Peter Eichhubl^1,2, Atilla Aydin^1
1 Rock Fracture Project, Dept. of Geological and Environmental Sciences, Stanford Univ., Stanford, CA 94305
² Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039

Blunt-tipped opening-mode fractures with large aperture to length ratios are characteristic of clinker that formed during natural in situ combustion of hydrocarbons in siliceous mudstone under near-surface conditions. Combustion alteration resulted in decomposition of clays and biogenic opal-A in the diatomaceous mudstone and in the formation of high-temperature minerals such as tridymite, cordierite, and calcic plagioclase. Formation of high-temperature minerals was accompanied by a complete textural reorganization and the formation of >1 micrometer pores at the expense of sub-micron pores inherited from the diatomaceous protolith. Pore growth and coalescence led to elongate pores and blunt-tipped macroscopic fractures and to ultimate brecciation. Bulk density increases with increasing alteration from initially 0.88 g/cm³ to 1.27 g/cm³ in sintered oxidized mudstone and to 1.85 g/cm³ in clinker. The 31% difference in bulk density or specific volume between sintered oxidized mudstone and clinker approximately equals the fracture porosity of 25-30% of brecciated clinker. Based on the comparison between specific bulk volume decrease and amount of fracture opening and the lack of extensional structures in the surrounding unaltered mudstone we suggest that fracture opening resulted from contraction of clinker due to porosity reduction and formation of denser mineral phases during combustion alteration and partial melting. We envision a process similar to high-temperature sintering of ceramics and metal powder compacts causing contraction and porosity reduction of up to 20%. Contraction is driven by the thermodynamic tendency of the system to reduce the energy of liquid and solid surfaces and may result in a tensile sintering stress of 1-20 MPa. Opening-mode fractures similar to those seen in clinker form if the contracting body is prevented from contracting freely either by adhering to a non-contracting substrate or due to heterogeneous sintering.

It is suggested that tensile forces between contacting mineral grains and between mineral grains and pore fluid contribute to subcritical fracture formation under a wide range of metamorphic, magmatic, and diagenetic conditions.