Production, redistribution and loss of helium and argon isotopes in a thick sedimentary aquitard-aquifer system (Molasse Basin, Switzerland)

Variation in the terrestrial isotopic composition and atomic weight of argon (IUPAC Technical Report)

“…A subsequent study [20] demonstrated improved analytical precision is possible, supported the relatively high 40 Ar/ 36 Ar ratio for atmospheric Ar of Lee et al [4], and confirmed isotopic variability among purified atmospheric Ar tank gases. Another subsequent study [21] supported the partially calibrated measurements of Lee et al [4] for both 40 Ar/ 36 Ar and 38 Ar/ 36 Ar. Most Ar isotope ratio measurements in the literature were made by static gas-source magnetic-sector mass spectrometry [22] and are expressed as simple molar ratios (e.g., 40 Ar/ 36 Ar, 38 Ar/ 36 Ar).…”

“…The stable isotopic composition of atmospheric Ar is essentially constant spatially and has not changed substantially on human time scales. An increase in the 40 Ar/ 38 Ar ratio of around 0.07 ‰/Ma was estimated on the basis of trapped air in ice representing the last 800 000 years and attributed to radiogenic 40 Ar degassing from the Earth [17]. For over 60 years, the best-calibrated measurement of the Ar isotope ratios in air was attributed to Nier [18], who reported a 40 Ar/ 36 Ar ratio of 296.0 and 38 Ar/ 36 Ar ratio of 0.1880.…”

“…An increase in the 40 Ar/ 38 Ar ratio of around 0.07 ‰/Ma was estimated on the basis of trapped air in ice representing the last 800 000 years and attributed to radiogenic 40 Ar degassing from the Earth [17]. For over 60 years, the best-calibrated measurement of the Ar isotope ratios in air was attributed to Nier [18], who reported a 40 Ar/ 36 Ar ratio of 296.0 and 38 Ar/ 36 Ar ratio of 0.1880. Confusion resulted when Nier's isotope ratios were converted to mole fractions, then reconverted to ratios, with the result that 295.5 was adopted as a conventional atmospheric 40 Ar/ 36 Ar ratio for geochronology [9].…”

“…The annotation "r" indicates the range in isotopic composition of normal terrestrial material prevents a more precise standard atomic weight being given, despite the fact that measurements on individual samples may be more precise. Even so, some sources of Ar with large concentrations of radiogenic 40 Ar, and some with large concentrations of nucleogenic 38 Ar and 36 Ar, were not included within the standard atomic-weight uncertainty limits. The annotation "g" indicates geological specimens are known in which the element has an isotopic composition not included within the expanded standard atomic-weight uncertainty.…”

“…Selected other nuclear reactions yielding stable Ar isotopes 35 Cl (n, γ) 36 Cl (β-) 36 Ar 35 Cl (a, p) 38 Ar 37 Cl (n, γ) 38 Cl (β-) 38 Ar Atomic masses are given in unified atomic mass units, u, with uncertainties in the last 2 digits in parentheses [3].…”