The Role of Noble Gases in Defining the Mean Residence Times of Fluids within Precambrian Crustal Systems

Abstract:

Brines rich in N₂, H₂, CH₄ and He hosted within Precambrian crustal rocks are known to sustain microbial life [1]. The geological systems containing these brines have the potential to isolate organisms over planetary timescales and so can provide unique insight into the diversity and evolution of terrestrial life [1-3]. Long considered geological outliers, the prevalence of systems containing these ancient, deep fracture waters is only now being revealed. Recent studies demonstrate the Precambrian crust which accounts for ~70% of total crustal surface area has a global hydrogen production comparable to marine systems [2]. In addition to H₂-producing reactions (e.g. radiolysis and serpentinization), a diversity of CH₄-producing reactions also occur in these systems through both microbial and water-rock interactions [1, 2]. However, the role these Precambrian systems have in global hydrogen and carbon cycles is poorly understood. For this we need good constraints on the origins, residence times and degree of microbial activity of the fluids within these systems as well as the degree of interaction with external systems. Fortunately, noble gases are ideal for this role [1,3].

Previous noble gas analysis of N₂, H₂, CH₄ and He-rich fluid samples collected at 2.4 km depth from a Cu-Zn mine in Timmins, Ontario, identified isolated fracture fluids with the oldest residence times ever observed (>1.1 Ga) [3]. This study has been significantly expanded now to fluids from an even greater depth (3 km) at Timmins, and from two new mines in the Sudbury Basin. Preliminary data from the deeper Timmins level indicate a new closed system with ¹³⁶Xe/¹³⁰Xe ratios 93% above modern air values (20% at 2.4 km) and an early atmosphere ¹²⁴Xe/¹³⁰Xe signal approaching the age of the host rock (~2.7 Ga) [4]. In comparison, the Sudbury system indicates exchange with an external source, being highly enriched in helium (30% gas volume) but with a low fissiogenic ¹³⁶Xe/¹³⁰Xe excess (10-38% above air). Through xenon and other noble gas data we present comparisons of mean fluid residence ages and fluid evolution for these closed and open systems.

[1] Lippmann-Pipke et al. (2011) Chem. Geol. 283 287-296. [2] Sherwood Lollar et al. (2014) Nature 516 379–382. [3] Holland et al. (2013) Nature 497 357-360. [4] Pujol et al. (2011) Earth. Planet. Sc. Lett. 308 298-306.