CHAPTER 12: COMBINED APATITE FISSION TRACK AND U–Pb DATING BY LA–ICP–MS AND ITS APPLICATION IN APATITE PROVENANCE ANALYSIS David M. Chew Department of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland [email protected] and Raymond A. Donelick Apatite to Zircon, Inc., 1075 Matson Road, Viola, Idaho 83872-9709, U.S.A. [email protected] INTRODUCTION To date, the majority of apatite provenance studies Apatite is a common accessory mineral in have focused on detrital thermochronology of igneous, metamorphic and clastic sedimentary apatite using the fission track or (U–Th)/He thermo- rocks. It is a nearly ubiquitous accessory phase in chronometers (e.g., Bernet & Spiegel 2004). Apatite igneous rocks, due in part to the low solubility of can incorporate nearly half of the elements in the P2O5 in silicate melts and the limited amount of periodic table in its crystal structure and many trace phosphorus incorporated into the crystal lattices of elements in apatite display a large range of the major rock-forming minerals (Piccoli & Candela concentrations. Unlike zircon, the trace element 2002). Apatite is common in metamorphic rocks of partition coefficients in igneous apatite are typically pelitic, carbonate, basaltic, and ultramafic compos- very sensitive to changes in magmatic conditions ition and is found at all metamorphic grades from (Sha & Chappell 1999) and therefore the trace transitional diagenetic environments to migmatite element chemistry of detrital apatite provides a link (Spear & Pyle 2002). Apatite is also virtually to the parent igneous rock type in provenance ubiquitous in clastic sedimentary rocks (Morton & studies (Jennings et al. 2011). Hallsworth 1999). Zircon provenance studies have been Apatite is widely employed in low temperature revolutionized in the last decade by the advent of thermochronology studies with the apatite fission the LA–ICP–MS U–Pb method, which offers low track and apatite (U–Th)/He thermochronometers cost, rapid data acquisition and sample throughput yielding thermal history information in the 60– compared to the ID–TIMS or ion microprobe U–Pb 110°C (Laslett et al. 1987) and 55–80°C (Farley methods (e.g., Košler & Sylvester 2003). LA–ICP– 2000) temperature windows, respectively. Apatite MS U–Pb dating of apatite is more challenging as has also been employed in high temperature apatite typically yields lower U and Pb thermochronology studies, which demonstrate that concentrations and higher common Pb to radiogenic the U–Pb apatite system has a closure temperature Pb ratios which nearly always necessitate common of ca. 450–550°C (Chamberlain & Bowring 2000, Pb correction. In contrast to the well documented Schoene & Bowring 2007). Apatite has also been polycyclic behavior of the stable heavy mineral employed in Lu–Hf geochronology studies (Barfod zircon, apatite is unstable in acidic groundwater and et al. 2003) and as an Nd isotopic tracer (Foster & weathering profiles and has only limited mechanical Vance 2006, Gregory et al. 2009). stability in sedimentary transport systems (Morton Detrital apatite analysis has many potential & Hallsworth 1999). It therefore more likely applications in sedimentary provenance studies. represents first cycle detritus, and hence U–Pb Detrital thermochronology is a provenance tool apatite dating would yield complementary which deciphers the thermotectonic history of information to U–Pb zircon provenance studies. source regions (typically orogenic belts) by Fission track and U–Pb dating are therefore two studying the chronology of their erosional products. of the most useful (and most rapid) techniques in Mineralogical Association of Canada Short Course 42, St. John’s NL, May 2012, p. 219-247 219 APATITE FISSION TRACK AND U–PB DATING BY LA–ICP–MS apatite provenance studies. They yield linear defects are commonly referred to as fission complementary information, with the apatite fission tracks, and are enlarged using a standardized track system yielding low-temperature exhumation chemical etching process so they can be observed ages and the U–Pb system yielding high- under an optical microscope (Price & Walker 1962). temperature cooling ages which help constrain the The technique is widely applied to apatite, zircon timing of apatite crystallization. This chapter and titanite because they contain sufficient U focuses on integrating apatite fission track and (typically >10 ppm) to generate a statistically useful U–Pb dating by the LA–ICP–MS method. The use quantity of spontaneous fission tracks over of LA–ICP–MS to calculate U concentrations in geological time. By comparing the density of fission fission track dating dramatically increases the speed tracks with the U content of the mineral, an of analysis and sample throughput compared to the apparent fission track age can be calculated. A conventional external detector method, as well as fission track age provides an estimate of the time avoiding the need for neutron irradiation (e.g., that has elapsed since the mineral cooled through