On the microboudin paleopiezometer: Application to glaucophane in metachert from the Sanbagawa Belt, Wakayama, Japan

Although microboudin paleopiezometers using minerals have been proposed to estimate differential stress, they generally provide only relative stress values. In this study, we propose a method to determine both the magnitude of differential stress and its temporal evolution by re-evaluating the microboudin paleopiezometer. We focus on glaucophane in metachert from the Sanbagawa Belt in Wakayama, Japan. Thin sections were prepared parallel to the XZ plane. To efficiently measure the aspect ratios and interboudin gaps of several hundred glaucophane microboudins, we developed a Python-based image-analysis method. Quartz crystallographic fabrics and grain sizes were measured using EBSD. Among 253 analyzed glaucophane grains, 192 grains were boudinaged. The mean aspect ratios are 7.32 for boudinaged grains and 3.47 for intact grains. Quartz c -axis fabrics exhibit a Type I cross-girdle pattern with a mean grain size of 8.08 μm. We revised the previously proposed microboudin paleopiezometer to better satisfy its mechanical assumptions. By applying the quartz dynamic recrystallization grain-size paleopiezometer to the measured grain size, the differential stress is estimated to be 116 MPa and the corresponding shear stress 82 MPa. This shear stress corresponds to a glaucophane aspect ratio of 4.91. Using the revised microboudin paleopiezometer, the tensile strength of glaucophane is calculated to be 1611 MPa. Furthermore, assuming that the interboudin gap represents strain and that the tensile strength is constant, we reconstructed the temporal evolution of the differential stress. The stress–strain relationship indicates that deformation in the Sanbagawa Belt occurred under relatively low differential stress (~70 MPa) during the early stages of exhumation, followed by a rapid increase to ~120 MPa during the final stage, which abruptly terminated plastic flow. The total strain is estimated to be ~0.8. These results demonstrate that the glaucophane microboudin paleopiezometer provides a quantitative framework for reconstructing stress evolution in the subduction-related metamorphic belt. • A new LJ-T microboudin paleopiezometer is proposed and quantitatively validated. • Semi-automated Python image analysis enables rapid, reproducible boudin measurements. • Tensile strength of glaucophane is estimated for the first time (1611 MPa). • Differential stress in the Sanbagawa Belt increased from ~70 to ~120 MPa during exhumation. • The method reconstructs temporal stress evolution beyond conventional quartz piezometers.