Fission-track analysis of detrital zircon.
Résumé
Zircon has become one of the most important minerals for studying sediment provenance and the exhumation history of orogenic belts. The reason for this utility is that zircon is common in many igneous, metamorphic, and sedimentary rocks, it is resistant to weathering and abrasion, and it can be dated with various isotopic methods having reasonable high concentrations of uranium and thorium (Fig. 1). Techniques used to date detrital zircon include U/Pb and (U-Th)/He dating, but in this chapter we focus exclusively on fission-track (FT) analysis. FT analysis allows age determination of single zircon grains that may have cooling ages between several hundred thousand to a billion years or more. The datable range depends on individual uranium content and cooling history of a zircon grain. Fission tracks in zircon have an effective annealing temperature of ~240° C +/- 30° C in natural systems (Hurford 1986; Brandon et al. 1998; Bernet et al. 2002). Therefore most detrital zircon are fairly resistant to thermal annealing in typical sedimentary basins after deposition, while the other low-temperature thermochronometers anneal at lower temperatures common in sedimentary basins (i.e. Helium dating and apatite FT) and therefore more readily have compromised provenance information (Fig. 2). Consequently, the strength of detrital zircon fission-track (DZFT) analysis lies in the fact that this method provides robust cooling ages of source terrains. The ability of zircon to retain information about the most recent thermal history of a source area is invaluable in elucidating the processes and system response in a range of geodynamic settings, especially the evolution of orogenic belts. This characteristic makes DZFT dating superior to U/Pb dating when the objective is to link sedimentation to the uplift and exhumation history of the source terrain. U/Pb dating of single crystals provides crystallization ages (or zircon growth during metamorphism), which typically pre-date the latest orogenic cycle. This long-term memory is partly due to the fact that zircon is so robust that recycling is common and it is typical for zircons to be polycyclic, even in crustal melts. As such, a U/Pb age on a detrital zircon may have little bearing on the nature of the immediate source rock, but may be ideal for understanding the long-term record of crustal formation. In addition, U/Pb ages of detrital zircon rarely allow the determination of exhumation rates and because of the possibility of multiple recycling of zircon grains their U/Pb ages can only vaguely be assigned to non-distinct source regions. Therefore, FT ages tend to be directly related to actively evolving source terrains. Consequently, as we explain below, DZFT analysis is a method ideally suited for: a) tracing the provenance of clastic sediments; b) determining stratigraphic ages in volcanically active areas; c) studying the long-term exhumation history of convergent mountain belts with little active volcanism, and d) dating low-temperature thermal events. Some interesting recent work has been aimed at combining DZFT with U/Pb or (U-Th)/He dating on the same grains or samples (see below, i.e. Reiners et al. in review). In this chapter we explain basic aspects of DZFT analysis, and provide some practical considerations on sampling techniques in the field and laboratory analysis. We then show how results can be presented and discuss the interpretation of fission-track grain-age (FTGA) distributions in several different applications. Finally, we give an overview of the current developments in DZFT analysis and end by outlining some outstanding issues that need further attention.
Domaines
Géochimie
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