The importance of melt extraction for tracing mantle heterogeneity
The importance of melt extraction for tracing mantle heterogeneity
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DOI：
10.1016/j.gca.2008.10.015
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发表时间：
2008-12
影响因子：
5
通讯作者：
A. Stracke;B. Bourdon
中科院分区：
文献类型：
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作者：
A. Stracke;B. Bourdon
对地幔岩和海洋玄武岩的大量同位素和微量元素研究表明，地球的地幔是不均匀的。海洋玄武岩的同位素变化表明，在全球和局部尺度上，大多数地幔源是由两种或两种以上组分组成的复杂组合，具有孤立的长期化学演化。在海洋玄武岩中观测到的同位素和高度不相容元素比值的范围通常被认为直接反映了它们的地幔源。因此，同位素阵列的端点代表了下伏地幔中不同组分的同位素组成，然后用于推断地幔非均质性的起源。本文提出了一个非均质地幔源的熔融模型，探讨了同位素和微量元素特征是如何以及在多大程度上从源传递到熔体的。我们使用最近的辉石岩和橄榄岩的熔融和划分实验约束来模拟含辉石岩的橄榄岩的熔融。特定辉石岩熔融特征的识别使辉石岩熔体的指纹化，并确认了地幔岩性非均质性的重要性。模式结果以及对所选MORB和OIB组（如东太平洋隆起、冰岛、特里斯坦达库尼亚、高夫和圣赫勒拿岛）微量元素同位素系统的计算与观测结果的比较进一步表明，不同源组分的相对丰度、固相温度的差异，特别是程度等因素，对不同源组分的富集程度有显著影响。熔体聚集方式和深度范围从根本上影响了关键微量元素和同位素比值（如Ba/Th、La/Nb、Sr/Nd、La/Sm、Sm/Yb、143Nd/144Nd）的关系。这样做的原因是，地幔中存在的任何非均质性都是平均的，或者，根据熔体混合过程的有效性，甚至在熔融和熔体提取过程中都是均匀的。因此，在喷发的熔体中反映地幔非均质性的程度不仅是来源和熔融引起的变异性的函数。它还取决于熔化和熔体萃取过程中的混合程度，因此在很大程度上取决于所考虑的元素的相对不相容性。喷发熔体中所观察到的微量元素变化可能大于或小于其地幔源，这取决于所研究元素的不相容性。另一方面，喷发熔体的同位素变异性通常小于其地幔源。因此，熔体萃取过程中的熔体混合对相对变化程度有重要影响，从而影响同位素与微量元素比率之间的相关性程度。总的来说，当熔体在一个有限的深度范围内提取时，如海洋岛屿玄武岩的情况，与在更大的深度范围内提取熔体（洋中脊，特别是像冰岛这样以脊为中心的热点）的情况相比，预计微量元素和同位素比率之间的相关性更低。虽然最富集的熔体的同位素组成可能与富集的源组分的同位素组成非常接近，但即使是最富集的洋中脊玄武岩也可能低估了富集的地幔组分的同位素枯竭。这些观测结果表明，用在海洋玄武岩中观测到的化学和同位素范围来直接代表相应地幔源的化学和同位素范围可能会产生误导，因为这种假设只对均匀地幔源严格成立。除了确定来自不同地幔源的熔体的来源或分区相关差异外，还可以推断出…
Numerous isotope and trace element studies of mantle rocks and oceanic basalts show that the Earth’s mantle is heterogeneous. The isotopic variability in oceanic basalts indicates that most mantle sources consist of complex assemblages of two or more components with isolated long-term chemical evolution, on both global and local scales. The range in isotope and highly incompatible element ratios observed in oceanic basalts is commonly assumed to directly reflect that of their mantle sources. Accordingly, the end-points of isotope arrays are taken to represent the isotopic composition of the different components in the underlying mantle, which is then used to deduce the origin of mantle heterogeneity. Here, a melting model for heterogeneous mantle sources is presented that investigates how and to what extent isotope and trace element signatures are conveyed from source to melt. We model melting of a pyroxenite–bearing peridotite using recent experimental constrains for melting and partitioning of pyroxenite and peridotite. Identification of specific pyroxenite melting signatures allows finger-printing of pyroxenite melts and confirm the importance of lithological heterogeneity in the Earth’s mantle. The model results and the comparison of the calculated and observed trace element–isotope systematics in selected MORB and OIB suites (e.g. from the East Pacific Rise, Iceland, Tristan da Cunha, Gough and St.Helena) further show that factors such as the relative abundance of different source components, their difference in solidus temperature, and especially the extent, style and depth range of melt aggregation fundamentally influence the relationship between key trace element and isotope ratios (e.g. Ba/Th, La/Nb, Sr/Nd, La/Sm, Sm/Yb,143Nd/144Nd). The reason for this is that any heterogeneity present in the mantle is averaged or, depending on the effectiveness of the melt mixing process, even homogenized during melting and melt extraction. Hence to what degree mantle heterogeneity is reflected in the erupted melts is not only a function of source and melting-induced variability. It also depends on the extent of mixing during melting and melt extraction and thus strongly on the relative incompatibility of the elements considered. The observed trace element variation in erupted melts can be greater or smaller than that of their mantle sources, depending on the incompatibility of the elements investigated. The isotopic variability in erupted melts, on the other hand, is generally smaller than that of their mantle source. Melt mixing during melt extraction consequently has an important influence on the relative extent of variation, and hence the degree of correlation between the isotope and trace element ratios. Overall fewer correlations between trace element and isotope ratios are expected whenever melts are extracted from a restricted depth range, as is the case for ocean island basalts, than for cases where melts are extracted over a larger depth interval (mid ocean ridges and especially ridge centered hotspots like Iceland). While the isotopic composition of the most enriched melts may correspond closely to those of the enriched source component, even the most depleted mid ocean ridge basalts are likely to underestimate the isotopic depletion of the depleted mantle component. These observations imply that using the chemical and isotopic range observed in oceanic basalts as directly representative of that in the corresponding mantle source can be misleading, since this assumption is strictly true only for homogeneous mantle sources. In addition to identifying source or partitioning-related differences in melts from different mantle sources, inferring the …