Source and movement of helium in the eastern Morongo groundwater Basin: The influence of regional tectonics on crustal and mantle helium fluxes

Abstract

We assess the role of fracturing and seismicity on fluid-driven mass transport of helium using groundwaters from the eastern Morongo Basin (EMB), California, USA. The EMB, located ∼200 km east of Los Angeles, lies within a tectonically active region known as the Eastern California Shear Zone that exhibits both strike-slip and extensional deformation. Helium concentrations from 27 groundwaters range from 0.97 to 253.7 × 10 −7 cm 3 STP g −1H 2O, with corresponding 3He/ 4He ratios falling between 1.0 and 0.26 R A (where R A is the 3He/ 4He ratio of air). All groundwaters had helium isotope ratios significantly higher than the crustal production value of ∼0.02 R A. Dissolved helium concentrations were resolved into components associated with solubility equilibration, air entrainment, in situ production within the aquifer, and extraneous fluxes (both crustal and mantle derived). All samples contained a mantle helium-3 ( 3He m) flux in the range of 4.5 to 1351 × 10 −14 cm 3 STP 3He cm −2 yr −1 and a crustal flux (J 0) between 0.03 and 300 × 10 −7 cm 3 STP 4He cm −2 yr −1. Groundwaters from the eastern part of the basin contained significantly higher 3He m and deep crustal helium-4 ( 4He dc) concentrations than other areas, suggesting a localized source for these components. 4He dc and 3He m are strongly correlated, and are associated with faults in the basin. A shallow thermal anomaly in a >3,000 m deep graben in the eastern basin suggests upflow of fluids through active faults associated with extensional tectonics. Regional tectonics appears to drive large scale crustal fluid transport, whereas episodic hydrofracturing provides an effective mechanism for mantle-crust volatile transport identified by variability in the magnitude of degassing fluxes ( 3He m and J 0) across the basin.