Late metasomatic addition of garnet to the SCLM: Os-isotope evidence

Archean cratons are underlain by highly depleted subcontinental lithospheric mantle (SCLM). However, xenolith and xenocryst data [1; references therein] suggest that Archean SCLM has been extensively refertilized by metasomatic processes, with the addition of Fe, Ca, and Al to depleted protoliths. The distribution of sub-calcic garnets in the SCLM beneath the Siberian craton suggests (1) sub-calcic garnets and diamonds are metasomatic phases in the cratonic SCLM; (2) the distribution of both phases is laterally heterogeneous on relatively small scales and related to ancient structural controls [2]. Re-Os isotopic compositions of sulfide inclusions in lherzolitic Cr-pyropes from four Siberian middle Paleozoic diamond mines have been determined by laser ablation MC-ICP-MS: Mir (n=17) and Internationalnaya (n=109), Malobotuobiya field, Udachnaya (n=17), Daldyn field, and Nyurbinskaya (n=12), Nakyn field. Most analysed sulfides ( approximately 84%) have very low Re/Os ratios (<0.07), and their Re-depletion ages (T (sub RD) ) fall between 2.2 and 3.0 Ga (+ or -0.03 Ga, mean 2s analytical uncertainty). 10 to 15% of the sulfides give younger T (sub RD) down to 600 Ma. Our previous study [3] of sulfide inclusions in megacrystic olivines from the Udachnaya pipe suggests that most of the SCLM beneath the Daldyn kimberlite field formed at 3-3.5 Ga, and that lithosphere formation culminated at ca 2.9 Ga. Our new data suggest that refertilization of the highly depleted SCLM and the introduction of Cr-pyrope garnet occurred between 2.2 and 3.0 Ga; little garnet was present before 3 Ga. Pyropes with young sulfides (between approximately 1.9 and approximately 2.2 Ga) may have crystallised during the amalgamation of the Siberian craton in Paleoproterozoic time.