Plume impingement on the Siberian SCLM: Evidence from Re–Os isotope systematics

Abstract

We report Re–Os and platinum group element (PGE) systematics for a suite of 16 mantle peridotites from the Udachnaya (360Ma) and Obnazhennaya (160Ma) kimberlite pipes, Siberia. Xenoliths from these pipes bracket the thermal climax of the Siberian plume, which is represented by the emplacement of the ~250Ma Siberian Flood Basalts (SFBs). Thus, these xenoliths represent snapshots of the sub-continental lithospheric mantle (SCLM) before and after plume modification. Pre-plume Udachnaya peridotite xenoliths generally display unradiogenic Os-isotopes with respect to CI-chondrite (expressed by γOs, the percentage difference between the Os-isotope composition of a sample and the average chondrite composition; 187Os/188Os — 0.127), coupled with low [Pd/Ir]N, for both whole-rock and olivine mineral-fraction analyses. Such signatures are typical of an ancient depleted cratonic mantle that underwent melt extraction. The preservation of unradiogenic Os-isotope compositions (γOs −5 to −14), coupled with low (<0.4) 187Re/188Os ratios, provides robust melt extraction age estimates, ranging from ~3Ga to ~1.2Ga. This indicates that craton stabilization/growth events not only occurred during the Archean, but also extended into the Proterozoic. A number (4) of post-plume Obnazhennaya peridotites display 187Os/188Os ratios (>0.1292), which overlap the convecting mantle range. At first glance, these observations are in agreement with garnet chemistry data, which indicate that high-degrees of silicate-melt percolated through the lithosphere during the emplacement of the SFB. However, Obnazhennaya olivine mineral-separates display ‘depleted’ systematics (>Fo 92 and low [Pd/Ir]N), consistent with ‘pristine’ melt residues. We suggest that these Obnazhennaya xenoliths represent ‘newly formed’ residues associated with partial melts extracted from the impinging Siberian plume on the SCLM. During plume impingement, thermo-chemical erosion of the lithosphere is thought to be an important process, acting to remove the original depleted material. In the space created, new refractory residues may be able to infill this void; i.e., plume-subcretion. Importantly, two Obnazhennaya peridotites display very unradiogenic Os-isotope compositions (γOs −9 and −10) indicating that ancient depleted lithosphere is preserved after the emplacement of the SFB. Overall, we suggest that plume impingement may not be ubiquitous within the SCLM, leaving a lithosphere that is characterized by both its original depleted residues and newly formed, plume related residues.In addition, two samples from Obnazhennaya and one from Udachnaya, contain highly radiogenic 187Os/188Os ratios (γOs>+85). Such radiogenic values cannot be accounted for by kimberlitic or plume-related metasomatism. Instead, these samples reflect metasomatism from fluids that derived from a source characterized by long-term high 187Re/188Os, such as recycled subducted material (i.e., oceanic crust and sediments). These lithologies were introduced into the SCLM during craton growth events from ~1.8 to 3.0Ga. The identification of such samples reflects an important metasomatic source within the SCLM. Overall, these two suites of mantle peridotites reveal the complex evolution of the Siberian cratonic lithosphere, from birth in the Archean, followed by major depletion events during the Proterozoic, and ending with major tectonothermal perturbation during the impingement of the Siberian plume.