Mechanisms and rates of 4He, 40Ar, and H2 production and accumulation in fracture fluids in Precambrian Shield environments

Abstract

Fluids residing in fractures in Precambrian Shield environments can contain gases with concentrations up to 30% 4He, 6% 40Ar, and more than 50% H2 depending on the location (expressed as % volume of the free gas phase post-exsolution). These gases are produced through two distinct mechanisms; 1. Radioactive decay of naturally occurring U, Th, and K which produces H2 via radiolysis of water and the noble gases (e.g. 4He and 40Ar) and, 2. Hydration reactions with mafic and ultramafic minerals in the host rock that produce H2. He production is economically important as known reservoirs of 4He and 3He, resources integral to medical applications and different manufacturing sectors, are rapidly depleting. H2 meanwhile has recently been posited as an emerging, clean, energy source for a post-hydrocarbon energy future. 40Ar, though more common, also has important applications in research and industry. The accumulation of 4He and 40Ar can also be used to provide vital constraints on fluid residence times, crucial to understanding planetary processes. In both planetary science and astrobiology there is also keen interest in H2 as an energy source supporting subsurface microbial metabolisms and/or abiotic organic synthesis. For both H2 and 4He, innovative exploration strategies are required to understand their production and accumulation in the deep crust. At present few studies have specifically focussed on establishing the relationship between these two reactive and conservative elements in the deep subsurface, despite their cogenetic nature.We investigate the dependence of H2, 4He, and 40Ar production rates on rock type and geologic setting. Through collating and combining previously published dissolved gas data with substantial new data for specific localities, conservative estimates of H2 production are generated for a range of sites across the Canadian Shield. The results support the prevailing theory that H2 production dominantly depends on the associated lithology. This study also identifies broadly homogeneous 4He and 40Ar production rates based on a relatively homogeneous range of U, Th, and K concentrations in the source rocks. As a consequence, H2/4He ratios are relatively low in felsic environments where H2 is mostly produced radiolytically, whereas they are elevated in mafic environments due to additional H2 production through hydration reactions (e.g. serpentinization). Extrapolating to Precambrian terrains globally this study estimates the total annual production of 4He and 40Ar from these environments specifically to the worldwide budget and reports here a global production in these environments comparable to both production in the entire oceanic crust and to the total net outgassing of these elements at Mid-Ocean Ridges.