Abstract
Metamorphic reactions influence the evolution of the Earth’s crust in a range of tectonic settings. For example hydrous mineral dehydration in a subducting slab can produce fluid overpressures which may trigger seismicity. During reaction the mechanisms of chemical transport, including water expulsion, will dictate the rate of transformation and hence the evolution of physical properties such as fluid pressure. Despite the importance of such processes, direct observation of mineral changes due to chemical transport during metamorphism has been previously impossible both in nature and in experiment. Using time-resolved (4D) synchrotron X-ray microtomography we have imaged a complete metamorphic reaction and show how chemical transport evolves during reaction. We analyse the dehydration of gypsum to form bassanite and H2O which, like most dehydration reactions, produces a solid volume reduction leading to the formation of pore space. This porosity surrounds new bassanite grains producing fluid-filled moats, across which transport of dissolved ions to the growing grains occurs via diffusion. As moats grow in width, diffusion and hence reaction rate slow down. Our results demonstrate how, with new insights into the chemical transport mechanisms, we can move towards a more fundamental understanding of the hydraulic and chemical evolution of natural dehydrating systems.
Highlights
The study of metamorphism is underpinned by thermodynamics; that a system will tend towards a state of minimum energy and reach equilibrium with its environment[1, 2]
The use of 4D synchrotron X-ray microtomography provides new opportunities in the experimental investigation of metamorphism by allowing direct microstructural and mineralogical information to be gathered on the micron scale as a reaction proceeds
The X-ray microtomographic data have a voxel size of 1.3 μm which is sufficient to image growing grains in detail, and the contrast in absorption allows for segmentation of the evolving pore space as it is distinct from the solid phases
Summary
The overall porosity in the sample at this time is only 4.6%, indicating that efficient expulsion of H2O is achievable after a relatively small amount of reaction Solid volume changes during a transformation produce transient porosity and our datasets show that the transport of ions must occur via diffusion through the wide moats produced by the reaction itself (Fig. 3). There is a strong agreement between this previous dataset and the reaction rate observed under the experimental conditions of this study (see Supplementary equations and Supplementary Figure 4), highlighting the importance of pore fluid pressure as a rate–controlling parameter. The acceleration in grain growth coincides with the rapid increase in connectivity between 202–255 minutes (Fig. 2) because the excess pore fluid pressure, which slows the reaction, is able to dissipate. If the transient porosity is maintained during reaction, the early expulsion of fluids and slowing reaction rate suggest that greatest chance of seismicity is early in the reaction rather than at it maximum rate
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