doi:10.1016/j.gca.2004.08.018 Argon isotope fractionation induced by stepwise heating MARIO TRIELOFF, 1, *MARTINA FALTER, 2 ALEXEI I. BUIKIN, 1,3,4 EKATERINA V. KOROCHANTSEVA, 4 ELMAR K. JESSBERGER, 5 AND RAINER ALTHERR 1 1 Mineralogisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany 2 Institute for Solid State & Material Research, PO Box 270016, D-01171 Dresden, Germany 3 Moscow State University, Vorobyevy Gory, 119899, Moscow, Russia 4 Vernadsky Institute for Geochemistry, Kosygin St. 19, 119991 Moscow, Russia 5 Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany (Received April 27, 2004; accepted in revised form August 18, 2004) Abstract—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 40 Ar- 39 Ar 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 38 Ar/ 36 Ar and 40 Ar/ 36 Ar 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 40 Ar/ 36 Ar 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 40 Ar- 39 Ar 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. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION Determining the isotopic composition of noble gases in rocks is of utmost importance for a variety of scientific questions. For example, submarine basalt glasses, diamonds, or peridotitic rocks contain mantle derived noble gas isotopes that give essential information on the evolution of the mantle-atmo- sphere system (Ozima and Podosek, 2002). Interstellar carbon- rich grains contain noble gas isotopes characteristic of specific nucleosynthetic processes (Ozima and Podosek, 2002). 40 Ar- 39 Ar dating requires precise argon isotope analyses of K- bearing minerals (McDougall and Harrison, 1999). Noble gas extraction by stepwise heating is widely used to separate dif- ferent noble gas components bound in phases with different retentivity, e.g., more or less disturbed in situ radiogenic com- ponents and gases trapped from atmospheric or other external sources. However, the implicit assumption that an isotopically uni- form component is extracted homogeneously during laboratory heating is not supported by theoretical considerations that pre- dict subtle kinetic isotopic fractionation of noble gases during stepwise heating, as the diffusion coefficient D is proportional to the inverse square root of the atomic mass of the diffusing species (Wert and Zener, 1949; Zähringer, 1962; Funk et al., 1967; Ozima and Podosek, 2002). This indicates easier extrac- tion of lighter isotopes during stepwise heating. This effect is almost universally neglected, as the predicted isotopic variation is relatively small and hardly proven compellingly yet. Hence, to unequivocally prove its existence, high-precision isotope analysis is required. Regarding the potential importance of this effect for 40 Ar- 39 Ar dating, argon isotopes are particularly interesting. A most favourable isotope ratio is 38 Ar/ 36 Ar, as it is universal and hardly varies in nature except for very rare and specific cases (Ozima and Podosek, 2002). However, 38 Ar is the least abundant argon isotope, so high-precision measure- ments require rocks with high argon concentrations. Precam- brian 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 com- position (Cherdyntsev and Kolesnikov, 1965) or isotopically fractionated atmosphere of unknown age (Rison, 1980). For * Author to whom correspondence should be addressed (trieloff @min.uni-heidelberg.de). Geochimica et Cosmochimica Acta, Vol. 69, No. 5, pp. 1253–1264, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 1253