Trace element fractionation and isotope ratio variation during melting of a spatially distributed and lithologically heterogeneous mantle
Trace element fractionation and isotope ratio variation during melting of a spatially distributed and lithologically heterogeneous mantle
复制标题
DOI：
10.1016/j.epsl.2020.116594
复制
发表时间：
2020-12
影响因子：
5.3
通讯作者：
Yan Liang
中科院分区：
文献类型：
--
作者：
Yan Liang
地幔中玄武岩源区具有化学和岩性的不均一性。在地幔减压熔融过程中，不同地幔源区的矿物模式不断变化，导致微量元素在熔融柱中不同岩性中的体积分配系数的时空变化，进而影响微量元素在部分熔融体和残余固体中的分馏。这个问题可以通过跟踪熔化柱中固体的运动来量化。本研究提出了一种新的熔融模型，可用于跟踪的空间和时间变化的矿物模式，熔融反应，体积分配系数，微量元素浓度在岩性不均匀的熔融柱。得到了一个含时完全分数熔化模型的简单解析解。新模型的基本特征阐明通过熔融的两个岩性地幔，包括斑点的斜方辉石丰富的岩性在上涌二辉橄榄岩地幔的案例研究;和应用程序的Sr-Nd-Hf同位素比值变化的玄武岩中大西洋中脊。部分熔融的两个岩性地幔的结果在大的时间变化不兼容的微量元素浓度和Sr-Nd-Hf同位素比值的汇集熔体。熔融柱中不同岩性的部分熔体混合产生富集和贫化熔体，在Sr-Nd-Hf同位素比值相关图中形成混合环。这些混合环旋转远离混合线定义的二元混合模型，是一个独特的功能熔化的空间异质性地幔。混合环的形成可以追溯到熔融柱中富集岩性单元的位置和间距。岩性不均匀性的作用是改变微量元素的体积分配系数从其值在二辉橄榄岩地幔中的新的值，在辉石地幔，这改变了微量元素在熔融柱中的亏损程度。通过岩性的不均匀性改变体积分配系数随时间和空间的变化，可以导致更大的变化，在Sr-Nd-Hf同位素比值和高度不相容的微量元素浓度在汇集熔体。研究结果为系统研究空间分布和岩性不均匀地幔减压熔融过程中微量元素分馏和同位素比值变化提供了框架。
The source region of basalts in the mantle is chemically and lithologically heterogeneous. During decompression melting of a spatially distributed and lithologically heterogeneous mantle, mineral modes of distinct mantle sources vary continuously, resulting in spatial and temporal variations in the bulk partition coefficient of a trace element in different lithologies in the melting column, which in turn affects the fractionation of the trace element in partial melt and residual solid. This problem can be quantified by following the motion of solid in the melting column. This study presents a new melting model that can be used to keep track of spatial and temporal variations of mineral mode, melting reaction, bulk partition coefficient, and trace element concentration in the lithologically heterogeneous melting column. Simple analytical solutions for a time-dependent perfect fractional melting model are obtained. Essential features of the new model are elucidated through case studies of melting a two-lithology mantle that consists of blobs of orthopyroxene-rich lithology in the upwelling lherzolitic mantle; and an application to Sr-Nd-Hf isotope ratio variations in basalts from the Mid-Atlantic Ridge is presented. Fractional melting of the two-lithology mantle results in large temporal variations in incompatible trace element concentrations and Sr-Nd-Hf isotope ratios in the pooled melt. Mixing of fractional melts derived from different lithologies in the melting column produces enriched and depleted melts that form mixing loops in Sr-Nd-Hf isotope ratio correlation diagrams. These mixing loops rotate away from mixing lines defined by the binary mixing model and are a unique feature of melting a spatially heterogeneous mantle. Formation of the mixing loop can be traced to the location and spacing of the enriched lithological units in the melting column. The role of lithological heterogeneity is to change the bulk partition coefficient of a trace element from its values in the lherzolitic mantle to new values in the pyroxenitic mantle, which alters the extent of depletion of the trace element in the melting column. Changing bulk partition coefficient with time and space through lithological heterogeneity can result in greater variabilities in Sr-Nd-Hf isotope ratios and highly incompatible trace element concentrations in the pooled melt. Results from this study establish a framework for systematic studies of trace element fractionation and isotope ratio variation during decompression melting of a spatially distributed and lithologically heterogeneous mantle.