Zircon (ZrSiO4) is by far the most commonly utilized mineral for U–Pb dating, that is frequently found in the above-mentioned ash layers. It allows determining the age of a targeted stratigraphic horizon Figure 1.4. Evolution of carbonate carbon isotopic composition (δ13Ccarb) and oxygen isotopic composition of biogenic phosphate (δ18Ophos) during the Smithian and Spathian (Early Triassic) in Pakistan. Taken from Goudemand et al. (2019), red dashed line from Romano et al. (2013). Positive δ13Ccarb values coincide with higher δ18Ophosvalues, and negative δ13Ccarbvalues coincide with lower δ18Ophosvalues, Fluctuations in δ18Ophos
indicating changes in sea surface temperatures, variation in δ13Ccarb reflecting changes in the isotopic composi-tion of marine carbonate. VPDB, Vienna PeeDee Belemnite; VSMOW, Vienna Standard Mean Oceanwater.
at an optimal precision and accuracy. Zircon appears to be resistant to chemical and physical alteration in various geological settings (e.g., Scherer et al., 2007; Finch and Hanchar, 2003). In conjunction with low diffusion rates, its content of minor- and trace elements, chemical- as well as isotopic information are retained (Scherer et al., 2007; Finch and Hanchar, 2003). Additionally, high concentrations of acti-nides (e.g., Uranium, Thorium) and exclusion of common Pb during crystallization makes zircon a val-uable tool in high precision dating and for detailed studies of magmatic processes. However, the chem-ical and physchem-ical properties of zircon and its ability to incorporate and retain trace elements are largely determined by its crystal structure (Finch and Hanchar, 2003, and references therein). However, due its high concentration in actinides, zircon suffers from radiation damage due to α-decay causing a disor-dered crystal lattice, that provides pathways for Pb to diffuse through the lattice (Nasdala et al., 2005) (Fig. 1.5). The loss of radiogenic lead occurs along altered zones of the crystal lattice as a consequence of the structural damage (Fig. 1.5b), which is not following the rules of temperature-activated (“Fickian”) volume diffusion (Cherniak and Watson, 2000; Lee et al., 1997) but occurring along “short circuit”
pathways (Bowring et al., 2006), and is facilitated by its high solubility in diagenetic or hydrothermal fluids percolating the zircon lattice (Geisler et al., 2003, 2002). The loss of radiogenic lead from the zircon crystal structure biases the apparent U-Pb age to younger ages.
The age of zircon can be determined on the basis of its U-Pb ratio. Natural uranium contains three isotopes: 238U (99.3 wt. %), 235U (0.72 wt. %) and 234U (0.006 wt. %) (Priest, 2001), with their corresponding decay constants λ238 = 1.55125 x 10-10 yr-1, and λ235 = 9.8485 x 10-10 yr-1 (Jaffey et al., 1971). Among all geochronologic decay schemes, the U-decay constants are the most precisely and ac-curately determined (Jaffey et al., 1971; Mattinson, 2010; Schoene et al., 2006). Furthermore, the natural uranium isotope ratio had been redetermined to 238U/235U = 137.818 ± 0.045 (2σ) (Hiess et al., 2012).
The parent uranium decays to stable daughter isotopes Pb by emitting alpha particles;
(1) 238U 206Pb + 8 𝛼 (4He) (2) 235U 207Pb + 7 𝛼 (4He) (3) 232Th 208Pb + 6 𝛼 (4He)
Respectively, this allows calculating U-Pb age by the following equations,
Eq. 1: 𝑃𝑏 called a Concordia diagram (Wetherill, 1965). The Concordia diagram allows the recognition of potential complications such as inheritance, Pb loss (“open-system”), or initial daughter product disequilibrium that all result in a discordant analysis (Schoene, 2014). A concordant analysis plotting within error on this curve indicates a “closed system” of the dated mineral. However the discordance between the
206Pb/238U and 207Pb/235U isotopic systems is recently challenged as a valid indicator for Pb-loss, due to
small sample size, very low concentration of Pb* and/or by low radiogenic/common lead ratios of single zircon analysis resulting in elevated analytical uncertainties of 207Pb/235U ratios (e.g., Schoene et al 2014).
