Large atmospheric noble gas excesses in a shallow aquifer in the Michigan Basin as indicators of a past mantle thermal event

Abstract

Significant atmospheric noble gas excesses in aquifer systems have systematically been linked to increased hydrostatic pressure, either due to increased water table levels or due to the development of ice cover. Measured noble gases (Ne, Ar, Kr, and Xe) in the shallow Saginaw aquifer in the Michigan Basin display both moderate (∼20–60% Ne excess) and large (∼80−>120% Ne excess) excesses of atmospheric noble gases with respect to air saturated water for modern recharge conditions. All large atmospheric noble gas excesses are located in the main discharge area of the Michigan Basin, in the Saginaw Lowlands region.Here, through a step-by-step analysis, we first show that large atmospheric noble gas excesses in the Saginaw aquifer do not result from increased hydrostatic pressure but, instead, are the result of vertical transport of atmospheric noble gases that are believed to have escaped from deep Michigan Basin brines following a past thermal event of mantle origin. Subsequently, we show that the atmospheric noble gas pattern of the entire Michigan Basin strata appears to result from two distinct end-members: (a) an end-member represented by the deepest, most depleted brines from which most of the atmospheric noble gases escaped; and (b) an end-member with excess atmospheric noble gas values above those displayed by the Saginaw samples. The latter is unconstrained due to the dilution effect exerted by recharge water. Using a Rayleigh distillation model we further show that the greater enrichment of lighter relative to heavier noble gases in the Saginaw aquifer in the Saginaw Lowlands area is compatible with either diffusion or solubility related mechanisms. These findings reinforce the notion that a past thermal event is indeed responsible for the atmospheric noble gas excesses found in the Saginaw aquifer in the Saginaw Lowlands area. They are also consistent with and reinforce previous findings with respect to the occurrence of a thermal event of mantle origin in the Michigan Basin.