Ground waters with unradiogenic 87 Sr/ 86 Sr ratios in the Great Artesian Basin, Australia Kenneth D. Collerson,* William J. Ullman,* T. Torgersen* Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra, A.C.T., Australia ABSTRACT Ground water from the major Jurassic aquifer in the Great Artesian Basin, Australia, shows significant variation in calcium and strontium concentration and strontium isotopic composition with age and distance from the recharge area. Hydrologically young bore waters in the eastern part of the basin exhibit surprisingly unradiogenic 87 Sr/ 86 Sr ratios (0.7045 to 0.7054). By contrast, hydrologically older waters in the central and discharge regions of the basin have significantly more radiogenic isotopic compositions (0.7060 to 0.7118). Waters with unradiogenic 87 Sr/ 86 Sr ratios are interpreted to be the result of reaction between recharge water in the aquifer and lithologies that have juvenile 87 Sr/ 86 Sr compositions. These juvenile strontium isotope signatures most likely reflect interaction with Cenozoic mafic igneous rocks which occur on the eastern side of the basin. More radiogenic waters in the basin are the result of addition of 87 Sr derived by in situ dissolution of aquifer silicates. 100 km INTRODUCTION The strontium concentration and isotopic composition of natural waters and salts that pre- cipitate from them are important geochemical tracers of hydrologic processes with application in many geologic environments. Doe et al. (1966), Stettler (1977), Stettler and AUegre (1979), and Elderfield and Greaves (1981) have demonstrated that the isotopic composition of strontium in geothermal brines can be used to model relative contributions of strontium from mantle and crustal sources as a result of fluid and rock or fluid and fluid interaction. Varia- tions in strontium concentration and isotopic composition of ground water from regional aquifers have yielded information about sources of salts in brines (Chaudhuri, 1978; Starinsky et al., 1983; Stueber et al., 1984) and about the isotopic composition of ancient seawater (Sta- rinsky et al., 1980). We present calcium and strontium concentra- tions and 87 Sr/ 86 Sr ratios for ground waters from an extensive Jurassic aquifer in the Great Artesian Basin, Australia. The waters exhibit highly variable calcium and strontium concen- trations and 87 Sr/ 86 Sr ratios. These differences in chemistry reflect regional geologic control on water flow in the basin. GEOLOGY AND HYDROGEOLOGY The Great Artesian Basin, one of the world's largest confined ground-water basins, underlies 1.7 x 10 6 km 2 of the Australian continent, ex- tending from Queensland and northern New South Wales into the Northern Territory and South Australia (Fig. 1). The basin is structur- *Present addresses: Collerson—Earth Sciences Board, University of California, Santa Cruz, California 95064; Ullman—Oceanography Program, College of Marine Studies, University of Delaware, Lewes, Del- aware 19958; Torgersen—Department of Marine Sciences, University of Connecticut-Avery Point, Groton, Connecticut 06340. Figure 1. Location and geologic setting of Great Artesian Basin, Australia, showing di- rections of flow lines 1 and 3 (after Habermehl, 1980) and locations of ground-water samples analyzed in this study (see Table 1). Distribu- tion of Cenozoic volcanics on the eastern flank of basin is also shown (from Ewart, 1961, 1982). GEOLOGY, v. 16, p. 59-63, January 1988 59