Radiation damage ages: practical concept or impractical vision?: reply to two comments on 'Metamictisation of natural zircon: accumulation versus thermal annealing of radioactivity-induced damage', and further discussion

Abstract

In a study of little to moderately radiation-damaged zircons from various localities, Nasdala et al. (2001) have proposed that the relation between the Raman band broadening and the time-integrated a-dose can be used to examine the accumulation of alpha-event damage in unknown zircons. Comparing Raman-dose data pairs of unknowns with those of un-annealed zircons would provide an indication for whether or not the unknowns have completely stored their a-damage. As one among seven examples, the examination of gem zircons from Sri Lanka gave strong evidence for partial structural reconstitution through thermal annealing experienced by these zircons. Two discussions on this paper were received. John I. Garver commented FT (fission track) measurements on Sri Lankan zircons, which underline the incomplete storage of radiation damage in these samples. Thorsten Geisler and Robert T. Pidgeon wrote a fairly critical discussion. They did not question that the comparison of the present degree of metamictisation with the selfirradiation dose may provide information on complete or incomplete a-damage storage, which was the main conclusion of Nasdala et al. (2001). In fact, Geisler and Pidgeon (2002, this volume) stated that such comparison provides even more precise information, and they hold to the general practicability of ‘‘radiation damage ages’’. Other issues raised by these two authors, however, are not directly related to the content of the article by Nasdala et al. (2001) but criticise the conclusions of previous publications. The discussion by Geisler and Pidgeon (2002, this volume) is for the most part rather an addendum to the article of Geisler et al. (2001) and re-discusses their observations and conclusions. In replying to Geisler and Pidgeon (2002, this volume), we are therefore obliged to refer to Geisler et al. (2001), too. In the present paper, we will reply to the two discussions and give further relevant comments. Our responses will focus on points raised by Garver (2002, this volume) and Geisler and Pidgeon (2002, this volume) which are related to the article by Nasdala et al. (2001), namely, the interpretation of Raman spectra, their use to estimate radiation damage and their potential application for geochronological purposes. Critical aspects in the work of Geisler et al. (2001) and the discussion by Geisler and Pidgeon (2002, this volume) will only be discussed if relevant for the issues above.