High-pressure and temperature deformation experiments interfaced with acoustic emission (AE) monitoring have been conducted to study transformational faulting in MnGeO olivine, which transforms to the β phase, isostructural to wadsleyite. Metastable MnGeO olivine exhibits a marked embrittlement behavior at temperatures between 800 and 1100 K, emitting numerous AEs. At each temperature, brittle deformation is characterized by a two-stage process: (1) a “preparation” stage with numerous diffusedly located low-magnitude AEs and large values (>2), and (2) a failure stage where larger-magnitude AEs form a planar distribution with values about 1. Microstructure analysis reveals extensive kink band development in olivine grains in the recovered samples. Kink band boundaries (KBBs), with a typical thickness of ∼100 nm, are filled with a nanometric β-MnGeO “gouge”. A dense array of secondary shear localizations is often present within the kink bands, suggesting significant shear deformation therein. The combined observations suggest that faulting in metastable MnGeO olivine is a self-similar process, from grain-scale to the sample-scale. Both observed embrittlement behavior and the microstructure of metastable MnGeO olivine are essentially identical to those in MgGeO olivine we have reported previously, indicating that the physical mechanism of faulting in metastable olivine is insensitive to the specific crystallographic structure of the high-pressure phase. The low values (about 1) observed in the faulting process in our experiments are similar to those of deep focus earthquakes in cold subduction zones. Our observed mechanism explains deep focus seismicity in cold metastable mantle wedges, provided that the self-similarity assumption holds to geological scales.

中文翻译：

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Mn2GeO4 从橄榄石到瓦兹利石结构的转换断层：对深源地震物理机制的启示


高压和温度变形实验与声发射（AE）监测相结合，研究了 MnGeO 橄榄石的相变断层，它转变为 β 相，与瓦兹利石同构。亚稳态 MnGeO 橄榄石在 800 至 1100 K 的温度下表现出明显的脆化行为，发出大量 AE。在每个温度下，脆性变形都具有两个阶段的过程：(1) 一个“准备”阶段，具有大量分散分布的低幅度 AE 和大值 (>2)，以及 (2) 失效阶段，其中较大幅度的 AE AE 形成平面分布，其值约为 1。微观结构分析揭示了回收样品中橄榄石晶粒中广泛的扭结带发育。扭结带边界 (KBB) 的典型厚度为 ∼100 nm，充满了纳米级 β-MnGeO“凿痕”。扭结带内通常存在密集的二次剪切定位阵列，表明其中存在显着的剪切变形。综合观察结果表明，亚稳态 MnGeO 橄榄石中的断层是一个从晶粒尺度到样品尺度的自相似过程。观察到的亚稳 MnGeO 橄榄石的脆化行为和微观结构与我们之前报道的 MgGeO 橄榄石基本相同，表明亚稳橄榄石断层的物理机制对高压相的特定晶体结构不敏感。我们实验中观察到的断裂过程中的低值（约1）与冷俯冲带深源地震的低值相似。我们观察到的机制解释了冷亚稳地幔楔中的深焦点地震活动，前提是自相似性假设适用于地质尺度。