Methodology

Apatite fission track thermochronology

Fission tracks are radiation damage trails that form in non-conductive materials (e.g. minerals and glasses) by the spontaneous fission of 238U. On geological timescales, fission tracks in apatite anneal rapidly at temperatures above 120°C and the annealing rate is very low at temperatures below 60°C, with the 60-120°C temperature window defined as the apatite Partial Annealing Zone or PAZ (e.g. Gallagher et al. 1998). Other factors such as apatite composition (particularly Cl content) can influence the track annealing process (e.g. Green et al. 1986). In addition, the distribution of track lengths is critical in interpreting fully the fission track age (Gleadow et al. 1986a). 
The apatite mineral separates in this study were embedded in small epoxy resin disks (15 mm diameter × 1.5 mm deep) and polished to expose internal apatite grain surfaces. Spontaneous tracks were revealed by acid etching using 5.5 M HNO3 at 21°C for 20s.  The grain mounting and etching procedures are very similar to the protocols described by Donelick et al. (2005). Fission track counting and length measurements were carried out using a Zeiss AxioImager Z1m microscope equipped with an Autoscan automated stage system in Trinity College Dublin. Where possible at least 20 grains were counted per sample, while 100 horizontal confined tracks were measured for the track length distribution (Gleadow et al. 1986b; Gallagher et al. 1998).  The orientation of the horizontal confined tracks relative to the c-axis was also determined as both the annealing and etching of fission tracks in apatite are anisotropic (Green 1981; Barbarand et al. 2003a; Ketcham et al. 2007). The long axes of the fission-track etch pits (termed Dpar) were also measured as a proxy for the compositional dependence of annealing (Carlson et al. 1999; Barbarand et al. 2003b; Ketcham et al. 2007). 
Apatite uranium concentrations were measured using a Photon Machines Analyte Excite 193 nm ArF Excimer laser ablation system coupled to a Thermo Scientific iCAP Qc ICPMS in Trinity College Dublin. The analytical protocol employed is described in detail in Chew and Donelick (2012), and integrates uranium concentration measurements for apatite fission track dating with REE and trace element analyses along with the isotopes required for required for U-Th-Pb geochronology.  The time interval for the baseline correction was 15 s and the analysis was 45 s.  The laser spot size was set at 30 m with a repetition rate of 4Hz and a fluence of 3.1 mJ/cm2 to yield spot depths typically 10 m deep.  A HelEx 2-volume sample cell was used with a He carrier gas flow of 800 mL/min.  Ar and N2 as an additional di-atomic gas to enhance sensitivity (flow rate of 6 mL / min) were admixed downstream. Typically 28 isotopes were measured in each apatite sample.   Of these isotopes, 43Ca was used as the internal standard to correct for drift in instrument sensitivity and variations in ablation volume during sessions. Common trace elements in apatite (e.g. Cl, Mn, Sr and Y) and the REE were determined along with the seven isotopes (200Hg, 204,206,207,208Pb, 232Th and 238U) commonly required for U-Th-Pb geochronology.  Data reduction for both trace element concentration measurements and U-Th-Pb geochronology was undertaken using the freeware Iolite package of Paton et al. (2011). The Iolite approach involves processing an entire analytical session (typically > 5 h) of data, which is not only more efficient but also greatly improves the consistency and reliability of data reduction. 
Trace element concentration measurements were undertaken using the “Trace Elements” data reduction scheme in Iolite.  Cl concentration measurements followed the analytical protocol of Chew et al. (2014a).  The 35Cl background-corrected signals for each apatite analysis were normalized to the internal standard (43Ca) and then sample-standard bracketing was employed using apatites of known Cl concentration (Bamble chlorapatite (6.56 wt% Cl, Chew et al., 2014a) and Durango fluorapatite (0.37 wt% Cl, Chew et al., 2014a)).  
Uranium concentration in unknown apatites followed the analytical protocol of Donelick et al. (2005).  In contrast to the ICPMS absolute calibration approach of Hasebe et al. (2004), which employs the 238U fission-decay constant, a fission track registration factor and a calibration factor for etching and observation, Donelick et al. (2005) used a modified zeta calibration approach (cf. Hurford and Green 1983) for ICPMS apatite U concentration measurements.  The method assumes that the apatite 43Ca signal intensity during a given LA-ICP-MS session acts as a proxy for the volume of apatite ablated, and hence the apatite 238U/43Ca ratio yields relative U concentration measurements.  In this study an extensive primary LA-ICPMS session was undertaken on crystals of Durango apatite (31.44 ± 0.18 Ma (2σ), McDowell et al., 20005) that were previously counted for fission tracks to yield a primary LA-ICPMS zeta factor.  The uncertainty on the calibration procedure (including the age uncertainty and the counting statistics related to the number of spontaneous tracks counted in the Durango standard) were propagated through to the final age calculation. These same crystals of Durango apatite were then analysed in subsequent LA-IPCMS (along with apatite unknowns for fission-track dating) to yield a session-specific calibration factor on the primary zeta value.  Inter-session drift in both the primary and subsequent LA-ICPMS sessions was corrected for by monitoring the 238U/43Ca ratio of NIST612 standard glass. The main difficulty in apatite fission-track dating by LA-ICPMS is ensuring that the uranium distribution with the portion of sample that generates fission tracks on the polished apatite surface is quantified spatially, both horizontally (i.e. along the surface of the grain mount) and vertically.  The spontaneous track density in the samples analysed in this study was high enough to detect possible U zonation, while depth-related variations in U concentration were accounted for here by incorporating a function within the Iolite “Trace elements” data reduction scheme that weights appropriately the 238U/43Ca ratio with depth.  As LA–ICPMS is a destructive technique high quality 3-D images of all the counted grains were archived for potential reinvestigation in the future.
Apatite U-Th-Pb geochronology was undertaken using a modified version of VizualAge data reduction scheme (Petrus and Kamber 2012) for Iolite.  This data reduction scheme (VizualAge_UcomPbine) can correct for variable amounts of common Pb in any U–Pb accessory mineral standard as long as the standard is concordant in the U/Pb (and Th/Pb) systems after common Pb correction (Chew et al. 2014b).   A ca. 1 cm sized crystal of Madagascar apatite (Thomson et al. 2012) which has yielded a weighted average ID-TIMS concordia age of 473.5 ± 0.7 Ma (Cochrane et al. 2014) was used as the primary apatite reference material in this study. McClure Mountain syenite apatite (207Pb/235U TIMS age of 523.51 ± 1.47 Ma; Schoene and Bowring, 2006) and Durango apatite (31.44 ± 0.18 Ma, McDowell et al. 2005) were employed as secondary standards.  Common Pb correction employed weighted mean average 207Pb-corrected ages.  An iterative approach based on terrestrial Pb evolution models (Stacey and Kramers 1975) was used to calculate the initial Pb isotopic composition for detrital apatite samples as described in Chew et al. (2011).

