Raman scattering from metamict zircon: comments on “Metamictisation of natural zircon: accumulation versus thermal annealing of radioactivity-induced damage” by Nasdala et al. 2001 (Contribution to Mineralogy and Petrology)141: 125–144

Abstract

In a recent paper, Nasdala et al. (2001) reported the results of a study of metamictization and the annealing process in natural zircons using micro-Raman spectroscopy. The main objective of this paper was to show that the linewidth (given as full width at half maximum) of the asymmetrical m3 Si–O stretching band of zircon near 1,008 cm and its radiation-dose dependence could be used to recognize zircons which have been annealed in their geological history. The authors demonstrated that a comparison of the line-width versus a-decay dose relationship obtained from natural, unannealed rhyolite zircons with those obtained from unknown samples provides a potential means of examining the low-temperature history of the host rock. This concept, which has previously been proposed by Pidgeon et al. (1995, 1998), opens new perspectives for the interpretation of discordant U–Pb ages and for the reconstruction of the thermal history. However, the main conclusion drawn by Nasdala et al. (2001) about the physical effect of annealing of radiation-damaged zircon rests upon a simplification of the physical meaning of the linewidth of the m3 Si–O band. The major purpose of this comment is (1) to show that the linewidth of this band is not only related to the short-range order as suggested by Nasdala et al. (2001), but that it is complexly dependent on the microstructure of metamict zircon, (2) to discuss their conclusions about the recovery process in metamict zircon during annealing, (3) to demonstrate that the physical significance of the linewidth combined with the phonon frequency – an aspect which was ignored by Nasdala et al. (2001) – opens new perspectives for the concept of radiation-damage ages as a geochronological tool, and (4) to further re-evaluate the high potential of radiation-damage ages, which contrasts with the rather cautious conclusions drawn by Nasdala et al. (2001).