Trace element transport during dehydration processes in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts
Trace element transport during dehydration processes in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts
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DOI：
10.1016/s0012-821x(97)00018-6
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发表时间：
1997-04
影响因子：
5.3
通讯作者：
T. Kogiso;Y. Tatsumi;S. Nakano
中科院分区：
文献类型：
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作者：
T. Kogiso;Y. Tatsumi;S. Nakano
在上地幔P/T条件下，对天然斜长角闪岩进行脱水实验，以研究微量元素在俯冲洋壳岩石圈脱水过程中的迁移。在角闪岩脱水过程中，Pb、Nd和Rb比U、Th、Sm和Sr更容易被水溶液迁移。结果表明，俯冲洋壳的脱水作用可能导致U/Pb、Th/Pb和Sm/Nd比值的大幅度升高，而Rb/Sr比值的降低。这最终导致更高的Pb和Nd同位素比值，和较低的Sr同位素比值在俯冲洋壳比现在的MORB源地幔，在地幔中的隔离足够长的时间。因此，在一些洋岛玄武岩中观察到的非常高的Pb同位素比值，被称为HIMU，可以很容易地通过将古老的俯冲地壳纳入其地幔源来实现。然而，Sr和Nd同位素比值不能解释的大规模混合物的古老俯冲洋壳与亏损或原始地幔，但需要显着分馏的Nd-Sr比值在俯冲洋壳与地幔物质混合之前。可能的过程，以产生类似的HIMU的Sr和Nd同位素组成可能涉及下地幔条件下的再循环俯冲玄武岩地壳的部分熔融和原始地幔的部分熔融再施肥。
Dehydration experiments on natural amphibolite have been carried out under upper mantle P/T conditions, in order to examine transportation of trace elements during dehydration processes in the subducted oceanic lithosphere. Pb, Nd, and Rb are more readily transported by aqueous fluids during amphibolite dehydration than UTh, Sm, and Sr, respectively. The results indicates that the dehydration of subducted oceanic crust may result in large increases in U/Pb, Th/Pb and Sm/Nd ratios, and a decrease in Rb Sr ratios of subducted oceanic crust. This ultimately leads to higher Pb and Nd isotopic ratios, and lower Sr isotopic ratios in the subducted oceanic crust than the present MORB source mantle, given a sufficiently long periods of isolation in the mantle. It follows that the very high Pb isotopic ratios observed in some ocean island basalts, known as HIMU, can be readily achieved by incorporation of ancient subducted crust into their mantle source. However, Sr and Nd isotopic ratios cannot be explained by bulk mixture of the ancient subducted oceanic crust with depleted or primitive mantle, but require significant fractionation of Nd Sr ratios in the subducted oceanic crust before mixing with mantle material. Possible processes to produce Sr and Nd isotopic compositions similar to that of HIMU may involve partial melting of recycled subducted basaltic crust under lower mantle conditions and refertilization of primitive mantle by the partial melt.