Table 2.

Estimates of 40Ar outgassing rates from the solid Earth by various processes

 Calculating the rate of atmospheric 40Ar increase Ar content of the atmosphere 1.65 × 1018 mol Rate of 40Ar increase (this article) 0.066 ‰/Myr Resulting outgassing rate 1.1 × 108 mol/yr Calculating 40Ar degassing from the continental crust From chemical weathering [K] of river water (35) 44 μmol/liter Global water discharge 3.74 × 1016 liters/yr Average K-Ar age of weathering basement rocks (36) K-Ar age = 1.12 Ga, ≡ 40Ar/K = 2.7 × 10−7 mol/g Fraction of K released because of weathering of basement rocks (36) 34% Average K-Ar age of weathering sediment (30, 37) K-Ar age = 0.5 Ga, ≡ 40Ar/K = 1.0 × 10−7 mol/g Fraction of K released due to weathering of sediments (38) 66% Outgassing by chemical weathering 0.40 × 108 mol/yr From mechanical weathering Mass of total suspended solids (37, 39) 3 × 1015g/yr K concentration in TSS (35) 2 wt % Average age (30, 37) 0.5 Ga, 40Ar/K = 1.0 × 10−7 mol/g Outgassing by mechanical weathering 0.06 × 108mol/yr Diffusion from sediments Degassing model to explain difference between measured K-Ar and stratigraphic ages (31) 0.05–0.13 × 108mol/yr From metamorphism First-order constant b for degassing to match average basement K-Ar age (40) 3.7 × 10−10 yr−1 Mass of crust processed per year (assuming a constant fraction of the crust is degassed per year) 7.5 × 1015 g/yr Concentration of K in crust (41) 1.5 wt % Average age of processed rocks 1.12 Ga 40Ar/K ratio 2.7 × 10−7 mol/g 40Ar released by metamorphism 0.3 × 108 mol/yr Subtotal 40Ar flux from continents 0.81–0.89x108 mol/yr Box model calculations of crustal degassing Degassing rate from crustal age and 40Ar retention of Armstrong (27) 1.68–2.24 × 108mol/yr Degassing rate from crustal age and 40Ar retention of Taylor and McClennan (28) 1.28–1.62 × 108 mol/yr Degassing rate calculated from crustal age and 40Ar retention of Allegre et al. (1) 0.73–0.86 × 108mol/yr Degassing rate calculated from crustal age and 40Ar retention of Coltice et al. (29) 0.79–1.01 × 108mol/yr Total range of model continental crust fluxes 0.79–2.24 × 108mol/yr Calculating 40Ar outgassing from the mantle From midocean ridges Rate of 3He outgassing (12) 422 mol/yr MORB 3He/4He (8RA) 1.1 × 10−5 MORB source 4He/40Ar (see text) 1.9 Outgassing at midocean ridges 0.20 × 108 mol/yr From hotspots Max. rate of volcanism, incl. seamounts (23) 1–12% of MOR Concentration assuming average 20 RA (see text) 1 × MORB Intraplate hotspot outgassing (upper limit) 0.024 × 108 mol/yr From subduction zones Volcanic production relative to MORB (22) 5% 0.01 × 108 mol/yr From extension zones Groundwater He data for extensional basins (21) 0.04 × 108 mol/yr Total 40Ar flux from mantle 0.27 × 108 mol/yr Total 40Ar degassing from mantle and continental crust based on present fluxes (1.08–1.16) × 108 mol/yr Total flux/flux inferred from ice core data 1.1 ± 0.1 × 108 mol/yr