a Hasebe et al. 2004). Other chemical composition specific temperature window (referred to as the data (e.g., trace elements and the REE) can be partial annealing zone or PAZ). Apatite, zircon and acquired at the same time as U concentration data, titanite each have their own specific PAZ. The and these data are very useful in detrital apatite apatite PAZ is estimated at 60–110°C although this provenance studies. Trace element compositional varies with apatite composition (Green et al. 1986, data are also highly useful for characterizing the Carlson et al. 1999, Barbarand et al. 2003). At thermal annealing kinetics of fission tracks in temperatures higher than the PAZ, there is sufficient apatite and can be used in detailed time– energy to completely anneal (or remove) fission temperature history modeling using programs such tracks via thermally activated diffusion of the as HeFTy (Ketcham 2005). relocated ionic species in the lattice. At Our analytical protocol for combined apatite temperatures lower than the PAZ, there is fission track and U–Pb dating by LA–ICP–MS is insufficient energy to cause significant repair of outlined following summaries of both methods. Our fission tracks. Fission tracks are partially annealed approach is intentionally broad in scope, and should at temperatures within the PAZ. be applicable to any quadrupole or rapid-scanning To calculate the apparent fission track age of a magnetic-sector LA–ICP–MS system. Potential mineral we need to i) estimate the amount of 238U applications of combined apatite fission track and decay that is recorded in the mineral lattice and U–Pb dating and future trends in apatite provenance which is given by the spontaneous fission track analysis are discussed at the end of this chapter. density, and ii) estimate the amount of 238U (the parent isotope). The spontaneous fission track OVERVIEW OF APATITE FISSION TRACK density is calculated by counting etched fission DATING tracks by optical microscopy. In most fission track Fission track dating is a widely used technique dating studies the amount of 238U is obtained after for reconstructing the low-temperature thermal irradiation of the samples by thermally activated histories of upper crustal rocks. It has been used to neutrons in a nuclear reactor. This approach, study the tectonic and thermal histories of referred to as the external detector method (e.g., compressional and extensional margins, the stability Gleadow 1981) is described in the next section. of continental interiors, the thermal histories of In addition to the fission track age which yields sedimentary basins and their source regions, information on the timing of cooling through the landscape evolution, and to constrain the timing of PAZ, the apatite fission track method also yields ore mineralization events. The method has been information on the nature of the cooling path. This described and reviewed elsewhere (e.g., Gleadow information is obtained from the distribution of 1981, Gleadow et al. 1986a, Gleadow et al. 1986b, confined fission track lengths in a sample (Gleadow Green 1988, Gallagher et al. 1998, Donelick et al. et al. 1986a). Confined fission tracks are horizontal 2005) and this chapter only presents a brief (or <10° from horizontal) tracks that lie in a c-axis overview of the technique. prismatic section, such that both ends of the track Fission track dating is based on the are visible entirely within the polished and etched spontaneous fission decay of 238U which produces apatite crystal without altering the focal depth. linear defects in the lattice of U-bearing minerals Unannealed, spontaneous fission track lengths in (Fleischer et al. 1975, Price & Walker 1963). These natural apatite grains typically range between ~14.5 220 D.M. CHEW & R.A. DONELICK and 15.5 µm depending on its chemical composition known and homogeneously distributed U contents. (Gleadow et al. 1986a). For example, an apatite The induced fission tracks registered in all the grain which exhibits a fission track population with muscovite detectors are then chemically etched. mean track lengths in this range and a narrow Subsequently, the 238U content of any specific variation in track length distribution (e.g., <1.5 µm), portion of an apatite crystal can be determined by would be interpreted to have cooled relatively counting the induced fission tracks in the muscovite rapidly from a temperature ≥ 110°C to a detector that was in contact with that region of the temperature ≤60°C at the time indicated by the crystal. apparent apatite fission track age. A shorter mean track length with a broad standard distribution Advantages and disadvantages of an LA–ICP– indicates that the sample resided in the PAZ for a MS-based approach significant period of time since the formation of the LA–ICP–MS-based U concentration measure- oldest fission tracks (Gleadow et al. 1986b). The ments in apatite fission track dating were first apatite fission track age and track length described in detail by Hasebe et al. (2004).