Fission-track Analysis of Detrital Zircon

Abstract

INTRODUCTIONZircon 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 2 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...

Mesozoic and Cenozoic Cooling History of the Qiangtang Block, Northern Tibet, China: New Constraints from Apatite and Zircon Fission Track Data

“…Previous studies show that many factors (including radiation damage, U concentration, time, temperature, cooling rate, and pressure) can affect the annealing properties of ZFT (Bernet and Garver 2005). Of these factors, radiation damage has the greatest effect.…”

Fission track thermochronology of Neogene plutons in the Principal Andean Cordillera of central Chile (33-35°S): Implications for tectonic evolution and porphyry Cu-Mo mineralization

“…As a reference, Tagami and Shimada (1996) have shown that zircon fission track ages in sandstones surrounding a rapidly cooled granitic intrusion become totally reset within a distance of ~3 km from a steep-dipping contact, although this distance can vary depending on several factors, including the size and shape of the intrusives, the permeability of the wallrock, the depth of emplacement, and the presence of fluids (e.g., Adriasola et al, 2006). The zircon fission track ages are interpreted to represent the time of post-emplacement cooling to temperatures of around 280±30°C (e.g., Tagami and Shimada, 1996;Bernet and Garver, 2005;Reiners and Brandon, 2006), and the biotite 40 Ar/ 39 Ar closure temperature can be estimated as 320±30°C using Dodson (1973) and parameters presented by Harrison et al (1985) and McDougall and Harrison (1999). Therefore, the overall coincidence of the zircon fission track ages with biotite 40 Ar/ 39 Ar ages (Table 4) is consistent with in situ post-magmatic rapid cooling of the host intrusions.…”

“…However, experimental data show that there is a wide temperature range, from 160 to 380°C, for the temperature bounds for the zircon partial annealing zone, which are strongly dependent on cooling rates and alpha radiation damage of zircon crystals (Bernet and Garver, 2005;Tagami, 2005;Reiners and Brandon, 2006). Thus, as a reasonable approximation for this work, a temperature of 280±30°C has been plotted against the zircon fission track age in study to produce a single temperature-time point on a thermal history path (e.g., Tagami and Shimada, 1996;Thomson et al, 2001;Reiners and Brandon, 2006;Adriasola and Stökhert, 2008).…”

Intense localized rock uplift and erosion in the St Elias orogen of Alaska

“…The key finding is that samples from the Seward-Malaspina glacier have a significant fraction of ZFT ages of <3 Myr, whereas samples farther west have increasingly larger percentages of older ages (>6 Myr). Analysis of these data using binomial peak-fitting routines 15 yields age populations for each sample. A remarkable aspect of the cooling ages in the Seward-Malaspina samples is that single-grain ZFT ages are as young as 0.4 Myr and define an age-population peak at 2-3 Myr that comprises 25-41% of the dated grains.…”