Sr and Nd Isotope Decoupling in the Mantle Xenoliths - Evidence for Carbonatitic Fluid/Melt Percolation Metasomatism

Garnet and spinel peridotite and pyroxenite xenoliths from three upper Cretaceous basanitic explosion pipes in the Minusa region, southern Siberia have been studied to characterize off-cratonic upper mantle near the southwestern boundary of the Siberian craton. A lithospheric mantle section was constructed based on EMP and LA-ICP-MS analyses of minerals in representative xenoliths (Malkovets et al., 2000). Here we report results of a Sr-Nd isotope and trace-element study of metasomatized spinel-peridotite xenoliths from Kongarov and Krasnoozersk basanitic explosion pipes (Malkovets et al., 2001). Some of those xenoliths plot off the Sr-Nd mantle array and show strong fractionation of elements with similar compatibility. We argue that those features can be explained by chromatographic effects of carbonatitic melt/fluid percolation in the peridotite mantle. The contents of compatible to moderately incompatible elements in minerals of peridotites indicate that most of those rocks are residues after partial melting and melt extraction. The initially depleted xenoliths bear a record of later metasomatic events that produced enrichments in the incompatible-trace-element concentrations of pyroxenes. Trace-element compositions of clinopyroxene define two principal enrichment patterns. Type-I Cpx is characterized by very-high La/Ce and La/Nd ratios and lower concentrations of the middle REE and Sr, relative to Type-II Cpx. On primitive mantle-normalized, trace-element-distribution diagrams, Type-I Cpxs have nearly flat HREE-MREE patterns, with moderate depletions from Eu to Nd, and a steep La-Ce inflection. Type-II Cpx has lower HREE concentrations, with a continuous increase in normalized REE concentrations from Ho to Ce. Both types of Cpx have negative Ti, Zr, Hf, and strong negative Nb anomalies, in addition to small to moderate positive Sr anomalies. Sr-Nd isotopic compositions of mineral separates from the xenoliths indicate an unusual type of enrichment, with Sr decoupled from Nd. This results in Type-I Cpx having high ⁸⁷Sr/⁸⁶Sr but low ¹⁴³Nd/¹⁴⁴Nd values, thereby resulting in displacement of the data points to the right of the mantle array on Sr-Nd isotopic plots. Formation of the two groups of Cpxs may be explained by the chromatographic effect during the percolation of carbonatitic melts/fluids through the peridotitic substratum. The chromatographic models imply the selective removal of elements with high Cpx/melt distribution coefficients, e.g., HREEs from a percolating melt. Those elements would interact with the Cpx from peridotite resulting in the progressive enrichment of the melt in incoherent elements at the percolation front. Since the cpx/melt K_D for Sr is lower than that for Nd, the front of Sr enrichment moves faster than the front of Nd enrichment. Thus, a zone can be formed in which the host peridotite has more radiogenic Sr. This would explain the position of the data points to the right of the mantle trend in the Sm-Nd isotopic plot. Type-I Cpx forms at the melt-percolation front. However, Type-II Cpx forms at later stages of the same metasomatic event and reflects larger degrees of the equilibrium with the percolating carbonatitic fluids/melts.