Significance of the whole rock Re-Os ages in cryptically and modally metasomatised cratonic peridotites: Constraints from HSE-Se-Te systematics

The Re-Os isotopic system is the geochronometer of choice to constrain the timing of lithospheric mantle root formation and reconstruct the evolution of Earth's dynamics from the mantle perspective. In order to constrain the effects of metasomatic processes on the Re-Os isotopic system, eleven peridotites from the Letlhakane kimberlite pipe were investigated for whole rock major and trace elements, highly siderophile elements (HSE), Se, Te and Os-187/Os-188 signatures. These spinel peridotites (SP), garnet peridotites (GP), garnet-phlogopite peridotites (GPP) and phlogopite peridotites (PP) experienced cryptic metasomatism and the GP-GPP-PP additionally constitute a sequence of increasing modal metasomatism. The cryptically metasomatised SP appear devoid of base metal sulphides (BMS) and show suprachondritic Se/Te ratios (15-40) and extremely Pd- and Pt-depleted HSE patterns. These features are characteristic of high-degree partial melting residues. Their Os-187/Os-188 signatures are thus considered to be inherited from the partial melting event. This implies a Neoarchean (2.5-2.8 Ga, T-RD eruption) stabilisation of the Letlhakane mantle root and supports the Letlhakane mantle root being a westerly extension of the Zimbabwe cratonic root. The modally metasomatised peridotites contain BMS whose abundance significantly increases from the GPP to the GP and PP. The BMS-poor GPP are only slightly richer in Pt and Pd than the BMS-free SP but have similarly high Se/Te ratios. The BMS-rich GP and PP exhibit significant enrichments in Pt, Pd, Se, Te resulting in HSE-Se-Te signatures similar to that of the Primitive Upper Mantle (PUM). Addition of 0.001-0.05 wt.% metasomatic BMS +/- PGM (platinum group minerals, i.e., Pt-tellurides) to highly refractory residues, such as the Letlhakane SP, reproduce well the HSE-Se-Te systematics observed in the BMS-poor and BMS-rich modally metasomatised peridotites. In the GPP, the negligible addition of metasomatic BMS +/- PGM did not dist