The crystallographic preferred orientation (CPO) of olivine, specifically the type-B characterized by c-axes aligned parallel to lineation and b-axes concentrated perpendicular to foliation, is essential for explaining the trench-parallel seismic anisotropy in the forearc regions of subduction zones. However, its origin remains a subject of ambiguity and controversy. In this study, we present experimental findings on the formation of a type-B olivine CPO through the dehydration of foliated serpentinite under a compressive stress at a pressure of 300 MPa and temperature of 700–750°C. Our results reveal a progressive evolution of olivine CPO, transitioning from a type-C fabric to a type-B fabric, with increasing grain size and dehydration level. The type-B CPO observed in coarse-grained olivine within fully dehydrated samples primarily arises from mechanisms involving anisotropic growth, grain rotation, and oriented coalescence of newly formed, small olivine grains following the decomposition of antigorite under a compressive stress. This study provides the first experimental evidence for a novel, low-temperature dynamic dehydration mechanism, in contrast to the mechanism of high-temperature plastic flow, for explaining the development of type-B olivine CPO in forearc regions. Hence, it contributes significantly to our understanding of the formation of olivine CPO with implications for seismic anisotropy in subduction zone forearcs.

中文翻译：

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弧前区叶蛇纹石动态脱水诱导橄榄石 B 型晶体择优取向

橄榄石的晶体择优取向 (CPO)，特别是 B 型橄榄石，其特征是 c 轴平行于线理排列，b轴垂直于叶理集中，对于解释俯冲带弧前区域的沟槽平行地震各向异性至关重要。然而，它的起源仍然是一个含糊不清和有争议的话题。在这项研究中，我们提出了通过叶状蛇纹岩在 300 MPa 压力和 700-750°C 温度的压应力下脱水形成 B 型橄榄石 CPO 的实验结果。我们的结果揭示了橄榄石 CPO 的逐步演变，从 C 型织物转变为 B 型织物，并且晶粒尺寸和脱水水平不断增加。在完全脱水样品中的粗粒橄榄石中观察到的 B 型 CPO 主要源于涉及各向异性生长、晶粒旋转以及叶蛇纹石在压应力下分解后新形成的小橄榄石晶粒的定向聚结的机制。这项研究为一种新颖的低温动态脱水机制（与高温塑性流动机制相反）提供了第一个实验证据，以解释弧前区域 B 型橄榄石 CPO 的发育。因此，它对我们了解橄榄石 CPO 的形成以及对俯冲带弧前地震各向异性的影响具有重要意义。