Experimentelle und Analytische Planetologie
Argon isotope fractionation induced by stepwise heating
Noble gas isotopes are widely used to elucidate the history of the rocks in which they have been trapped, either from
distinct reservoirs or by accumulation following radioactive decay. To extract noble gases from their host rocks,
stepwise heating is the most commonly used technique to deconvolve isotopically different components, e.g.,
atmospheric, in situ radiogenic, or excess radiogenic from mantle or crustal reservoirs. The accurate determination of
the isotopic composition of these different components is of crucial importance, e.g., for ages obtained by
40Ar-39Ar stepheating plateaus. However, diffusion theory-based model calculations
predict that the stepwise thermal extraction process from mineral phases induces isotope fractionation and, hence,
adulterates the original composition. Such effects are largely unconsidered, as they are small and a compelling
experimental observation is lacking. We report the first unequivocal evidence for significant mass fractionation of
argon isotopes during thermal extraction, observed on shungite, a carbon-rich Precambrian sedimentary rock. The
degree of fractionation, as monitored by 38Ar/36Ar and
40Ar/36Ar ratios, very well agrees with theoretical predictions assuming an inverse square
root dependence of diffusion coefficient and atomic mass, resulting in easier extraction of lighter isotopes. Hence,
subatmospheric 40Ar/36Ar ratios obtained for argon extracted at low temperatures may
not represent paleoatmospheric argon.
Shungite argon resembles modern atmospheric composition, but constraints on the timing of trapping appear
difficult to obtain, as shungites are multicomponent systems. In 40Ar-39Ar stepwise
heating, the isotope fractionation effect could cause systematic underestimations of plateau ages, between 0.15 and
0.4% depending on age, or considerably higher if samples contain appreciable atmospheric Ar. The magnitude of
this effect is similar to the presently achieved uncertainties of this increasingly precise dating technique. Our results
also indicate the importance of thermally activated diffusion as a possible fractionation mechanism, e.g., for
hydrothermal gas exhalations, or for carbonaceous carrier phases such as "Q" in meteorites that have
been suggested as carriers of highly fractionated noble gas residues from the early solar nebula.
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