Os–Hf–Sr–Nd isotope and PGE systematics of spinel peridotite xenoliths from Tok,SE Siberian craton: Effects of pervasive metasomatism in shallow refractory mantle

Os–Hf–Sr–Nd isotopes and PGE were determined in peridotite xenoliths carried to the surface by Quaternary alkali basaltic magmas in the Tokinsky Stanovik Range on the Aldan shield. These data constrain the timing and nature of partial melting and metasomatism in the lithospheric mantle beneath SE Siberian craton. The xenoliths range from the rare fertile spinel lherzolites to the more abundant, strongly metasomatised olivine-rich (70–84%) rocks. Hf–Sr–Nd isotope compositions of the xenoliths are mainly within the fields of oceanic basalts. Most metasomatised xenoliths have lower ¹⁴³Nd / ¹⁴⁴Nd and ¹⁷⁶Hf / ¹⁷⁷Hf and higher ⁸⁷Sr / ⁸⁶Sr than the host basalts indicating that the metasomatism is older and has distinct sources. A few xenoliths have elevated ¹⁷⁶Hf / ¹⁷⁷Hf (up to 0.2838) and plot above the Hf–Nd mantle array defined by oceanic basalts.¹⁸⁷Os / ¹⁸⁸Os in the poorly metasomatised, fertile to moderately refractory (Al₂O₃ ≥ 1.6%) Tok peridotites range from 0.1156 to 0.1282, with oldest rhenium depletion ages being about 2 Ga. The ¹⁸⁷Os / ¹⁸⁸Os in these rocks show good correlations with partial melting indices (e.g. Al₂O₃, modal cpx); the intercept of the Al–¹⁸⁷Os / ¹⁸⁸Os correlation with lowest Al₂O₃ estimates for melting residues (～0.3–0.5%) has a ¹⁸⁷Os / ¹⁸⁸Os of ～0.109 suggesting that these peridotites may have experienced melt extraction as early as 2.8 Gy ago. ¹⁸⁷Os / ¹⁸⁸Os in the strongly metasomatised, olivine-rich xenoliths (0.6–1.3% Al₂O₃) ranges from 0.1164 to 0.1275 and shows no apparent links to modal or chemical compositions. Convex-upward REE patterns and high abundances of heavy to middle REE in these refractory rocks indicate equilibration with evolved silicate melts at high melt / rock ratios, which may have also variably elevated their ¹⁸⁷Os / ¹⁸⁸Os. This inference is supported by enrichments in Pd and Pt on chondrite-normalised PGE abundance patterns in some of the rocks. The melt extraction ages for the Tok suite of 2.0 to 2.8 Ga are younger than oldest Os ages reported for central Siberian craton, but they must be considered minimum estimates because of the extensive metasomatism of the most refractory Tok peridotites. This metasomatism could have occurred in the late Mesozoic to early Cenozoic when the Tok region was close to the subduction-related Pacific margin of Siberia and experienced large-scale tectonic and magmatic activity. This study indicates that metasomatic effects on the Re–Os system in the shallow lithospheric mantle can be dramatic.