Porosity evolution determines available reaction mechanisms in water-rock interactions

Fluid-aided mineral replacement plays a key role in metamorphic reactions and metasomatic mass transfers. Within the scope of this study we investigate the role of pore space evolution in the hydrothermal phase transition from gypsum to bassanite. We monitored the partial dehydration of gypsum to bassanite in-situ using an X-ray-transparent flow-through reaction cell (Kahl et al., 2016) during long-term hydrothermal percolation experiments. By repeated, intermittent X-ray microtomography (µ-CT) scans, we surveyed the evolution of the porous system created by the dissolution-reprecipitation process. The quantitative 3D image analysis of the obtained 4D image material shows that incipient bassanite formation takes place in domains well within the interior of the gypsum crystal, presumably located along nano- or microcracks (i.e. at the scale of grain boundaries of the selenite host, and maybe enhanced due to the presence of fluid inclusions), and not directly at the gypsum – fluid interface. After larger volumes of interconnected pores have formed in later stages of the experiment, bassanite nucleation becomes insignificant and bassanite growth is now the dominant fixation mechanism of hemihydrate. The fabric of the final reaction product is controlled by these later-stage elongate bassanite crystals that are oriented along [001] of the former gypsum crystal. Careful data analysis reveals that processes which strongly depend on transient characteristics of the fluid-hosting fabric component are easily obliterated from the rock record. Concerning the predictive numerical simulation of mineral replacement processes in general, these results reveal that the boundary conditions underlying mineral nucleation may not be deduced from observations of the resulting fabric of mineral growth.

 

Kahl, W.-A., Hansen, C., and Bach, W. (2016) A new X-ray-transparent flow-through reaction cell for a mu-CT-based concomitant surveillance of the reaction progress of hydrothermal mineral-fluid interactions. Solid Earth, 7(2), 651-658.