TRAVERTINE- DEPOSITING MOUND SPRINGS OF SOUTH AUSTRALIA: KEY FOR CHARACTERIZING GROUNDWATER MIXING AND PALEOHYDROLOGY IN THE GREAT ARTESIAN BASIN

Geochemistry of water and gas in mound springs provides a window into groundwater mixing in the Great Artesian basin (GAB), Australia. Elevated ³He/⁴He gas values, termed “xenowhiffs”, provide unequivocal evidence for deep fluid sources that have been introduced into the groundwater system in the last several million years and hence an active mantle-to-groundwater fluid linkage. Evaluating fluid and gas mixing requires use of multiple tracers. We estimate the external (deeply derived) CO₂ in water samples from both travertine (a.k.a. tufa) mound springs and artesian bores using water chemistry and C isotope data. Contributions from dissolution of carbonate in the aquifer (C_carb=Ca+Mg-SO4) is distinguished from contributions from biological/organic sources (d13C= -28) versus mantle sources (d13C= -5). Of the external C, mixing models using CO₂/³He values of 9 x 10⁹ (Warberton Spring) to 2 x 10¹⁰ (Bubbler Spring) can be used to model contributions of CO₂ from the asthenospheric mantle (MORB end member taken as 2 x 10⁹) versus lithosphere. Elevated ⁸⁷Sr/⁸⁶Sr values at Dalhousie Spring indicate fluid-rock interactions in granitic crust and small volume, but geochemically potent, crustal contributions to the endogenic fluids. U-Series dates indicate persistent deposition of travertine mound springs (conceptualized as “chemical volcanoes”) at discrete vent sites for millions of years. Travertine- depositing springs are windows into active and heterogeneous groundwater mixing. Major ion chemistry suggests different highly variable water chemistry spring to spring, different endmember endogenic fluids, and variable mixing proportions in different subbasins. Travertine mound springs, coupled with the associated mound and platform travertine rock record, thus collectively provide a rich record that can be used to link the present hydrologic system to paleohydrology of the GAB over the last several million years. Microbiology sampling of springs is underway to also evaluate niches for Archea and Bacteria associated with these springs. The overall goal is to test a model for interactions between mantle and deep crustal fluid inputs, neotectonic pathways, groundwater mixing, groundwater quantity and quality, and unique microbiology in the near-surface hydrologic systems.