Crustal noble gases in deep brines as natural tracers of vertical transport processes in the Michigan Basin

Noble gas concentrations and isotopic ratios are presented for 38 deep (similar to 0.5-3.6 km) brine samples in the Michigan Basin. These brine samples clearly show the presence of an important crustal component of He-4, Ne-21, Ar-40, and Xe-136. Both Ar-40(crust) and Xe-136(crust) display the presence of a strong vertical gradient along the sedimentary strata of the basin. We show that the in situ production for these two gases within the sedimentary strata is insufficient to account for the observed crustal component in the Michigan brines. These point to the presence of a deep, external source for crustal noble gases, likely the Precambrian crystalline basement beneath the Michigan Basin. Furthermore, observed elemental ratios of crustal noble gases (He-4/Ar-40, Ne-21/Ar-40, He-4/Xe-136, and Ne-21/Xe-136) in these brines vary over several orders of magnitude with respect to the expected production ratios from the crystalline basement rocks and display a systematic pattern within the basin. Specifically, samples above the Salina Group (shallow formations) are relatively enriched in He-4(crust) and Ne-21(crust) with respect to Ar-40(crust) and Xe-136(crust), as opposed to those below the massive Salina evaporite layer (deeper formations) which exhibit complementary patterns. We show that such a general trend is best explained by a Rayleigh-type elemental fractionation model involving upward transport of crustal noble gases and associated elemental fractionation processes, controlled by both diffusion- and solubility-related mechanisms. As previously indicated by the mantle and atmospheric noble gas signatures in these same Michigan brine samples, release of deep crustal noble gases into the basin is yet another independent indicator pointing to the occurrence of a past thermal event in the basin. We suggest that recent reactivation of the ancient midcontinent rift system underneath the Michigan Basin is likely responsible for the upward transport of heat and loss of the atmospheric noble gas component, as well as release of crustal (still ongoing) and mantle noble gases into the basin via deep-seated faults and fracture zones. Such a model also supports an internal heat source hypothesis as being largely responsible for the existence of past high temperatures in the basin without involvement of large-scale brine migration from peripheral forming orogenic fold belts.