Hidden intact coesite in deeply subducted rocks

The stabilization of coesite is a diagnostic indicator of ultrahigh-pressure metamorphism and in many cases it implies that a rock has been subducted to a minimum depth of 80 km. Coesite typically occurs as rare relicts in rigid host minerals, but most commonly transforms into α-quartz pseudomorphs during exhumation. The abundance of coesite-bearing rocks in orogens worldwide is a contentious issue in the petrological community, despite evidence from numerical modeling that suggests that coesite formation should be a common geological process during ultrahigh-pressure metamorphism. This knowledge gap must be addressed to improve the understanding of the geological aspects of subduction-zone geodynamics. Here we report that minuscule coesites (<20 μm) occur as abundant inclusions in garnet-rich layers from the Italian Western Alps. The discovery of such intact inclusions may fill the gaps in the predicted and observed abundances of coesite worldwide. Through integrated approaches with resolutions down to the nano-scale, we show that these garnet-hosted inclusions are composed entirely of coesite. Our results suggest that common coesite-derived quartz pseudomorphs are less typical structures in ultrahigh-pressure metamorphic rocks and the minuscule coesite in many rocks may be overlooked because of its size. These findings open up new research directions for constraining the extent of deeply subducted rocks and their rheology.