Rock Fracture and Mass Transfer as Coupled Processes.

Peter Eichhubl, Atilla Aydin
Rock Fracture Project, Dept. of Geological and Environmental Sciences, Stanford Univ., Stanford, CA 94305

Fracture formation is associated with mass transfer in chemically reactive subsurface environments. In this presentation, we describe the formation of opening-mode fractures in fine-grained carbonate- and silica-cemented sedimentary rocks under shallow and intermediate burial diagenetic conditions. We also consider fracture formation in clinker associated with combustion-metamorphism of siliceous shale. In organic-rich carbonate-cemented siltstone, large-aperture fractures occur as septarian veins that form radial and concentric fracture arrays. The opening and propagation of these fractures is accompanied by dissolution of pore-filling carbonate in the siltstone and carbonate reprecipitation in the fracture space. We interpret that carbonate dissolution and reprecipitation is controlled by the decomposition of dispersed organic matter with increasing burial. In siliceous mudstone, the formation of large-aperture fractures is related to the dissolution of metastable opal-A and opal-CT and the reprecipitation of silica as fracture-filling quartz. In both diagenetic systems, the opening-displacement of large-aperture fractures appears to be accommodated by dissolution of pore-filling cement and consecutive pore collapse. Fracture opening in clinker is observed to correlate with the reduction in sub-micron porosity that is accompanied by formation of high-temperature minerals. Whereas solution mass transfer during carbonate and silica diagenesis takes place in an aqueous pore fluid, we infer that fracture formation in clinker is accompanied by diffusion and solution mass transfer in a partial melt and possible minor bulk melt movement.

In all three examples documented here we infer that chemical mass transfer is responsible for fracture opening in excess of elastic opening displacements that are characteristic of brittle fractures. Where mass transfer is highly effective and strains are large such as in clinker, mass transfer leads to ductile fracture by growth and coalescence of voids to macroscopic blunt-tipped fractures. We believe that these or related processes are likely to occur in sub-crustal depths and are responsible for the shape and localization of ascending magmatic bodies.