Abstract 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.

中文翻译：

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空间分布和岩性非均质地幔熔融过程中的微量元素分馏和同位素比变化

摘要 地幔玄武岩的源区具有化学和岩性非均质性。空间分布、岩性非均质的地幔在减压熔融过程中，不同地幔源的矿物模式不断变化，导致熔融柱内不同岩性微量元素的体分配系数发生时空变化，进而影响分馏过程。部分熔体和残余固体中的微量元素。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，可用于跟踪岩性非均质熔融柱中矿物模式、熔融反应、体积分配系数和微量元素浓度的时空变化。获得了时间相关的完美分数熔化模型的简单解析解。新模型的基本特征通过对两个岩性地幔的熔化案例研究来阐明，该地幔由上涌的红长石地幔中富含斜方辉石的岩性团块组成；并介绍了大西洋中脊玄武岩中 Sr-Nd-Hf 同位素比变化的应用。两种岩性地幔的部分熔融导致不相容的微量元素浓度和汇集熔体中 Sr-Nd-Hf 同位素比率的巨大时间变化。在熔化塔中混合来自不同岩性的部分熔体会产生富集和贫化熔体，在 Sr-Nd-Hf 同位素比相关图中形成混合回路。这些混合回路旋转远离由二元混合模型定义的混合线，是熔化空间异质地幔的独特特征。混合回路的形成可以追溯到熔融柱中富集岩性单元的位置和间距。岩性非均质性的作用是将微量元素的体分配系数从其在红岩地幔中的值改变为辉石岩地幔中的新值，从而改变熔融柱中微量元素的消耗程度。通过岩性异质性随时间和空间改变体积分配系数会导致 Sr-Nd-Hf 同位素比的更大变化和汇集熔体中高度不相容的痕量元素浓度。