Mixing Loops, Mixing Envelopes, and Scattered Correlations among Trace Elements and Isotope Ratios Produced by Mixing of Melts Derived from a Spatially and Lithologically Heterogenous Mantle
Mixing Loops, Mixing Envelopes, and Scattered Correlations among Trace Elements and Isotope Ratios Produced by Mixing of Melts Derived from a Spatially and Lithologically Heterogenous Mantle
复制标题
空间和岩性异质地幔熔体混合产生的混合环、混合包络线以及微量元素和同位素比率之间的分散相关性
DOI：
10.1093/petrology/egac092
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发表时间：
2022
影响因子：
3.9
通讯作者：
Liang, Yan
中科院分区：
文献类型：
--
作者：
Liang, Yan
混合已被广泛用于解释玄武岩的放射性同位素比值和高度不相容的微量元素变化的非均匀地幔熔融。通过考虑端元组分的质量平衡，建立了与地幔源区端元的物理状态和空间分布无关的二元混合模型。玄武岩中放射性同位素比值和高度不相容的微量元素的变化也取决于地幔源区化学和岩性不均匀性的大小和空间分布。在这里，我们提出了一个新的混合模型和混合计划，考虑到的大小，空间位置和熔融历史的富集地幔（EM）和亏损地幔（DM）包裹在熔融柱。我们展示了如何Sr，Nd和Hf的浓度和同位素比在聚合或汇集熔体收集在熔融柱的顶部作为一个功能的EM包裹在熔融柱的位置而变化。随着EM包裹在上涌熔融柱中位置的改变，汇集熔体的成分不遵循二元混合模型所期望的单一混合曲线。相反，它们定义了一个混合环，该混合环具有由组成空间中的两个端点连接的富集分支和耗尽分支。混合回路的起源可以追溯到熔融柱中的四种类型的EM分布或配置。混合回路的形状取决于EM与DM的相对熔化速率以及熔化柱中EM包裹的数量和间距。在汇集的熔体中富集和贫化的分支取样的概率与地幔源中富集和贫化物质的体积分数成正比。混合池熔体从一束熔融柱的结果在同位素比相关图中的混合信封。混合包线是研究幔源熔体化学变化的有效工具。作为应用，我们考虑了大洋中脊玄武岩中87 Sr/86 Sr与143 Nd/144 Nd和143 Nd/144 Nd与176 Hf/177 Hf的散射相关。我们表明，这种相关性自然产生的熔融的空间异质性地幔。
Mixing has been widely used in the interpretation of radiogenic isotope ratios and highly incompatible trace element variations in basalts produced by melting of a heterogeneous mantle. The binary mixing model is constructed by considering mass balance of endmember components, which is independent of physical state and spatial distribution of the endmembers in the mantle source. Variations of radiogenic isotope ratios and highly incompatible trace elements in basalts also depend on the size and spatial distribution of chemical and lithological heterogeneities in the mantle source. Here we present a new mixing model and a mixing scheme that take into account of the size, spatial location, and melting history of enriched mantle (EM) and depleted mantle (DM) parcels in the melting column. We show how Sr, Nd, and Hf concentrations and isotope ratios in the aggregated or pooled melt collected at the top of the melting column vary as a function of location of the EM parcel in the melting column. With changing location of the EM parcel in the upwelling melting column, compositions of the pooled melt do not follow a single mixing curve expected by the binary mixing model. Instead, they define a mixing loop that has an enriched branch and a depleted branch joined by two extreme points in composition space. The origin of the mixing loop can be traced back to four types of EM distribution or configuration in the melting column. The shape of the mixing loop depends on the relative melting rate of the EM to that of the DM and the number and spacing of EM parcels in the melting column. Probabilities of sampling the enriched and depleted branches in the pooled melt are proportional to volume fractions of the enriched and depleted materials in the mantle source. Mixing of pooled melts from a bundle of melting columns results in mixing envelopes in the isotope ratio correlation diagrams. The mixing envelope is a useful tool for studying chemical variations in mantle-derived melts. As an application, we consider scattered correlations in87Sr/86Sr vs.143Nd/144Nd and143Nd/144Nd vs.176Hf/177Hf in mid-ocean ridge basalts. We show that such correlations arise naturally from melting of a spatially heterogeneous mantle.