Elemental and Stable Isotope Chemistry of Cuttings and Core Samples From SAFOD Drill Hole

We have analyzed the major and minor element chemistry and stable isotope composition of samples from the SAFOD drill hole in order to document fluid-rock interactions in the San Andreas and associated fault zones. To date, we have analyzed samples from three sections of the drill hole that contain faults, mineralogical changes, drilling breaks, and/or anomalous gas shows. The elemental chemistry of samples from 10450 to 10530 ft interval (measured depth, MD) does not vary significantly. Only magnesium, yttrium, and barium increase and calcium decreases moderately down hole across this interval. The yttrium and barium variability is potentially due to variable contamination of barite-bearing mud in the cuttings. In samples from 10860 to 11000 ft (MD), iron, magnesium, and potassium increase and calcium decreases moderately down hole. In sample interval 11400 to 11540 ft (MD), iron increases and calcium decreases slightly down hole. The other major and minor elements in the XRF analyses do not vary much across these three intervals. These variations in elemental chemistry correspond to mineralogy changes across these intervals. Carbon and oxygen isotope values of carbonate veins and breccias from the 10450 and 11400 ft intervals range from 0 to +8 per mil (carbon) and 14 to 20 per mil (oxygen), consistent with carbonate precipitation from fluids that had isotopically exchanged with silicates. More work is being done to elucidate the origin of the fluid and the extent of fluid-rock interactions in the fault zones.