 wt%) and high mg# (80.6–82.6) garnet xenocrysts are common in the southern diamondiferous kimberlite fields; 2) garnet xenocrysts with low content of TiO₂ (0.06–0.26 wt%) and relatively high values of mg# (78.8–81.7), which prevail in the northern ‘barren’ kimberlite fields; and 3) three anomalous northern kimberlite fields (Chomurdakh, Ogoner-Yuryakh, Toluopka) characterized by the predominance of garnet xenocrysts with high TiO₂ content (0.53–0.78 wt%) at relatively low mg# (76.9–78.3).<p> <p>It is reasonable to assume that relatively thin cratonic mantle lithosphere beneath the three anomalous kimberlite fields underwent intense metasomatic overprinting by melts and fluids injected from the underlying asthenosphere, which changed the compositions of peridotitic garnets significantly. An interpretation of the data presented in this study is that the generally high TiO₂ contents of kimberlites in the northern YaKP (>1.5 wt% TiO₂) are a primary magmatic feature of asthenospheric origin because the lithospheric mantle traversed by these kimberlite magmas is TiO₂ depleted. We propose a model in which the relatively thin SCLM of the northern Siberian craton provided less opportunity for high-TiO₂ asthenospheric kimberlite melts to interact and change compositions on their way to the Earth’s surface. The high-TiO₂ kimberlites of the northern YaKP may thus represent a good approximation of the primary compositions of natural kimberlite melts.