The importance of silicate weathering of a sedimentary aquifer in arid Central Australia indicated by very high 87Sr/86Sr ratios

With the exception of brine systems and crystalline-rock aquifers, groundwater 87Sr/86Sr ratios rarely differ by more than ±0.01 from the Phanerozoic seawater value of 0.70923. Groundwater 87Sr/86Sr ratios from Central Australia (0.72562–0.76248) are the highest ever recorded for a low salinity, sedimentary aquifer. We propose a model for the evolution of these 87Sr/86Sr ratios and demonstrate how the lowest 87Sr/86Sr ratios reflect water–rock interactions with predominantly carbonate minerals, while the highest 87Sr/86Sr ratios are ultimately controlled by weathering of old primary silicate minerals. Detailed analysis of aquifer core material identified albite and microcline as the most prevalent primary silicates and where these minerals were abundant, relatively high Rb concentrations were measured. High Rb combined with long time scales since deposition has produced very high 87Sr/86Sr ratios in the rocks and subsequently the groundwater. Therefore, strontium isotopes can provide further constraints for chemical models of arid zone groundwater systems, particularly when silicate hydrolysis is believed to significantly contribute to dissolved solute compositions.