Mantle-melt Evolution (Dynamic Source) in the Origin of a Single MORB Suite: a Perspective from Magnesian Glasses of Macquarie Island
Mantle-melt Evolution (Dynamic Source) in the Origin of a Single MORB Suite: a Perspective from Magnesian Glasses of Macquarie Island
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DOI：
10.1093/petrology/43.10.1909
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发表时间：
2002-10
影响因子：
3.9
通讯作者：
Vadim Kamenetsky;R. Maas
中科院分区：
文献类型：
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作者：
Vadim Kamenetsky;R. Maas
利用西南太平洋中新世麦夸里岛蛇绿岩中的镁质玄武岩玻璃样品的地球化学和Sr-Nd-Pb同位素数据集，研究了源成分和源演化对洋中脊玄武岩（MORB）熔融物化学组成的影响。这些眼镜：（1）显示出清晰的母-子关系;（2）允许简单地重建初始熔体组成;（3）显示出异常的组成多样性（如K2O/TiO2 0.09-0.9; La/Yb 1.5-22; Pb-206/Pb-204 18.70-19.52）;（4）保存了与微量元素富集程度（如La/Sm）相关的常量元素和同位素组成的变化。MORB成因的传统模型调用地幔的熔融，是异质的小规模，其次是二元混合的亲石元素富集熔融批次。这种类型的模型无法解释的麦夸里岛玻璃的组成，主要是因为不相容的元素比（如Nb/U，Sr/Nd）和铅同位素比值的变化与富集程度的非系统性。我们建议，个别熔融批次产生的瞬时“父母”地幔共生体，不断变化的熔融和熔融提取的进行。这种“动力源”的概念结合了小尺度地幔不均匀性和部分熔融的模型。一个动力源是一个局部平衡地幔固体和相关的熔体部分组合。因此，整合了众多熔融批次特征的普通MORB岩浆往往掩盖了动态源的化学和同位素特征。这项研究表明，同位素比值差混合MORB熔体是一个复杂的功能的动态源演化，在一个单一的MORB套件内的同位素比值的范围并不一定需要不同的组件的混合。
The effects of source composition and source evolution during progressive partial melting on the chemistry of mantle-derived mid-ocean ridge basalt (MORB) melts were tested using a comprehensive geochemical and Sr-Nd-Pb isotopic dataset for fresh, magnesian basaltic glasses from the Miocene Macquarie Island ophiolite, SW Pacific. These glasses: (1) exhibit clear parent-daughter relationships; (2) allow simple reconstruction of primary melt compositions; (3) show exceptional compositional diversity (e.g. K2O/TiO2 0.09-0.9; La/Yb 1.5-22; Pb-206/Pb-204 18.70-19.52); (4) preserve changes in major element and isotope compositions, which are correlated with the degree of trace element enrichment (e.g. La/Sm). Conventional models for MORB genesis invoke melting of mantle that is heterogeneous on a small scale, followed by binary mixing of variably lithophile element-enriched melt batches. This type of model fails to explain the compositions of the Macquarie Island glasses, principally because incompatible element ratios (e.g. Nb/U, Sr/Nd) and Pb isotope ratios vary non-systematically with the degree of enrichment. We propose that individual melt batches are produced from instantaneous 'parental' mantle parageneses, which change continuously as melting and melt extraction proceeds. This concept of a 'dynamic source' combines the models of small-scale mantle heterogeneities and fractional melting. A dynamic source is an assemblage of locally equilibrated mantle solids and a related melt fraction. Common MORB magmas that integrate the characteristics of numerous melt batches therefore tend to conceal the chemical and isotopic identity of a dynamic source. This study shows that isotope ratios of poorly mixed MORB melts are a complex function of the dynamic source evolution, and that the range in isotope ratios within a single MORB suite does not necessarily require mixing of diverse components.