MOLYBDENUM ISOTOPE COMPOSITION OF SIBERIAN TRAPS BASALTS: IMPLICATIONS FOR SOURCE CONTRIBUTIONS

The Siberian Traps Large Igneous Province (STLIP) is the most voluminous continental magmatic event of the Phanerozoic. The STLIP has been linked to the Permian–Triassic mass extinction, and has produced the world-class Norilsk and Talnakh Cu–Ni–PGE magmatic sulphide ore deposits. Despite the numerous studies that have characterized the petrogenesis of the STLIP, questions surrounding the mantle source remain. To address this knowledge gap, we report new Sr, Nd, and Hf radiogenic isotopes, and Mo stable isotopes to characterize the mantle source of the STLIP. We characterized eleven volcanic formations (approximately 2 km of lava thickness), two sills, and three potential contaminants — anhydrite, coal, and black shale from the Norilsk region. The ⁸⁷Sr/⁸⁶Sr_(i) and ¹⁴³Nd/¹⁴⁴Nd_(i) data are consistent with previous work from the STLIP basalts, and can be interpreted as magma–crust interaction between a melt from a relatively depleted mantle source (εNd_(i) of ~4), and granitic and/or metasedimentary rock (εNd_(i) <- 15). In εNd–εHf space, the STLIP basalts plot along the terrestrial array.

The δ⁹⁸Mo (values normalized to NIST SRM3134) in the STLIP basalts range from -0.6‰ to 0.07‰ (mean=-0.17, 2sd=0.21, n=37). The Norilsk ore body has δ⁹⁸Mo ~ 0.0. The coupled Mo-Sr-Nd-Hf isotope compositions are inconsistent with assimilation of anhydrite (δ⁹⁸Mo = 0.35‰), coal (δ⁹⁸Mo = 0.06‰ to -0.15‰) or black shale (δ⁹⁸Mo= -0.08‰) due to the relatively low Mo concentrations of the later. δ⁹⁸Mo shows a weak correlation with crustal assimilation, assuming a continental crust with δ⁹⁸Mo = 0.15‰. However, the most radiogenic, high-Mg Gudchinchinski lavas, with high Gd/Yb ratios ~ 2.6 that imply a source within garnet stability field (>80 km depth), show a large range of δ⁹⁸Mo (-0.4 to 0.05‰), compared to the upper mantle (~-0.21 +/-0.06 ‰). When combined with the low Mo/Ce (<0.01-0.03) ratios of these lavas, we infer a subduction-modified source for the plume, that comprises a dehydrated eclogite with low δ⁹⁸Mo and Mo/Ce.