Sr-Nd-Pb isotopic compositions of peridotite xenoliths from Spitsbergen: Numerical modelling indicates Sr-Nd decoupling in the mantle by melt percolation metasomatism

Abstract

Several spinel peridotite xenoliths from Spitsbergen have Sr-Nd isotopic compositions that plot to the right of the 'mantle array' defined by oceanic basalts and the DM end-member (depleted mantle, with low 87 Sr/86 Sr and high 143Nd/144 Nd). These xenoliths also show strong fractionation of elements with similar compatibility (e.g. high La/Ce), which cannot be produced by simple mixing of light rare earth element-depleted peridotites with ocean island basalt-type or other enriched mantle melts. Numerical simulations of porous melt flow in spinel peridotites applied to Sr-Nd isotope compositions indicate that these features of the Spitsbergen peridotites can be explained by chemical fractionation during metasomatism in the mantle. 'Chromatographic' effects of melt percolation create a transient zone where the host depleted peridotites have experienced enrichment in Sr (with a radiogenic isotope composition) but not in Nd, thus producing Sr-Nd decoupling mainly controlled by partition coefficients and abundances of Sr and Nd in the melt and the peridotite. Therefore, Sr-Nd isotope decoupling, earlier reported for some other mantle peridotites worldwide, may be a signature of metasomatic processes rather than a source-related characteristic, contrary to models that invoke mixing with hypothetical Sr-rich fluids derived from subducted oceanic lithosphere. Pb isotope compositions of the Spitsbergen xenoliths do not appear to be consistently affected by the metasomatism.