Novel Radiogenic Noble Gas Signatures in the Crust: The Krypton Factor

Abstract:

Crustal noble gases are a combination of starting concentrations, fluxes, and radiogenic in-growth. Their inert nature allows constraints to be put on recharge conditions, fluid migration and mixing, and fluid residence times. ⁴He and ⁴⁰Ar, produced through radioactive decay of U, Th, and K, are the primary isotope systems used for estimation of residence times due to their relatively high production rates and straightforward analytical techniques. Where geologic systems can be isolated over extended (Ma-Ga) timescales, (e.g. Precambrian shield systems), measurable excesses of ²¹Ne, ²²Ne, and ^131-136Xe can also accumulate and provide additional lines of evidence for long mean residence times of the in-situ fluids. To date, krypton has remained the outlier, with production rates typically too low to be readily differentiated from air-like ratios. As a result, krypton has been used as a geochronology tool only sparingly, principally focusing only on ‘short-lived’ isotopes like ⁸¹Kr and ⁸⁵Kr, which decrease in abundance as a function of isolation from the atmosphere.

In 2019 fluid and gas samples were collected from a fracture fluid network found 3 km below surface in the gold- and uranium-producing Moab Khotsong Mine in the Witwatersrand Basin, S. Africa. These fluids have elevated salinities, high levels of reduced gas (H₂, CH₄, C₂H₆) and δ¹⁸O and δ²H water isotopes to the left of the meteoric water line, a classic signal for crystalline shield brines globally. Analysis of noble gas samples reveals high concentrations of radiogenic He, Ne, Ar, isotopes, with radiogenic signatures similar to those observed at Kidd Creek Mine, Canada, where Ma-Ga residence times have been documented. Using these noble gas data along with supporting geochemical observations, and U, Th, and K data, we evaluate the residence times of these fluids in context of other shield sites. The highly enriched U content of the host rocks at Moab Khotsong may explain several novel features of these fluids, including some distinct noble gas signatures. Intriguingly, the Xe isotopic ratios are much greater than observed elsewhere and represent the highest ratios measured to date in free fluids. Most importantly, initial models suggest the presence of a radiogenic Kr excess (⁸⁶Kr); the first of its kind ever observed in a natural crustal system.