DUCTILE OPENING-MODE FRACTURE BY PORE GROWTH AND COALESCENCE DURING COMBUSTION ALTERATION OF SILICEOUS MUDSTONE

Abstract

Opening-mode fractures with blunt tips and large maximum apertures are characteristic of clinker that formed by combustion alteration of siliceous mudstone. These fractures are inferred to result from pore growth and coalescence, with initially circular pores inherited from the diatomaceous protolith. Circular pores grow preferentially in an en-echelon arrangement and coalesce to elongate pores and blunt-tipped fractures by thinning and rupture of bridges between pores. Coalescence of overlapping en-echelon pores causes fracture propagation in a zig-zag path that is considered indicative of significant inelastic deformation outside the immediate vicinity of the fracture tips. This process of fracture formation by void growth and coalescence is inferred to result from solution mass transfer and possible bulk melt movement during partial melting of clinker. Chemical mass transfer provides a mechanism for extensive inelastic deformation in the surrounding host rock concurrent with fracturing that is considered characteristic of ductile fracture. The preferred elongation of coalescing pores and local rupture of pore bridges is explained by a tensile sintering stress due to the thermodynamic tendency of the system for energy minimization of solid and liquid surfaces. It is suggested that ductile fracture processes that are accompanied by extensive inelastic deformation lead to opening-mode fractures with large apertures in a variety of crustal settings including metamorphic and magmatic systems as well as in reactive diagenetic environments, thus affecting fracture-controlled transfer of heat and mass and the rheology of the Earth's crust.