Isotopic fractionation of argon during stepwise release from shungite

Argon isotope fractionation induced by stepwise heating

“…However, consistent with the suggestion by Rison (1980), our data refute, in detail, the hypothesis that subatmospheric 40 Ar/ 36 Ar ratios at low extraction temperatures in shungites represent paleoatmospheric argon. In contrast, our results demonstrate that the isotopic composition of argon trapped in Precambrian shungites is indistinguishable from modern atmospheric composition.…”

“…Isotopic fractionation as a cause of subatmospheric 40 Ar/ 36 Ar ratios at low-extraction temperatures was suggested in a previous study on shungites (Rison, 1980); however, only one 38 Ar/ 36 Ar ratio Ͼ1 below air was measured, which may be compared with this study that yielded six values lower than air by more than 4, and 14 values by more than 3. Regarding possible isotope fractionation effects of helium isotopes during stepwise degassing, previous studies resulted in different conclusions: While Kurz (1993, 1999) reported diffusion coefficients for magmatic 3 He and 4 He differing by factors of 1.09, 1.04, and 1.08 for olivine, pyroxene, and basaltic glass, respectively, Shuster et al (2003) reported no isotopic fractionation of proton-induced 3 He and radiogenic 4 He for Durango apatite.…”

“…In this case, it could constrain the evolution path of atmospheric argon, even if its composition is indistinguishable from modern argon. However, Rison (1980) argued, rather, for a recent exchange between shungites and present-day atmospheric argon: Based on shungite diffusion parameters, evaluated via standard Arrhenius diagrams from the thermal argon release spectrum assuming a uniform grain size of the argon carrier, Rison (1980) extrapolated the diffusion coefficient to room temperature and concluded that shungites should completely exchange argon at 20°C over timescales of less than 1 Ma.…”

“…However, 38 Ar is the least abundant argon isotope, so high-precision measurements require rocks with high argon concentrations. Precambrian shungites, sedimentary rocks with a remarkably high content of amorphous carbonaceous matter (see section 2. below and Borisov, 1957;Cherdyntsev and Kolesnikov, 1965;Firsova and Yakimenko, 1985;Volkova and Bogdanova, 1986;Parfen'eva et al, 1995), have exceptionally high abundances of trapped atmospheric argon, possibly of paleoatmospheric composition (Cherdyntsev and Kolesnikov, 1965) or isotopically fractionated atmosphere of unknown age (Rison, 1980). For example, shungite of type I has 10 -5 ccSTP/g 36 Ar (Cherdyntsev and Kolesnikov, 1965), compared to average sedimentary rocks with 6 ϫ 10 -8 ccSTP/g 36 Ar (Ozima and Podosek, 2002), or the normal potassium-rich silicates used in 40 Ar-39 Ar dating with typically between 10 -11 and 10 -9 ccSTP/g 36 Ar.…”

“…These lines have higher inclinations than an average line that is derived from total data of superposed reservoirs. Extrapolation of the average line with a lower slope-as done by Rison (1980)-will yield unrealistically high diffusion coefficients at room temperature, while individual reservoir lines with steeper slopes result in lower-diffusion coefficients and, hence, significantly slower exchange with the ambient atmosphere.…”