Clumped-Isotope Thermometry of Carbonate Veins from the SAFOD Borehole

Abstract:

We present clumped-isotope and stable-isotope data from carbonate veins obtained from the San Andreas Fault Observatory at Depth (SAFOD) borehole. A number of models proposed to explain the apparent weakness of the San Andreas Fault (SAF) require fluids to be present in the fault zone. However, little is known about the presence, source(s), temperature, and migration pathways of these fluids. We investigate spatial trends in isotopic composition of veins within meters of two actively deforming strands of the SAF - the southern deformation zone (SDZ) and central deformation zone (CDZ). Two populations of veins are present based on the isotopic data. The first group of veins with calcite δ¹⁸O values < +15 ‰ (VSMOW) and δ ¹³C values > +1 ‰ (VPDB) are present in foliated siltstone and shale cataclasites from 3186 to 3194 meters MD and in sheared siltstones and sandstones of the CDZ from 3297 to 3301 meters MD. Clumped-isotope analyses for a subset of samples from this vein set indicate temperatures between 72 and 99 °C and calculated pore fluid δ¹⁸O values of -3.4 to +0.1‰ (VSMOW). A second group of veins with δ¹⁸O values between +17 and +25 ‰ (VSMOW) and δ¹³C values between +1 and -18 (VPDB) is present in the serpentinite-bearing SDZ from 3196 to 3197 meters MD and in siltstones from 3302 to 3310 meters MD. Veins in the SDZ record temperatures from 80 to 118 °C and calculated pore fluid δ¹⁸O values of -0.3 to +3.1‰ (VSMOW). Both vein populations record temperatures less than or within uncertainty of present-day borehole temperature of ca. 120 °C. We propose the first group of veins formed by precipitating from fluids charged with soil CO₂ or biogenic methane that flowed along preexisting diagenetic fracture networks or fractures formed early in the evolution of the SAF. The second group of veins precipitated from fluids charged with thermogenic methane near present-day ambient temperatures and localized in a ~50 meter wide zone of damage along the actively deforming strands of the SAF. Our findings are consistent with localization of fluid flow leading to enhanced circulation of relatively cool reducing (hydrocarbon-rich) aqueous fluids, extensive fluid-rock interaction, and precipitation of Fe-, Ni-, Mg-, and Cr-oxides, hydroxides, sulfides, and weak phyllosilicates that may contribute to the low-strength behavior of the SAF.