Noble Gases Dissolved in Groundwaters of the Michigan Basin: Implications for Paleoclimatology, Hydrogeology, Tectonics and Mantle Geochemistry.

Abstract

This dissertation uses noble gases dissolved in groundwaters of the Michigan Basin as natural tracers of crustal fluids, to reconstruct paleoclimate, to study cross-formation flow, to trace the origin and evolution of deep brines, to investigate the thermal and tectonic history of this region, and to place constraints on the underlying mantle structure and convection models. Dissolved noble gases from the shallow Marshall aquifer reveal a paleo-temperature record since the end of last glacial maximum for southern Michigan, which suggests that groundwater recharge occurred under the Laurentide Ice Sheet. This record also identifies major global climatic oscillations such as the Bølling-Allerød warm phases in this region and suggests that an atmospheric circulation pattern distinct from today prevailed in the late Pleistocene. Analyses of He isotopes from the Marshall aquifer show unusually high He excesses for such shallow depths and yield He fluxes far greater than those reported in other sedimentary basins, suggesting the presence of a dominant vertical water flow component at greater depths. Of particular relevance is the contrast observed between high helium and low heat fluxes in this aquifer leading to helium/heat flux ratios greater than the expected radiogenic production ratio in certain areas, pointing to the occurrence of a major past thermal event in the basin. Subsequent analyses of all stable noble gases in deep Michigan Basin brines from eight different formations all confirmed the presence of a past mantle thermal event, most likely related to a recent reactivation of the Mid Continent Rift underneath the basin. More specifically, the atmospheric component shows a strong depletion pattern resulting from subsurface boiling and steam separation. This pattern is consistent with the presence of past high temperatures in the basin. The crustal component further supports the occurrence of the upward transport of noble gases released from the Precambrian basement beneath the basin. More importantly, the mantle component clearly reveals a primordial solar-like component in this stable continental region, where the presence of mantle plumes is highly unlikely. This suggests that a deep primordial mantle reservoir is not required to explain the presence of such components.