Atmospheric noble gas signatures in deep Michigan Basin brines as indicators of a past thermal event

Atmospheric noble gases (eg., Ne-22, Ar-36, Kr-84, Xe-130) in crustal fluids are only sensitive to subsurface physical processes. In Particular, depletion of atmospheric noble gases in groundwater due to boiling and steam separation is indicative of the occurrence of a thermal event and call thus be used to trace the thermal history of stable tectonic regions. We present noble gas concentrations of 38 deep brines (similar to 0.5-3.6 km) from the Michigan Basin. The atmospheric noble gas component shows a strong depletion pattern with respect to air saturated water. Depletion of lighter gases (Ne-22 and Ar-36) is stronger compared to the heavier ones (Kr-84 and Xe-130). To understand the mechanisms responsible for this overall atmospheric noble gas depletion, phase interaction models were tested. We show that this atmospheric noble gas depletion pattern is best explained by a model involving subsurface boiling and steam separation, and thus, consistent with the occurrence of a past thermal event of mantle origin as previously indicated by both high He-4/heat flux ratios and the presence of primordial mantle He and Ne signatures in the basin. Such a conceptual model is also consistent with the presence of past elevated temperatures in the Michigan Basin (e.g., similar to 80-260 degrees C) at shallow depths as suggested by previous thermal Studies in the basin. We suggest that recent reactivation of the ancient mid-continent rift system underneath the Michigan Basin is likely responsible for the release of both heat and mantle noble gases into the basin via deep-seated faults and fracture zones. Relative enrichment of atmospheric Kr and Xe with respect to Ar is also observed, and is interpreted as reflecting the addition of sedimentary Kr and Xe from associated hydrocarbons, following the hydrothermal event. This study pioneers the use of atmospheric noble gases in subsurface fluids to trace the thermal history of stable tectonic regions. (c) 2008 Elsevier B.V. All rights reserved.