Abstract
Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth’s internal dynamics. Element exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al–Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.
Highlights
Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth’s internal dynamics
We use a combination of back-scattered electron (BSE) imaging, high-resolution transmission electron microscopy (HR-TEM), focused ion beam (FIB) sectioning, and Nano-secondary ion mass spectrometer (SIMS) mappings to visualize dissolution of primary minerals resulting in the formation of solid amorphous material, element transport via the amorphous state in an interconnected, syn-metamorphic pore space, and precipitation of product phases by repolymerization of the amorphous material
Our observations demonstrate that the kinetics of these reactions is not limited by the solubility of the reactant in the fluid, as an amorphous solid material is formed directly from the depolymerized mineral
Summary
Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth’s internal dynamics Element exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth. We use a combination of back-scattered electron (BSE) imaging, high-resolution transmission electron microscopy (HR-TEM), focused ion beam (FIB) sectioning, and Nano-secondary ion mass spectrometer (SIMS) mappings to visualize dissolution of primary minerals resulting in the formation of solid amorphous material, element transport via the amorphous state in an interconnected, syn-metamorphic pore space, and precipitation of product phases by repolymerization of the amorphous material. Precipitation of the crystalline product by repolymerization of the amorphous material and the existence of an interconnected syn-reactive pore space further enhance mineral reaction rates
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