The analysis of U and Pb isotope ratios in zircon (ZrSiO4) by thermal ionization is considered the most precise and accurate geochronological dating technique. The technique enables to acquire ab-solute ages via tracer solutions (EARTHTIME tracer; 202Pb-205Pb-233U-235U (ET2535); 205Pb-233U-235U (ET535)) that are calibrated against a series of certified isotopic reference material and are traceable to SI units (Condon et al., 2015; McLean et al., 2015). U–Pb analysis by isotope-dilution thermal ionization mass spectrometry (ID-TIMS) involves a chemical pre-treatment prior dissolution of single grains or grain fragments, mixing with an isotopically-enriched tracer solution (e.g., Schoene and Baxter, 2017;
Schoene et al., 2010), ion exchange chromatography and TIMS analysis. Lead and uranium isotopes are separated from the dissolved sample by ion exchange chromatography, in order to improve ionization and to eliminate polyatomic or isobaric interferences from other elements (Krogh, 1973). The sample is then loaded with a Si-gel emitter onto a Re filament to improve the ionization efficiency of U and Pb (Gerstenberger and Haase, 1997). In an additional step, geochemical or isotopic information on exactly the same volume of dated material can be obtained such as concentrations of REE (TIMS-TEA), or the isotope composition of Hf, which allows to extract additional information of the crystallization history of a zircon grain, and/or of the tectonic setting of magmatism (Samperton et al., 2015; Schoene et al., 2012; Schoene et al., 2010; Schaltegger et al., 2002, 2009).
Reliable and meaningful ages require control on those factors that cause dispersion of ages, beside precision these concern accuracy, within-laboratory repeatability and interlaboratory reproduci-bility. The uncertainty on a measurement comprises non-systematic- (e.g., raw isotope measurements) and systematic uncertainties (e.g., tracer composition, decay constants) (Schoene, 2014).
Fig 1.5. a) Schematic illustration of radioactive decay in zircon. 1) 238U and 235U isotope decays to 2) stable 206Pb and 207Pb, respectively. Decay constants λ238 = 1.55125 x 10-10 yr-1, and λ235 = 9.8485 x 10-10 yr-1, after Jaffey et al.
(1971). b) crystal structures of zircon, including those be-fore and after radiation damage. Radioactive decay trans-fers a non metamict crystal with well-ordered crystal lattice to a metamict zircon with disordered crystal lattice, provid-ing pathways for Pb to diffuse through the lattice. c) molec-ular-dynamics simulation animation. b) and c) taken from Xu et al. (2012).
Considerable advances of ID-TIMS technique in the last 15 years concern the improved accu-racy and repeatability at a precision of about 0.1% (2) (Schoene, 2014) using single zircon crystals (usually <300 µm) or fragments. This has been achieved by lowering of procedural blanks, more precise and accurate, empirical calibration of the U-decay constants (Mattinson, 2010; Schoene et al., 2006), measuring the UO2 oxygen isotopic composition (Condon et al., 2015), improved understanding of initial daughter product disequilibrium (Schärer, 1984), as well as the correction for mass-dependent isotope fractionation during mass spectrometric measurements using double-isotope tracer solutions (McLean et al., 2015).
It is instrumental to minimize systematic uncertainties to allow a direct comparison of dates produced in different laboratories or using different protocols. The long term goal of an intercalibration between U-Pb-laboratories at the level of 0.1% of 206Pb/238U ages has been achieved by 1) the distribu-tion of the gravimetric EARTHTIME 202Pb-205Pb-233U-235U tracer solution calibrated against certified isotopic reference materials (Condon et al., 2015; McLean et al., 2015); 2) introduction of commonly used statistical data reduction software such as Redux and Tripoli (Bowring et al., 2011); and 3) the improved understanding of error contribution to the final age and its propagation (Schmitz and Schoene, 2007). Furthermore, repeated analysis of synthetic reference solutions (e.g., EARTHTIME 100, 500 and 2000) and/or well-characterized natural reference materials (such as Plešovice, Temora, R33, GJ-1) have been established as quality control of laboratory reproducibility and accuracy (Schaltegger et al., 2015).
This is indispensable to directly compare dates at the same precision from different high precision U-Pb-laboratories (Condon et al., 2015; McLean et al., 2015) or between different TIMS instruments (Schaltegger, 2018) without systematic error.
A breakthrough of the ID-TIMS technique to mitigate the impact of Pb-loss, has been the im-plementation of the so-called chemical-abrasion (Mattinson, 2005), that involves an annealing step at >
900 °C for 48 − 60 hours to restore the zircon crystal structure that is followed by a chemical treatment with hydrofluoric acid in order to remove domains compromised by Pb-loss, leaving an undisturbed (closed-system) residue. The method was successfully applied by various studies (e.g., Schoene et al., Figure 1.6. Schematic procedure of the CA-ID-TIMS technique. Zircon picture taken from Rioux et al. (2007).
2010; Mundil et al., 2004). However, the CA-technique is based on an empiric calibration and a clear understanding of the underlying mechanisms is still lacking. Moreover, the proposed protocol by Matti-son (2005) has been modified by different laboratories in terms of the applied temperature and duration of the chemical abrasion step. This makes the comparison of U-Pb dates difficult between laboratories and is a problem for the accuracy of the acquired dates. Therefore, a more thorough and quantitative, albeit empirical calibration of the experimental conditions during chemical abrasion is an important step forward for the U-Pb dating community. In the second chapter of the thesis an empirical approach is undertaken that will contribute to harmonise laboratory procedures to allow improved comparsion of the acquired U-Pb dates between high-precision U-Pb laboratories.