Alteration history of mudrock samples from the San Andreas Fault Observatory at Depth (SAFOD) drill hole: a TEM-XRD study

Studying rock alteration in active fault zones is key to understanding fault history and the interaction with mechanical, hydrological and mineralogical processes. In the SAFOD mudrocks at 3064 and 3436 m depths, abundant clay mineral phases, including mixed-layered varieties, occur in the matrix. At present, little is known about the mechanism and timing of their formation in relation to the faulting process. Based on a detailed TEM-XRD study, a preliminary assessment of the alteration history of the clay-rich rock chips is made by studying mineral character, hydration behavior, fabric and textural relationships. The mudrocks contain detrital grains of chlorite, muscovite and biotite, along with a range of authigenic illite, illite-smectite, chlorite- smectite and smectite. The predominant clay minerals are small illite packets (20-30 nm thickness) of a 1Md polytype. The illite constitutes about 75 percent of the illite-smectite mixed layer particles, showing a long- range ordering (R greater than 3). These authigenic mineral phases are probably formed during the movement of aqueous fluids along permeable fractures and veins by dissolution-precipitation reactions, and partly at the expense of larger (about 150 nm thick), deformed, detrital packets. The chlorite-smectite mixed layered phases formed via similar mechanisms, but in this case by direct alteration of detrital chlorite and biotite grains. Preliminary DTA heat flow results from a whole rock mudstone indicate a total loss of about 6 percent of total volatiles (about 2 percent interlayer water, about 4 percent crystalline water), most of which occurs during the dehydroxylation of clays (400-600 C). The timing of authigenic mineral growth in the matrix assemblages is considered to be coeval or older than localized mineralization along open fractures. The fracture precipitations are notably smectite rich and can be taken as an indication of more recent low temperature fluids of varying composition or more extensive dissolution associated with enhanced fluid flow. That these surface precipitations show some alignment along these small scale fault planes indicates a direct relationship between slip and mineral growth.