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<p>Intergranular coesite found in ultra-high pressure (UHP) eclogites provides critical insights into deep subduction and exhumation processes. However, its plastic deformation behavior remains not well understood. We report here the detailed deformation micro-ultramicrostructure of both intragranular and intergranular coesite, along with its surrounding minerals in UHP eclogites from the Yangkou Bay, Sulu UHP terrane. A combination analysis of electron backscatter diffraction (EBSD), focused ion beam (FIB), and transmission electron microscopy (TEM) was performed in this study. EBSD analysis reveals that retrograde quartz replacing both types of coesite displays palisade textures, whereas the quartz replacing intergranular coesite further develops bulging mosaic textures. TEM analysis shows coesite is nearly dislocation-free, while retrograde quartz exhibits abundant dislocations. The defect structures in retrograde quartz include free dislocations, dislocation tangles, and subgrain boundaries. The dislocation density, quantified from total dislocations, indicates that the retrograde quartz experienced plastic deformation under differential stress. Intragranular palisade quartz records 628–999 MPa differential stress, exceeding intergranular palisade values (517 MPa; 154–165 MPa with Brazil twins) and mosaic quartz (345–657 MPa). Lower differential stress in the intergranular coesite indicates grain boundary facilitated pressure release. Notably, the intergranular coesite is free of water-related defects; however, they were detected in the surrounding retrograde quartz. Based on these results, we proposed a “stress-controlled pressure vessel” model to explain the preservation mechanism of intergranular coesite, which can only survive among 2–3 omphacite grains during exhumation. In addition to a dry preservation environment, differential stress is a more important key factor governing the preservation of intergranular coesite. Our findings provide critical micro- to ultra-microstructural insights into the preservation mechanisms of this coesite type.</p>

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Microstructural investigations on the preservation mechanisms of intergranular coesite in UHP eclogite from Yangkou Bay, Sulu Region

  • 期刊论文

  • 发表日期：

    2026年3月25日

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  • Jinyu Zheng,
  • Tao Chen,
  • Junfeng Zhang,
  • Lu Wang,
  • Wenlong Liu,
  • Feng Shi,
  • Xiaohu Xuan,
  • Mingxing Gong,
  • Zhe Gong

摘要

Intergranular coesite found in ultra-high pressure (UHP) eclogites provides critical insights into deep subduction and exhumation processes. However, its plastic deformation behavior remains not well understood. We report here the detailed deformation micro-ultramicrostructure of both intragranular and intergranular coesite, along with its surrounding minerals in UHP eclogites from the Yangkou Bay, Sulu UHP terrane. A combination analysis of electron backscatter diffraction (EBSD), focused ion beam (FIB), and transmission electron microscopy (TEM) was performed in this study. EBSD analysis reveals that retrograde quartz replacing both types of coesite displays palisade textures, whereas the quartz replacing intergranular coesite further develops bulging mosaic textures. TEM analysis shows coesite is nearly dislocation-free, while retrograde quartz exhibits abundant dislocations. The defect structures in retrograde quartz include free dislocations, dislocation tangles, and subgrain boundaries. The dislocation density, quantified from total dislocations, indicates that the retrograde quartz experienced plastic deformation under differential stress. Intragranular palisade quartz records 628–999 MPa differential stress, exceeding intergranular palisade values (517 MPa; 154–165 MPa with Brazil twins) and mosaic quartz (345–657 MPa). Lower differential stress in the intergranular coesite indicates grain boundary facilitated pressure release. Notably, the intergranular coesite is free of water-related defects; however, they were detected in the surrounding retrograde quartz. Based on these results, we proposed a “stress-controlled pressure vessel” model to explain the preservation mechanism of intergranular coesite, which can only survive among 2–3 omphacite grains during exhumation. In addition to a dry preservation environment, differential stress is a more important key factor governing the preservation of intergranular coesite. Our findings provide critical micro- to ultra-microstructural insights into the preservation mechanisms of this coesite type.