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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Contamination and melt aggregation processes in continental flood basalts: constraints from melt inclusions in Oligocene basalts from Yemen
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Contamination and melt aggregation processes in continental flood basalts: constraints from melt inclusions in Oligocene basalts from Yemen

机译：大陆洪水玄武岩中的污染和熔体聚集过程：也门渐新世玄武岩中熔体包裹体的制约

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摘要

Melt inclusions from Oligocene continental flood basalts (CFB) erupted in Yemen provide unique insight into the timing and nature of the processes that lead to crustal contamination and melt aggregation in CFB magmas. Large variations in trace element indices that are sensitive to the degree and composition of assimilated crustal material (e.g. K_2O = 0.20-1.94 wt%, Ba = 13-543 ppm, K/Nb = 128-1603, Ba/Th = 9-303) are evident in many inclusions, even where these derive from the same host lava, and reflect the complexity of the processes that lead to contamination within individual CFB melting and melt-transport systems. The compositions of melt inclusions relate to differences in the degree of contamination, but in addition require that there is substantial heterogeneity in the composition of the contaminant material itself. Many inclusions also appear to contain more primitive melts than typical Yemen CFB lava compositions, and as such would be highly sensitive to addition of crustal materials. Overall melt inclusions provide a markedly better record of the diversity of melt compositions present within given CFB magma systems than the bulk compositions of erupted lavas. Highly contaminated melts (with K/Nb and Ba/Nb up to 957 and 22, respectively) trapped within fosterite-rich olivines (Fo_(85-90)) require very high rates of assimilation relative to crystal fractionation, with M_a/M_c values (the mass ratio of assimilated to crystallized material) 1. Such rapid assimilation may reflect decoupling of heat and mass transfer at the margins of larger magma chambers, within feeder dyke complexes, or at other sites where primitive magma is juxtaposed against wall-rocks that are already heated to temperatures near, or above, their solidus. In addition, relatively little assimilation appears to have occurred after crystallization of the phases that host melt inclusions, consistent with a thermal link between assimilation and phenocryst formation. Melt inclusions also show trace element variations related to mantle source compositions and mantle melting processes. Two inclusions with unusually Sr-rich and rare earth element-poor compositions are similar to those recognized from Mauna Loa, Hawaii, and may be related to melting of recycled gabbroic material within the upwelling Afar plume. In addition, many melt inclusions with crustally contaminated compositions also show large variations in trace element ratios that are essentially insensitive to crustal contamination (e.g. Zr/Y = 2-10) but are fractionated during progressive partial melting within an upwelling mantle column. The presence of both mantle and crustal-derived trace element signatures in the same inclusions demonstrates that melt transport systems in Yemen CFB were capable of transporting compositionally distinct melt batches, without complete mixing, through the asthenospheric and lithospheric mantle until final mixing and aggregation (and contamination) within crustal magma reservoirs. Thus, regardless of contamination, the ultimate compositions of many CFB lavas may be determined by magma mixing within the crust, rather than representing primitive compositions derived directly from the mantle.
机译：也门爆发的渐新世大陆洪水玄武岩（CFB）的熔体包裹体提供了导致CFB岩浆受到地壳污染和熔体聚集的过程的时间和性质的独特见解。对同化地壳物质的程度和组成敏感的微量元素指数的较大变化（例如K_2O = 0.20-1.94 wt％，Ba = 13-543 ppm，K / Nb = 128-1603，Ba / Th = 9-303在许多夹杂物中也很明显，即使这些夹杂物来自同一宿主熔岩，也反映出导致单个CFB熔融和熔体运输系统内污染的过程的复杂性。熔体夹杂物的组成涉及污染程度的差异，但另外还要求污染物材料本身的组成存在很大的异质性。与典型的也门CFB熔岩成分相比，许多夹杂物似乎还含有更多的原始熔体，因此对添加地壳材料非常敏感。总的熔体夹杂物提供了给定CFB岩浆系统内存在的熔体成分多样性的明显记录，比喷熔岩的整体成分更好。困在富含钙铁石的橄榄石（Fo_（85-90））中的高度污染的熔体（K / Nb和Ba / Nb分别高达957和22）需要相对于晶体分馏具有非常高的同化率，M_a / M_c值（同化物质与结晶材料的质量比） 1.这种快速同化可能反映出较大的岩浆室边缘，馈线堤复合体内或原始岩浆与壁并列的其他场所的热量和质量传递的解耦，已经加热到接近或高于其固相线温度的岩石。另外，在容纳熔体夹杂物的相结晶后，似乎很少发生同化，这与同化和异晶形成之间的热联系相一致。熔体包裹体还显示出与地幔源组成和地幔融化过程有关的微量元素变化。两种夹杂物具有异常丰富的Sr和稀土元素贫乏成分，类似于从夏威夷的莫纳罗阿（Mauna Loa）所识别的夹杂物，这可能与上升的Afar羽流中的再生辉长岩材料融化有关。另外，许多具有被壳污染的熔体夹杂物也显示出痕量元素比例的大变化，这些元素元素对地壳污染基本不敏感（例如Zr / Y = 2-10），但在上地幔柱内逐步进行部分熔融过程中被分馏。同一夹杂物中同时存在地幔和地壳痕量元素特征表明，也门CFB中的熔体传输系统能够通过软流层和岩石圈地幔传输组成完全不同的熔体批次，而无需完全混合，直到最终混合和聚集（和地壳岩浆储层中的污染）。因此，不管污染如何，许多CFB熔岩的最终成分可以通过地壳内的岩浆混合来确定，而不是代表直接来自地幔的原始成分。

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