Carbonatite Versus Silicate Melt Metasomatism Impacts Grain Scale 87 Sr/ 86 Sr and 143 Nd/ 144 Nd Heterogeneity in Polynesian Mantle Peridotite Xenoliths
Carbonatite Versus Silicate Melt Metasomatism Impacts Grain Scale 87 Sr/ 86 Sr and 143 Nd/ 144 Nd Heterogeneity in Polynesian Mantle Peridotite Xenoliths
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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包体中的晶粒 87 Sr/ 86 Sr 和 143 Nd/ 144 Nd 异质性
DOI：
10.1029/2021gc009749
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发表时间：
2021
期刊：
影响因子：
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通讯作者：
Bizimis, M.
中科院分区：
文献类型：
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作者：
Byerly, Benjamin L.;Jackson, M. G.;Bizimis, M.
地球上地幔在大长度尺度上具有同位素异质性，但这些异质性的下限尚未得到很好的量化。在地幔橄榄岩中观察到了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。同位素比质谱分析的最新进展使得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——存在于萨瓦伊岛（萨摩亚热点）和塔希提岛（社会热点）的熔岩中——表现出颗粒尺度的微量元素异质性，可能与捕获的流体和/或熔体包裹体有关。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，我们分离了单斜辉石、斜方辉石和（在地球化学上最丰富的捕虏岩中）橄榄石进行单颗粒87Sr/86Sr和143Nd/144Nd分析。我们在一些捕虏体中发现了极端的捕虏体内同位素异质性。例如，在一种捕虏体中，87Sr/86Sr 中的不同矿物颗粒范围为 0.70987 至 0.71321，143Nd/144Nd 中的相应变化范围为 0.512331 至 0.512462。然而，并非所有表现出痕量元素异质性的橄榄岩捕虏体都表现出同位素异质性。基于同位素和微量元素的耦合数据（即 87Sr/86Sr 与 Ti/Eu 的负倾斜趋势），我们认为碳酸岩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。这种碳酸岩成分从萨摩亚热点玄武岩在 87Sr/86Sr-143Nd/144Nd 空间定义的阵列中脱落，这表明萨摩亚热点中存在第二种独特的 EM2（富集地幔 II）成分，该成分在喷发产物中不易识别，而仅在地幔橄榄岩捕虏体中可见。
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‐grain87Sr/86Sr and143Nd/144Nd analyses. We find, in some xenoliths, extreme intra‐xenolith isotopic heterogeneity. For example, in one xenolith, different mineral grains range in87Sr/86Sr from 0.70987 to 0.71321, with corresponding variability in143Nd/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 in87Sr/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 in87Sr/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.