Continental degassing flux of ⁴He and its variability

Abstract

[1] The existing measures of the 4 He flux from the Earth's continental solid surface have been evaluated collectively. The lognormal mean of continental crustal flux measurements (n = 33) globally covering many geological environments is 4.18 × 10 10 4 He atoms m −2 s −1 with an estimated one sigma variance of */45X based on an assumption of symmetric error bars (lognormal distribution provides a standard deviation with a multiplication or division factor (*/) by which the mean may statistically vary). The range of the continental 4 He degassing flux (95th percentile) increases with decreasing time scales (to */∼10 6 X at 0.5 year) and decreasing space scales (to */∼10 6 X at 1 km). The statistics can be interpreted as reflecting natural variability and suggest that the mechanisms transporting the crustal helium degassing flux contain a high degree of both spatial and temporal variability. This lognormal mean of the continental degassing flux of 4 He as well as the (n = 271) estimate of degassing from Precambrian Shield lakes are both approximately equivalent to the radiogenic production rate for 4 He in the whole crust. Large-scale vertical mass transport in continental crust is estimated as scaled values of the order 10 −5 cm 2 s −1 for helium (over 2 Gyr and 40 km vertically) versus 10 −2 cm 2 s −1 for heat. This rate of mass transport requires not only release of He from the solid phase via diffusion, fracturing, or comminution but also an enhanced rate of mass transport facilitated by some degree of fluid advection. This further implies a separation of heat and mass during transport which will significantly influence the interpretations of heat and 3 He/ 4 He relations.

Dissolved noble gases and stable isotopes as tracers of preferential fluid flow along faults in the Lower Rhine Embayment, Germany

“…A groundwater residence time can be estimated for the Brühl groundwater samples using 4 He concentration data. In this study, a terrigenic-diffusive-flux-rate estimate of 4×10 9 atoms 4 He m −2 s −1 was taken from the literature presented by Torgersen (2010) for a similar hydrogeological setting to estimate helium input to the deep groundwater system (prior to recent upward movement along faults), but generally many values used in our calculation were under-constrained. Recent terrigenic helium inputs to the shallow aquifer system are assumed to be negligible; however, a 4…”

“…Dissolved noble gases and stable isotopes are particularly useful in studies of aquifer systems that contain pre-Holocene 'palaeo' groundwater (e.g. Vaikmäe et al 2001) or fluids that have a deep crustal origin such as those observed in the Great Artesian Basin in Australia (Torgersen andClarke 1985, 1987;Bethke et al 1999), the Paris Basin in France (Marty et al 1993(Marty et al , 2003Castro et al 1998;Lavastre et al 2010) and the Witwatersrand Basin in South Africa (Lippmann et al 2003;Lippmann-Pipke et al 2011). Dissolved helium has been used extensively for estimating groundwater residence times and tracing the origin of crustal fluids (Torgersen et al 1992;Gardner et al 2012).…”

Determining fluid migration and isolation times in multiphase crustal domains using noble gases

“…These data further preserve details about the hydrocarbon/water volume ratio and whether the system is open to gas loss (Ballentine et al, 1996;Zhou et al, 2005;Barry et al, 2016). Radiogenic noble gases, which are isotopically distinct from the air-derived noble gases dissolved in water, accumulate in groundwater systems and can be used to investigate mean fluid residence (e.g., Torgersen, 2010;Aggarwal et al, 2014). This technique has previously been applied to a number of systems, including fracture fluids in deep crustal basement rocks with mean residence times of >1 b.y.…”

technical report and journal articles

“…When the fluxes of Table 8.2 are evaluated with respect to their log normal mean He flux is approximately ±1.5 orders of magnitude (Torgersen, 2010; see also Fig. 8.9).…”

“…8.7). The net result of micro-scale fracturing coupled to larger scale (km) macro-fracturing is that retention of He from the Earth's crust to the atmosphere is comparable to the net in situ production by U-,Th-series elements alphadecay in 30-40km of crust (Torgersen, 1989;Torgersen, 2010). This is not equivalent to a steady-state loss of 4 He from the crust to the atmosphere as implied by as any measure of flux contains within it a characteristic time and space scale whereas steadystate implies that the flux is constant in time and space.…”