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 atmosphere1.65 × 1018 mol
Rate of 40Ar increase (this article)0.066 ‰/Myr
Resulting outgassing rate1.1 × 108 mol/yr
Calculating 40Ar degassing from the continental crust
From chemical weathering
    [K] of river water (35)44 μmol/liter
    Global water discharge3.74 × 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 weathering0.40 × 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 weathering0.06 × 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 rocks1.12 Ga
     40Ar/K ratio2.7 × 10−7 mol/g
     40Ar released by metamorphism0.3 × 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 fluxes0.79–2.24 × 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 ridges0.20 × 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 data1.1 ± 0.1 × 108 mol/yr