Mechanical twins form by the simple shear of the crystal lattice during deformation. In order to test the potential of narrow twins in monazite to record the timing of their formation, we investigated a ca. 1700 Ma monazite grain (from the Sandmata Complex, Rajasthan, India) deformed at ca. 980 Ma, by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atom probe tomography (APT). APT 208 Pb/ 232 Th ages indicate that the twin was entirely reset by radiogenic Pb loss during its formation at conditions far below the monazite closure temperature. The results are consistent with a model where Pb is liberated during rupture of rare earth element-oxygen (REE-O) bonds in the large [REE]O 9 polyhedra during twinning. Liberated Pb likely migrated along fast diffusion pathways such as crystal defects. The combination of a quantitative microstructural investigation and nanogeochronology provides a new approach for understanding the history of accessory phases.

中文翻译：

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独居石的机械孪晶排出放射性铅

机械孪晶在变形过程中通过晶格的简单剪切形成。为了测试独居石中窄双胞胎记录其形成时间的潜力，我们调查了一个约。1700 Ma 独居石颗粒（来自印度拉贾斯坦邦的 Sandmata Complex）大约在 980 Ma，通过电子背散射衍射 (EBSD)、透射电子显微镜 (TEM) 和原子探针断层扫描 (APT)。APT 208 Pb/ 232 Th 年龄表明，在远低于独居石闭合温度的条件下，双胞胎在其形成过程中因放射性铅损失而完全重置。结果与在孪晶过程中大 [REE]O 9 多面体中的稀土元素-氧 (REE-O) 键断裂期间释放 Pb 的模型一致。释放的 Pb 可能沿着快速扩散路径迁移，例如晶体缺陷。