Helium-4 characteristics of groundwaters from Central Australia: Comparative chronology with chlorine-36 and carbon-14 dating techniques

SummaryHelium isotope and concentration characteristics were determined for a suite of groundwater samples from the Amadeus Basin in Central Australia. Two study areas include a wellfield south of Alice Springs, and the Dune Plains and Mututjulu aquifers near Uluru. Measurements of 36Cl/Cl and 14C on the same sample suite enable us to assess the relative applicability of the three groundwater chronometers over a range of anticipated groundwater residence times (ages), and to investigate possible causes of discordant ‘ages’ derived from the different groundwater dating techniques.Results from the analyses of 39 groundwater samples reveal helium-4 (4He) concentrations that range from 0.80 to 98.8 (×10−7 cm3 STP g−1 H2O) in the Alice Springs samples, and from 0.47 to 65.6 (×10−7 cm3 STP g−1 H2O) in the Uluru samples. 4He concentrations yield uncorrected groundwater residence times (i.e. time since recharge) of between modern to >2500 ka (near Alice Springs) and modern to 1600 ka (near Uluru) assuming an effective porosity of 20%, and uranium and thorium contents of 1.7 and 6.1 ppm, respectively. 36Cl/Cl ratios on the same samples range from 93 to 158 (×10−15) (near Alice Springs) and from 80 to 335 (×10−15) (near Uluru) representing groundwater residence times near Alice Springs from modern to >200 ka, and from modern to >300 ka near Uluru. Percent modern carbon (pmc) on the same samples ranged from 64.9 to 12.5 pmc near Alice Springs, and from 93.5 to <2 pmc near Uluru. Corresponding 14C residence times for the Alice Springs samples range from modern to 13.0 ka, and near Uluru from modern to >30 ka.For the Amadeus Basin groundwater samples, the 4He method (uncorrected) over-estimates groundwater residence time compared to 36Cl and 14C techniques. This implies the presence of an extraneous He component or basal flux of He (J0). To reconcile groundwater 4He and 14C residence times, it is necessary to adopt J0 values between 0 and 30 (×10−8) cm3 STP He cm−2 a−1 which supplements in situ produced He within the aquifer. Adoption of J0 values over this range lowers 4He residence times relative to 36Cl and produces closer agreement between the 4He and 14C chronometers.The extraneous He component (basal flux) in Amadeus Basin samples is dominated by radiogenic crustal 4He without a detectable amount of mantle-derived 3He. We conclude that the stable tectonic regime, albeit with the presence of major faults within the basin, inhibits the input of mantle volatiles to the groundwater system so that in this region, diffusion is the dominant transport mechanism for He in the crust.