Abstract
Using knowledge of groundwater dynamics in the Carrizo aquifer and surrounding formations in southwest Texas, United States, 3He and 4He transport was modeled and calibration on concentrations achieved for both isotopes. Three components were isolated: atmospheric, in situ production and external deeper crustal and mantle components. Among the three major sources for these isotopes, 3He concentrations are primarily a binary mixture from atmospheric and deeper external sources. Unlike 4He, 3He resulting from in situ production is negligible over the entire domain. Tests of the distribution of R/ R a values ( R is the groundwater 3He/ 4He ratio; R a is the atmospheric 3He/ 4He ratio), show that relatively high R/ R a values (∼0.14) entering the aquifer are quickly lowered by extremely low (0.0025) in situ production ratios almost totally devoid of 3He. Such lessening gives rise to “typical crustal” R/ R a values of about 0.055. Such effect on R/ R a ratios increases as 3He and 4He move upward through the sedimentary succession. Further analysis on the R/ R a behavior suggests that low crustal R/ R a values have the potential to mask high R/ R a ratios carrying a substantial 3He mantle component, particularly, in multilayered aquifer systems. This suggests that a high mantle 3He component is present in sedimentary groundwater systems. Groundwater R/ R a values between 0.01 and 0.05 (considered as typical crustal values) may be the result of a significant (perhaps dominant) mantle component modified by low 3He/ 4He in situ production ratios. It further suggests that the continental crust may constitute a much more permeable environment to mantle 3He than previously thought. The occurrence of deeper magma bodies is not needed to explain the presence of mantle 3He in the crust, and implies that mantle 3He fluxes might be higher in crustal rocks than previously thought.
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