RE-OS, SM-ND, AND RB-SR ISOTOPE EVIDENCE FOR THICK ARCHEAN LITHOSPHERIC MANTLE BENEATH THE SIBERIAN CRATON MODIFIED BY MULTISTAGE METASOMATISM

A suite of peridotite xenoliths from kimberlites intruding the Siberian craton indicate the pressence of lithospheric mantle over 150 km thick at 350 Ma. We report Sr-Nd isotope data for minerals from the peridotite xenoliths together with osmium isotopic compositions for whole-rocks and two olivine separates. Additionally, the osmium isotopic composition of a carbonatite from Fort Portal, Uganda, has been measured in order to evaluate the effect of carbonatite metasomatism on mantle Re-Os systematics. Osmium isotope compositions of peridotite xenoliths from the Mir and Udachnaya kimberlites vary from those characteristic of the oceanic mantle, to considerably less radiogenic values (Os-187/Os-188, 0.16469 to 0.10812), comparable to those previously found in other crationic peridotites. In contrast, two eclogite xenoliths from Udachnaya have extremely radiogenic Os, Os-187/Os-188, up to 9.67. The lowest peridotite osmium isotopic compositions require Re depletion in the mid-Archaean (3.2 Ga) and this age is interpreted as the time of differentiation of the Siberian cratonic lithospheric mantle. Archaean depletion ages for spinel peridotites of relatively shallow origin and garnet peridotites and dunites containing diamond indicate that the depleted lithosphere reached from the Moho to 150 Km depth at this time and has been stable for 3 Ga. Re-Os and Sm-Nd model ages for two eclogite xenoliths are also in the range of 2.7 to 3.1 Ga and support an ancient origin for the Siberian lithosphere. The oldest peridotite depletion ages and the eclogite model ages correspond to the oldest crustal ages obtained from the Anabar Shield of the Siberian craton, and suggest that the initiation of major crust formation and stabilisation of a thick cratonic keel were coeval. In general, the Siberian low-temperature peridotites are not as enriched in incompatible elements as those from the Kaapvaal craton yet their diopsides possess similar, low Sm/Nd. The low incompatible element concentrations but LREE/MREE enrichment seen in some Siberian lherzolites suggest they may be the products of disequilibrium melting. Neodymium and strontium isotopic compositions of minerals from the peridotites are extremely heterogeneous (epsilon(Nd(350)), -55.1 to 491; Sr-87/Sr-86, 0.70253 to 0.72235). Subcalcic garnets of diamond inclusion-like composition within megacrystalline peridotites have epsilon(Nd(350)) values varying from -55.1 to -12.1. Depleted mantle model Nd ages are as old as 3.2 Ga permitting an ancient, enriched origin similar to that suggested for diamond inclusions (Richardson et al., 1984). Alternatively, consideration of the complex garnet Sm-Nd isotope systematics and the presence of unsupported radiogenic Sr together with marked trace element zonation (Shimizu et al., 1994) suggest that these subcalcic garnets crystallised recently (close to the time of kimberlite eruption) from ancient, LREE-enriched, high Rb/Sr precursors. We propose that the isotope systematics of subcalcic garnet diamond inclusions can also be interpreted in terms of a recent origin.