Fluid-assisted chemical and mechanical resetting of garnet: from experiments to the Adula nappe

Dr. Xin Zhong, Freie Universität Berlin, Berlin

Abstract

Garnet is commonly regarded as a robust recorder of metamorphic pressure–temperature history because of its slow diffusion and high mechanical strength. In this study, we investigate fluid-assisted resetting of garnet across the Adula nappe, Central Alps, using an integrated approach combining Raman and FTIR spectroscopy, EPMA, EBSD, and numerical modelling. Across more than 50 samples, Zr-in-rutile thermometry records a gradual north-to-south temperature increase from ~500 to 700 °C, whereas entrapment pressures obtained from quartz-in-garnet elastic barometry drop abruptly from up to ~2.3 GPa in the north and middle to ~1 GPa in the south. This transition coincides with garnet chemical homogenization, enhanced OH signatures, and recrystallization microstructures. Controlled heating experiments further show that aqueous fluid strongly accelerates time-dependent stress relaxation and deformation in garnet. Together, these results demonstrate that fluid infiltration can simultaneously enhance chemical transport and mechanical relaxation, overprinting the high-pressure metamorphic record during retrogression.