Argon accumulation and the crustal degassing flux of 40Ar in the Great Artesian Basin, Australia

Measurements of the 40Ar/36Ar ratio of dissolved argon from groundwaters in the Great Artesian Basin (GAB), Australia, indicate the accumulation of significant radiogenic 40Ar. This radiogenic 40Ar cannot be supplied by in situ production (40K-decay in the hydrologically conductive Hooray Sandstone) or by the dissolution of Hooray Sandstone with a finite K-40Ar age. As was found earlier for the 4He, the magnitude of the radiogenic 40Ar in the GAB requires a crustal degassing flux of 40Ar to be entering the aquifer. Within the errors of the data and assumptions regarding the composition and structure of the eastern Australian continental crust, this degassing flux approaches and may be equivalent to the whole crustal production of 40Ar. A flux of this magnitude is in agreement with 40Ar degassing estimates derived from atmospheric models. These measurements constitute the first direct measure of the 40Ar continental degassing flux. The distribution of radiogenic 40Ar from crustal degassing in the aquifer is discontinuous with time, distance, and 4He. This suggests that the crustal degassing processes for 40Ar and 4He are local, time dependent, and decoupled. This can be understood in terms of 1. the mineral and lithologic distribution of K, U, Th; 2. the release of 4He and 40Ar from the solid phase to the fluid phase by recoil, diffusion, and dissolution; 3. the release of 4He and 40Ar by stress fracturing; 4. the variability of tectonic and hydraulic stress; and 5. the separation of differentially released 4He and 40Ar by the rapid rate (>10-6 cm2 s-1; >0.1 cm yr-1) of crustal fluid transport required by metamorphism and crustal degassing. This further suggests that the crustal degassing process may be tectonically and hydraulically linked with other continental-scale fluid processes. (orig.)