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Interaction of Plate, Mantle, core and the magmatism in the earth's history

地球历史上板块、地幔、地核与岩浆作用的相互作用

基本信息

批准号：
12126202
负责人：
KAWAMURA Katsuyuki
金额：
$ 27.84万
依托单位：
Tokyo Institute of Technology
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research on Priority Areas
财政年份：
2000
资助国家：
日本
起止时间：
2000 至 2002
项目状态：
已结题

项目摘要

We made a fieldwork at Acasta granitic complex in Canada, which is the oldest igneous body in the world formed at 4 Ga. We revealed the sequence and timing of formation of igneous complex on the basis of zircon dating and geological evidences. We made a fieldwork also at Pilbara craton in Australia, which is one of the oldest granite-greenstone terrain formed at 2.8-3. 5 Ga. We investigated igneous activities both at mid-oceanic ridges and convergent margins at that age. Results indicate that Archean seawater included large amounts of carbon-dioxide, and it formed significant proportions of carbonate minerals in oceanic crust through hydrothermal activities. Such carbonated crust subducted at convergent margins carrying large amount of carbon into the deep interior. In addition, melt inclusion studies of 3.5 Ga komatiites from Zimbabwe revealed that plume mantle included lots of water at that time. We also performed high P-T experiments using Kawai-type apparatus in order to investigate the melting processes in the lower mantle. Melting phase relations both of pyrolite and basalt compositions were determined, and trace element partitioning were investigated. These experimental results suggest that CaSiO_3-perovskite plays predominant roles in the partitioning of trace elements in the lower mantle including melting process at the core-mantle boundary. The characteristic pattern of trace elements expected to be formed by melting at the bottom of the mantle is found in the xenolith clinopylroxenes in the plume mantle at Hawaii and south Pacific, suggesting that they originated from that region. We further performed ultra-high P-T experiments using laser-heated diamond anvil cell. We revealed that silica phase undergoes phase transition twice in the lower mantle, and α-PbO_2-type phase is stable at the lowermost mantle conditions.

我们在加拿大的阿里斯塔花岗杂岩体进行了野外考察，该杂岩体是世界上最古老的火成岩体，形成于4Ga。根据锆石定年和地质证据，揭示了岩浆杂岩形成的序列和时代。我们还在澳大利亚的皮尔巴拉克拉通进行了实地考察，这是最古老的花岗岩-绿岩地形之一，形成于2.8-3。5 Ga.我们调查了当时洋中脊和会聚边缘的火成岩活动。结果表明，太古代海水中含有大量的二氧化碳，并通过热液活动形成了大洋地壳中相当比例的碳酸盐矿物。这种碳酸盐地壳在会聚边缘俯冲，携带大量的碳进入深部。此外，对津巴布韦3.5Ga科马提岩的熔融包裹体研究表明，当时地幔柱中含有大量的水。为了研究下地幔的熔融过程，我们还利用Kawai型装置进行了高P-T实验。测定了地幔岩和玄武岩成分的熔融相关系，并研究了微量元素的分配。这些实验结果表明，CaSiO_3-钙钛矿在下地幔微量元素的分配中起着主导作用，包括核幔边界的熔融过程。在夏威夷和南太平洋地幔柱中的捕虏体单斜辉石中发现了可能由地幔底部熔融形成的微量元素特征模式，表明它们可能起源于该地区。我们进一步利用激光加热金刚石压砧进行了超高P-T实验。结果表明，硅相在下地幔中经历了两次相变，α-PbO_2型相在地幔最低层条件下是稳定的。