Direct evidence for upper mantle structure in the NW Pacific Plate : Microstructural analysis of a petit-spot peridotite xenolith
Direct evidence for upper mantle structure in the NW Pacific Plate : Microstructural analysis of a petit-spot peridotite xenolith
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DOI：
10.1016/j.epsl.2010.12.011
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发表时间：
2011-02
影响因子：
5.3
通讯作者：
Y. Harigane;T. Mizukami;T. Morishita;K. Michibayashi;N. Abe;N. Hirano
中科院分区：
文献类型：
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作者：
Y. Harigane;T. Mizukami;T. Morishita;K. Michibayashi;N. Abe;N. Hirano
晚中新世晚中新世西北太平洋板块上的碱性玄武质火山，起源于岩石圈底部。小斑点火山岩包围拉斑玄武岩，粗玄岩，辉长岩和地幔橄榄岩的碎片，提供了一个独特的窗口，整个俯冲洋岩石圈部分。我们在这里提供的第一个直接观察太平洋岩石圈的深部结构，使用显微结构分析的一个小点橄榄岩捕虏体。捕虏体是一种二辉橄榄岩，主要由粗粒和中等粒度的橄榄石，斜方辉石，单斜辉石，以及细粒度的尖晶石和斜方辉石的聚集体，可能代表更换的富含辉石的石榴石。一个强大的变形组构的标志是毫米大小的细长矿物和它们的晶体学优选取向的平行排列。橄榄石呈[010]纤维状，环带为[100]轴，最大值为[010]，垂直于叶理，这与大洋岩石圈底部高温条件下的压扭变形相一致。我们的显微构造观测和地震资料表明，西北太平洋岩石圈下部具有软流圈地幔流的早期结构。这一解释与传统的海洋岩石圈在冷却和板块对流过程中增厚的模型是一致的。弱各向异性的小点橄榄岩和强方位各向异性的地震数据在西北太平洋板块之间的差异意味着存在一个高度各向异性的组成部分，在深海岩石圈。
Petit-spots, the late Miocene alkali basaltic volcanoes on the Early Cretaceous NW Pacific Plate, originate at the base of the lithosphere. The petit-spot volcanic rocks enclose fragments of tholeiitic basalt, dolerite, gabbro, and mantle peridotite, providing a unique window into the entire section of subducting oceanic lithosphere. We provide here the first direct observations on the deep structure of the Pacific lithosphere using microstructural analyses of a petit-spot peridotite xenolith. The xenolith is a lherzolite that consists mainly of coarse- and medium-grained olivine, orthopyroxene, and clinopyroxene, as well as fine-grained aggregates of spinel and orthopyroxene that probably represent replaced pyrope-rich garnet. A strong deformational fabric is marked by a parallel alignment of millimeter-sized elongate minerals and their crystallographic preferred orientation. The olivine displays a [010] fiber pattern with a girdle of [100] axes and a maximum of [010] perpendicular to the foliation, a pattern which is consistent with a transpressional deformation in high temperature conditions at the base of oceanic lithosphere. Our microstructural observations and seismic data indicate that the lower part of the NW Pacific lithosphere possess an early stage structure of mantle flow at the asthenosphere. This interpretation is compatible with a conventional model in which oceanic lithosphere is thickened during cooling and plate convection. A discrepancy between the weak anisotropy in the petit-spot peridotite and the strong azimuthal anisotropy from the seismic data in the NW Pacific plate implies the existence of a highly anisotropic component in the deep oceanic lithosphere.