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 ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd analyses. We find, in some xenoliths, extreme intra-xenolith isotopic heterogeneity. For example, in one xenolith, different mineral grains range in ⁸⁷Sr/⁸⁶Sr from 0.70987 to 0.71321, with corresponding variability in ¹⁴³Nd/¹⁴⁴Nd 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 ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr-¹⁴³Nd/¹⁴⁴Nd 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.

中文翻译：

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碳酸岩与硅酸盐熔体交代作用影响波利尼西亚地幔橄榄岩捕虏体中晶粒尺度 87Sr/86Sr 和 143Nd/144Nd 的非均质性

地球上地幔在大尺度范围内具有同位素异质性，但这些异质性的下限尚未得到很好的量化。在地幔橄榄岩中已经观察到颗粒尺度的微量元素变异，这表明同位素异质性也可能被保留。同位素比质谱法的最新进展能够对非常小的样品（例如，纳克或更少的分析物）进行同位素分析，同时保持有意义的解释所需的精度。在这里，我们研究了四种橄榄岩捕虏体——它们位于来自 Savai'i（萨摩亚热点）和塔希提岛（社会热点）岛屿的熔岩中——它们表现出可能与被困流体和/或熔体包裹体有关的颗粒尺度微量元素异质性。为了评估这种异质性是否也反映在颗粒尺度同位素异质性中，⁸⁷ Sr/ ⁸⁶ Sr 和¹⁴³ Nd/ ¹⁴⁴ Nd 分析。我们在一些捕虏体中发现了极端的捕虏体内部同位素异质性。例如，在一种捕虏体中，⁸⁷ Sr/ ⁸⁶ Sr 的不同矿物颗粒范围为 0.70987 至 0.71321，相应的¹⁴³ Nd/ ¹⁴⁴ Nd 变化范围为 0.512331 至 0.512462。然而，并非所有显示微量元素异质性的橄榄岩捕虏体都表现出同位素异质性。基于耦合同位素和微量元素数据（即⁸⁷ Sr/ ⁸⁶Sr vs. Ti/Eu)，我们认为碳酸盐交代作用是造成我们观察到的捕虏体内同位素异质性的原因。这种碳酸岩组分从萨摩亚热点玄武岩的⁸⁷ Sr/ ⁸⁶ Sr- ¹⁴³ Nd/ ¹⁴⁴ Nd 空间中定义的阵列中脱落，这表明在萨摩亚热点中存在第二个不同的 EM2（富集地幔 II）组分，但不容易识别在喷发的产物中，但仅在地幔橄榄岩捕虏体中可见。