Imaging the textural evolution of gypsum through a metamorphic cycle

Metamorphic textures are often complex because they reflect a long and protracted history across a range of conditions. When these textures develop in settings where deformation also occurs it becomes increasingly difficult to accurately pull apart their spatiotemporal history.  

While experimental petrology and deformation studies have been instrumental in understanding metamorphic textures, traditional approaches only capture ‘before’ and ‘after’ states of metamorphic texture formation. This limitation has prevented direct observation of the dynamic evolution of internal reactions and processes within rocks as they occur throughout time. Fundamental questions remain unanswered, such as the spatial distribution of nucleation sites, the temporal evolution of growth patterns, and critically, how deformation influences these processes. Understanding these dynamics is particularly crucial along pressure-temperature-time (P-T-t) paths, where natural rocks preserve evidence of competing processes between textural inheritance and overprinting.

To address these questions, we conducted time-resolved microtomography (4D µCT) experiments on alabaster gypsum (Volterra, Italy) that cycled between dehydration, rehydration and dehydration to understand how complex metamorphic textures formed. We ran one experiment at hydrostatic confining conditions and another for comparison in a non-hydrostatic stress state. Initial results reveal differences between how the microstructures evolve in the two stress states. For the non-hydrostatic environment we have observed a sequence where any initial gypsum fabric is uniformly over-printed to form a foliated texture determined by the stress field, but the rehydration reaction occurs in both a localized and patchy fashion. This in-turn determines where any further dehydration occurs. These experiments give insight into how a rock texture can evolve through a prograde and retrograde metamorphism on a clockwise P-T-t path.