Dehydration breakdown of antigorite and the formation of B-type olivine CPO
Dehydration breakdown of antigorite and the formation of B-type olivine CPO
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叶蛇纹石脱水分解及B型橄榄石CPO的形成
DOI：
10.1016/j.epsl.2013.11.025
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发表时间：
2014
影响因子：
5.3
通讯作者：
Megumi Matsumoto
中科院分区：
文献类型：
--
作者：
Takayoshi Nagaya;Simon R. Wallis;Hiroaki Kobayashi;Katsuyoshi Michibayashi;Tomoyuki Mizukami;Yusuke Seto;Akira Miyake;Megumi Matsumoto
橄榄岩形成的接触变质作用和脱水分解叶蛇纹石片岩从日本中部的Happo地区，显示出强烈的橄榄石晶体择优取向（O 1 CPO）。缺乏中尺度变形结构与入侵和缺乏显微结构的证据，塑性变形的neoblastic颗粒表明，橄榄石CPO在这方面没有形成固态变形的结果。相反，原来的叶蛇纹石的方向和新形成的橄榄石的方向之间的良好的对应关系意味着CPO形成叶蛇纹石后的橄榄石拓扑生长。在俯冲带环境中，片理状蛇纹岩被拖到叶蛇纹石不再稳定的深度，可能通过类似的过程形成Ol CPO。Happo Ol CPO具有垂直于线理和叶理内的强轴集中-通常称为B型Ol CPO。平行于洋沟的地震快方向在许多收敛边缘观察到，并与这些地区的地幔楔中的B型Ol CPO的存在是一致的。实验工作表明，B型CPO可以在含水条件下在相对高的应力下通过位错蠕变形成。然而，天然和实验室样品的特性与B型OI CPO之间存在一些差异。(1)一些天然样品的B型CPO的形成条件（应力和温度）超出了实验预测的范围。(2)在变形实验中，晶体学c轴方向上的滑移是重要的，但在B型CPO的自然例子中还没有观察到。(3)实验工作表明，水的存在下，无论是高剪切应力或相对较低的温度是必不可少的B型CPO的形成。这些条件最有可能在俯冲边界附近实现，但这些区域也与蛇纹石化有关，这会阻止形成强烈的橄榄石CPO模式。我们发现，B型Ol CPO可以形成作为一个结果，静态拓扑生长的橄榄石高温分解后的片理化蛇纹岩。这些结果解决了实验和自然的例子B-型CPO之间的差异，并显示需要重新思考的形成过程中的橄榄石CPO收敛的边缘。叶蛇纹石后橄榄石的拓扑生长可以解释推断的分布在地幔楔中的B型Ol CPO比位错蠕变更成功。
Peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from the Happo area, central Japan shows a strong olivine crystallographic preferred orientation (Ol CPO). The lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural evidence for plastic deformation of neoblastic grains suggest that olivine CPO in this area did not form as a result of solid-state deformation. Instead, the good correspondence between the original antigorite orientation and the orientation of the newly formed olivine implies the CPO formed by topotactic growth of the olivine after antigorite. Ol CPO is likely to develop by a similar process in subduction zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer stable. The Happo Ol CPO has a stronga-axis concentration perpendicular to the lineation and within the foliation—commonly referred to as B-type Ol CPO. Seismic fast directions parallel to the ocean trench are observed in many convergent margins and are consistent with the presence of B-type Ol CPO in the mantle wedge of these regions. Experimental work has shown that B-type CPO can form by dislocation creep under hydrous conditions at relatively high stresses. There are, however, several discrepancies between the characteristics of natural and laboratory samples with B-type Ol CPO. (1) The formation conditions (stress and temperature) of some natural examples with B-type CPO fall outside those predicted by experiments. (2) In deformation experiments, slip in the crystallographicc-axis direction is important but has not been observed in natural examples of B-type CPO. (3) Experimental work suggests the presence of H2O and either high shear stress or relatively low temperatures are essential for the formation of B-type CPO. These conditions are most likely to be achieved close to subduction boundaries, but these regions are also associated with serpentinization, which prevents strong olivine CPO patterns from forming. We show B-type Ol CPO can form as a result of static topotactic growth of olivine after high-temperature breakdown of foliated serpentinite. These results resolve the discrepancies between experimental and natural examples of B-type CPO and show the need to rethink the formation process of olivine CPO in convergent margins. Topotactic growth of olivine after antigorite can account for the inferred distribution of B-type Ol CPO in the mantle wedge more successfully than dislocation creep.