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Introduction to Detrital Apatite and Zircon Fission-track Thermochronology

Matthias BERNET

Institut des Sciences de la Terre, University Grenoble Alpes, France

1.1. Introduction

The use of detrital apatite and zircon fission-track and (U-Th)/He dating or white mica 40Ar-39Ar dating of modern river and beach sediments or ancient sandstone is a common approach in sediment provenance, source rock exhumation and basin analysis studies. In this chapter, the focus is on detrital apatite and zircon fission-track dating. Both techniques are considered as low-temperature thermochronology dating techniques, with temperature sensitivities in the range of ~ 250–180°C for zircon and ~ 130–80°C for apatite in comparison to high-temperature dating techniques such as zircon or monazite U-Pb dating (Figure 1.1).

Figure 1.1. Overview of the temperature sensitivity range of selected isotopic dating techniques.

Apatite (density of 3.1–3.3 g/cm3) and zircon (density of 4.5–4.6 g/cm3) are heavy minerals in contrast to the density of quartz (2.65 g/cm3). Despite the highly variable apatite and zircon fertility of many upper crustal plutonic, volcanic and metamorphic rocks, both apatite and zircon are relatively common accessory minerals in many sand-sized clastic sediments and sedimentary rocks (Malusà and Garzanti 2019). Although apatite is susceptible to dissolution in acid depositional environments (e.g. bogs) or soils, and abrasion during fluvial transport, zircon is considered as ultra-stable, as long as the grains have not accumulated too much α-radiation damage and are metamict (Malusà and Garzanti 2019; Malusà and Fitzgerald 2020). Both minerals are chemically stable under diagenetic conditions during burial in sedimentary basins or during basin inversion and exhumation.

Since the first detrital zircon fission-track analysis studies in the 1980s (e.g. Hurford et al. 1984; Cerveny et al. 1988), detrital apatite and zircon fission-track dating have developed into standard techniques that have been applied successfully to many different geodynamic settings, for example, the European Alps, the Himalaya, the Tibetan plateau, the Andes, the Southern Alps of New Zealand, or in Alaska, for quantifying exhumation or erosion rates, determining the timing of tectonic events, or the thermal history of sedimentary basins (see examples and references below). Today, it is possible to combine fission-track dating with U-Pb dating and even with (U-Th)/He dating for double and triple dating of single grains, as well as chemical analyses such as Sr isotopes in apatites and Lu and Hf isotopes in zircon. The combination of different techniques on single grains may provide additional valuable information for constraining more precisely sediment provenance and source rock or basin thermal histories.

This chapter provides an introduction to (A) the basics of detrital fission-track analysis using the external detector method (EDM), (B) the underlying statistics for age calculations, data interpretation and the evaluation of detrital grain age distributions, and (C) applications of detrital thermochronology with some examples.

1.2. Principals of fission-track dating

1.2.1. Basics of single grain apatite and zircon fission-track analysis

Fission-track dating has been developed in the 1960s by Robert L. Fleischer, P. Burford Price and Robert M. Walker, three physicists of General Electrics in Schenectady, New York, USA, who worked on nuclear defects in solids. The formation of a fission track by spontaneous fission of 238U isotopes is in fact a very rare event as radioactive 238U isotopes normally decay by a series of α and β-decay steps to stable 206Pb (Figure 1.2(A)). The regular 238U to 206Pb decay is about 2 million times more common than the spontaneous fission decay of 238U into two isotopes of different mass that recoil from each other and leave a damage zone in the crystal structure, which is called a latent track (Table 1.1; Figure 1.2(B)) (Fleischer et al. 1975; Wagner and Van den haute 1992). Early on Fleischer, Price and Walker realized that the formation of latent damage zones by spontaneous fission of 238U isotopes in crystals of U-bearing minerals such as apatite, zircon or titanite could be of geological interest (e.g. Price and Walker 1962; Fleischer and Price 1964; Fleischer et al. 1975). The isotopes formed during the spontaneous fission events are not always the same. In fact, two of over 690 different possible isotopes, one heavier isotope with a mass number of up to 172 and a lighter isotope with a mass number as low as 66, are formed. Most of these newly formed isotopes are also radioactive and will decay over seconds, hours, days, months or years, but without causing additional fission damage. The formation of spontaneous fission-track damage in a U-bearing crystal follows a decay constant, just like regular α-decay, and the fission tracks that are formed because of the spontaneous fission decay of 238U are regarded as the daughter products of this decay event (Table 1.1). Given the isotopic abundance and the spontaneous fission-decay constant of 238U isotopes with respect to 235U, 234U and 232Th, only 238U is considered as important for fission-track dating, and the contributions of spontaneous fission tracks from 235U, 234U and 232Th spontaneous fission are negligible. In dependence of U content, fission tracks accumulate in crystals over time and will be at least partially preserved, if the ambient temperatures are below the so-called closure temperature (Tc).

Figure 1.2. (A) 238U-206Pb decay chain, with a series of eight α-decay steps and six β-decay steps and their half-lives. (B) Spontaneous fission of 238U and formation of latent fission tracks.

Table 1.1. Decay constants and half-lives of U and Th isotopes. Note: α for α-decay, s.f. for spontaneous fission decay (from Wagner and Van den haute (1992); Donelick et al. (2005))

1.2.2. Closure temperature concept

The Tc is the temperature at which an isotopic system closes to the loss of daughter products (Dodson 1973). In case of fission-track dating, it means that fission tracks are preserved and not lost to total annealing, which occurs at elevated temperatures above the closure temperature. The Tc of the apatite and zircon fission-track dating systems, and also other thermochronological systems such as (U-Th)/He or 40Ar/39Ar dating, depends primarily on cooling rate (Figure 1.3(A)). Equation [1.1] shows that the Tc is controlled by diffusion and can be calculated as follows:

where Ea is the activation energy, R is the gas constant, A is the cylinder shape of the crystal, τ is the characteristic time taken for the diffusivity to decrease by a factor e and Do/a2 is the diffusion. Because the calculation of τ also requires a Tc value (see equation [1.2]), it is necessary to find an iterative solution for Tc. Table 1.2 shows commonly used values of activation energy and diffusion parameters for apatite and zircon fission-track dating determined by laboratory diffusion experiments (see summary in Wagner and Van den haute (1992); Reiners and Brandon (2006)).

Equation [1.2] also shows why the cooling rate, the change of temperature (δT) over time (δt), has an important influence on the Tc. The parameter...