Abstract
To investigate the source of volatiles and their relationship to the San Andreas Fault System (SAFS), 18 groundwater samples were collected from wells near the Big Bend section of the SAFS in southern California and analyzed for helium and carbon abundance and isotopes. Concentrations of 4He, corrected for air-bubble entrainment, vary from 4.15 to 62.7 (×10−8) cm3STPg−1H2O. 3He/4He ratios vary from 0.09 to 3.52 RA (where RA=air 3He/4He), consistent with up to 44% mantle helium in samples. A subset of 10 samples was analyzed for the major volatile phase (CO2) — the hypothesized carrier phase of the helium in the mantle–crust system: CO2/3He ratios vary from 0.614 to 142 (×1011), and δ13C (CO2) values vary from −21.5 to −11.9‰ (vs. PDB).3He/4He ratios and CO2 concentrations are highest in the wells located in the Mil Potrero and Cuddy valleys adjacent to the SAFS. The elevated 3He/4He ratios are interpreted to be a consequence of a mantle volatile flux though the SAFS diluted by radiogenic He produced in the crust. Samples with the highest 3He/4He ratios also had the lowest CO2/3He ratios. The combined helium isotope, He–CO2 elemental relationships, and δ13C (CO2) values of the groundwater volatiles reveal a mixture of mantle and deep crustal (metamorphic) fluid origins. The flux of fluids into the seismogenic zone at high hydrostatic pressure may cause fault rupture, and transfer volatiles into the shallow crust.We calculate an upward fluid flow rate of 147mma−1 along the SAFS, up to 37 times higher than previous estimates (Kennedy et al., 1997). However, using newly identified characteristics of the SAFS, we calculate a total flux of 3He along the SAFS of 7.4×103cm3STPa−1 (0.33mol3Hea−1), and a CO2 flux of 1.5×1013cm3STPa−1 (6.6×108mola−1), ~1% of previous estimates. Lower fluxes along the Big Bend section of the SAFS suggest that the flux of mantle volatiles alone is insufficient to cause the super hydrostatic pressure in the seismogenic zone; however, results identify crustal (metamorphic) fluids as a major component of the CO2 volatile budget, which may represent the additional flux necessary for fault weakening pressure in the SAFS.
Published Version
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