Geochemistry of ultrahigh-pressure anatexis: fractionation of elements in the Kokchetav gneisses during melting at diamond-facies conditions
Geochemistry of ultrahigh-pressure anatexis: fractionation of elements in the Kokchetav gneisses during melting at diamond-facies conditions
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DOI：
10.1007/s00410-014-1002-x
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发表时间：
2014-04
影响因子：
3.5
通讯作者：
A. Stepanov;J. Hermann;A. Korsakov;D. Rubatto
中科院分区：
文献类型：
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作者：
A. Stepanov;J. Hermann;A. Korsakov;D. Rubatto
哈萨克斯坦的Kokchetav杂岩含有含石榴子石的片麻岩，这些片麻岩是在超高压（UHP）条件下由变质沉积岩部分熔融形成的。部分熔融和熔融提取从这些岩石的记录减少K2 O和FeO + MgO的restites的增加。Kokchetav超高压交代岩最具特征的微量元素特征是轻稀土元素（LREE）、Th和U的强烈亏损。这归因于独居石/褐帘石在熔体中的完全溶解和熔体提取的可变程度。与此相反，锆浓度保持近似恒定的所有片麻岩。利用实验测定的LREE和Zr在高压熔体中的溶解度，这些数据将熔化温度限制在约1，000 °C。大离子亲石元素（LILE）只有中度亏损的样品中，具有最低的U，Th和LREE含量，表明多硅白云母保留一些LILE的残留物。有些残橄岩显示Nb/Ta相对于原岩的增加。这进一步表明存在多硅白云母，与金红石相比，多硅白云母优先结合Nb而不是Ta。在Kokchetav片麻岩的超高压深熔过程中观察到的微量元素分馏是显着不同的低压restites，一般没有LREE和Th亏损的亏损报告。在超高压条件下熔化导致Sm/Nd比的增加，并在restite的Sm-Nd和Lu-Hf系统的去耦。这种残橄岩的进一步俯冲和与地幔岩的混合可能导致不同于大块陆壳的独特同位素储集层。
The Kokchetav complex in Kazakhstan contains garnet-bearing gneisses that formed by partial melting of metasedimentary rocks at ultrahigh-pressure (UHP) conditions. Partial melting and melt extraction from these rocks is documented by a decrease in K2O and an increase in FeO + MgO in the restites. The most characteristic trace element feature of the Kokchetav UHP restites is a strong depletion in light rare earth elements (LREE), Th and U. This is attributed to complete dissolution of monazite/allanite in the melt and variable degree of melt extraction. In contrast, Zr concentrations remain approximately constant in all gneisses. Using experimentally determined solubilities of LREE and Zr in high-pressure melts, these data constrain the temperature of melting to ~1,000 °C. Large ion lithophile elements (LILE) are only moderately depleted in the samples that have the lowest U, Th and LREE contents, indicating that phengite retains some LILE in the residue. Some restites display an increase in Nb/Ta with respect to the protolith. This further suggests the presence of phengite, which, in contrast to rutile, preferentially incorporates Nb over Ta. The trace element fractionation observed during UHP anatexis in the Kokchetav gneisses is significantly different from depletions reported in low-pressure restites, where generally no LREE and Th depletion occurs. Melting at UHP conditions resulted in an increase in the Sm/Nd ratio and a decoupling of the Sm–Nd and Lu–Hf systems in the restite. Further subduction of such restites and mixing with mantle rocks might thus lead to a distinct isotopic reservoir different from the bulk continental crust.