A tungsten isotope approach to search for meteoritic components in terrestrial impact rocks

The identification of meteorite impact structures on Earth is based on two main criteria: the presence of shock-metamorphic effects in the crater rock ejecta and/or the confirmation of an extraterrestrial (meteoritic) component in breccias or melt rocks. For the latter, both high elemental abundances of siderophile elements (especially the platinum group elements) with corresponding meteoritic inter-element ratios and the osmium (Os) and chromium (Cr) isotopic signatures characteristic of meteorites have been used successfully. Inspired by earlier suggestions of a meteoritic component in Archean rocks based on tungsten (W) isotope anomalies, here we explore the possible use of W-182, which has been produced by the decay of now extinct Hf-182(T-1/2 = 8.9 Ma), as a tracer of meteoritic component in terrestrial material Each group of meteorites has. W isotopic compositions that are distinct from each other and from the terrestrial crust. W-182 has already been used to try to identify the impactor at the K/T boundary by analyzing the sediments and Ni-rich spinel. in the present study, we broaden the field of investigation by choosing a different approach, namely analyzing a variety of known impact rocks. We measured the W isotope composition in four samples from different impact structures (Gardnos, Norway; Morokweng, South Africa; Vredefort, South Africa: Ries, Germany) as well as in two samples from a distal ejecta layer (K-T boundary samples from Cams, Austria. and Berwind Canyon, USA). All these samples are unambiguously impact-produced and in several of those materials a meteoritic component has unequivocally been identified by other geochemical proxies. In all these samples, the isotopic composition of W is identical with analytical error to that of the Earth's continental crust, and no W-182 anomalies are present, even in the samples containing a significant (percent level) meteoritic component. Therefore, we conclude that, in contrast to the Cr or Os isotopes, W isotopes are not suitable for the identification of meteoritic components in terrestrial rocks. (C) 2009 Elsevier B.V. All rights reserved.