Helium isotope studies in the Mojave Desert, California: implications for groundwater chronology and regional seismicity

We report helium isotope and concentration results for groundwaters from the western Mojave River Basin (MRB), 130 km east of Los Angeles, CA. The basin lies adjacent to the NW–SE trending San Andreas Fault (SAF) system. Samples were collected along two groundwater flowpaths that originate in the San Gabriel Mountains and discharge to the Mojave River located f 32 km to the northeast. Additional groundwater samples were collected from Mojave River Deposits underlying the Mojave River. The primary objective of this study is to identify and quantify crustal and mantle helium contributions to the regional groundwater system. A total of 27 groundwaters, sampled previously for chemistry and isotope systematics (including C activity) have measured helium concentrations that increase along flowpaths from 9.9 10 8 to 1.0 10 4 cm STP g 1 H2O. Concomitantly, He/He ratios decrease from 0.84RA to 0.11RA (RA equals the He/He ratio in air = 1.4 10 ). We did not record He/He ratios equivalent to crustal-production values (f 0.02RA) in any sample. Dissolved helium concentrations were resolved into components associated with solubility equilibration, air entrainment, mantle-derivation, in-situ production within the aquifer, and extraneous crustal fluxes. All samples contained the first four components, but only older samples had the superimposed effects of helium derived from a crustal flux. The radiogenic He component has chronological significance, and good concordance between He and C ages for younger groundwaters ( < 25,000 year) demonstrates the integrity of the He-chronometer in this setting. Helium-rich waters could also be dated with the He technique, but only by first isolating the whole crustal flux (3–10 10 6 cm STP cm 2 year ). Mantle-derived He (Hem) is present in all MRB samples irrespective of distance from the SAF. However, regional-aquifer groundwaters near the terminus of the flowpath have a significantly greater content of mantle-derived He in comparison with more modern samples. We propose that faults in the basin other than the SAF may be an additional source of mantle-derived helium. The large range in Hem concentrations may be related to fault activity; however, groundwaters with lower and more constant Hem contents may indicate that seismic activity along the SAF has been 0009-2541/$ see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2003.07.002 * Corresponding author. Fax: +1-858-822-3310. E-mail addresses: [email protected], [email protected] (J.T. Kulongoski), [email protected] (D.R. Hilton), [email protected] (J.A. Izbicki). 1 Now at U.S. Geological Survey, San Diego, CA 92123, USA. 2 Fax: +1-858-822-3310. 3 Fax: +1-858-637-9201. J.T. Kulongoski et al. / Chemical Geology 202 (2003) 95–113 96 relatively constant for the past 30,000 years, demonstrating that ancient groundwaters may serve as an archive for paleoseismic events. D 2003 Elsevier B.V. All rights reserved.