Carbonatite Versus Silicate Melt Metasomatism Impacts Grain Scale ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd Heterogeneity in Polynesian Mantle Peridotite Xenoliths

The Earth's upper mantle is isotopically heterogeneous over large lengthscales, but the lower limit of these heterogeneities is not well quantified. Grain scale trace elemental variability has been observed in mantle peridotites, which suggests that isotopic heterogeneity may be preserved as well. Recent advances in isotope ratio mass spectrometry enable isotopic analysis of very small samples (e.g., nanograms or less of analyte) while maintaining the precision necessary for meaningful interpretation. Here we examine four peridotite xenoliths-hosted in lavas from Savai'i (Samoa hotspot) and Tahiti (Societies hotspot) islands-that exhibit grain scale trace element heterogeneity likely related to trapped fluid and/or melt inclusions. To evaluate whether this heterogeneity is also reflected in grain scale isotopic heterogeneity, we separated clinopyroxene, orthopyroxene, and (in the most geochemically enriched xenolith) olivine for single-grain Sr-87/Sr-86 and Nd-143/Nd-144 analyses. We find, in some xenoliths, extreme intra-xenolith isotopic heterogeneity. For example, in one xenolith, different mineral grains range in Sr-87/Sr-86 from 0.70987 to 0.71321, with corresponding variability in Nd-143/Nd-144 from 0.512331 to 0.512462. However, not all peridotite xenoliths which display trace elemental heterogeneity exhibit isotopic heterogeneity. Based on coupled isotopic and trace element data (i.e., a negatively-sloping trend in Sr-87/Sr-86 vs. Ti/Eu), we suggest that carbonatitic metasomatism is responsible for creating the intra-xenolith isotopic heterogeneities which we observe. This carbonatitic component falls off the array defined in Sr-87/Sr-86-Nd-143/Nd-144 space by Samoa hotspot basalts, which suggests a second, distinct EM2 (enriched mantle II) component is present in the Samoa hotspot that is not readily recognized in erupted products, but is instead seen only in mantle peridotite xenoliths.