Continental degassing flux of 4He and its variability

Abstract

The existing measures of the 4He 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 × 1010 4He 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 4He degassing flux (95th percentile) increases with decreasing time scales (to */∼106X at 0.5 year) and decreasing space scales (to */∼106X 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 4He as well as the (n = 271) estimate of degassing from Precambrian Shield lakes are both approximately equivalent to the radiogenic production rate for 4He in the whole crust. Large‐scale vertical mass transport in continental crust is estimated as scaled values of the order 10−5 cm2 s−1 for helium (over 2 Gyr and 40 km vertically) versus 10−2 cm2 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 3He/4He relations.