 (U+Th)/He analysis

U+Th/He system is sensitive to temperature between 40 and 110°C (Shuster and Farley 2009). Radiation damage produced by α-recoil is also important in the diffusion behaviour of He because high U and Th concentration will produce high amount of damages and thus will enhance He retention (Green at al. 2006; Shuster et al. 2006; Flowers et al. 2009; Gautheron et al. 2009; Shuster and Farley 2009). Additionally, -ejection (recoil) leads to non-diffusive He loss in a zone within about 20 µm of the apatite crystal edge. Therefore, the measured (U+Th)/He age can be corrected by an FT factor, which is a function of the surface/volume ratio of the crystal grain (Farley et al. 1996, Gautheron et al. 2012). Alternatively, the -ejection process can be corrected for explicitly during thermal modelling (Meesters and Dunai 2002). Other factors such as grain size or cooling rate will also influence the diffusion of He (e.g. Farley 2000).
Apatites were picked under a Nikon SMZ 1500 microscope under dark field illumination, plane-polarized light and cross-polarized light with a rotating stage at a magnification of 110.  Apatite grains were selected according to their shape, size, and lack of obvious fractures or inclusions. The selected crystals were grouped into between three to five separate aliquots (typically containing one and very occasionally two crystals) for each sample and put into Pt tubes. Helium was degassed using a laser and measurement was performed by a quadrupole mass spectrometer. Re-heating was undertaken to confirm total degassing and the possible presence of unidentified U-Th rich inclusions during picking (Farley 2002). After degassing, the samples were spiked with 235U and 230Th and U and Th concentrations were determined in solution by ICP-MS. All procedures are described in detail in Foeken et al. (2006). The minimum uncertainty on the calculated ages of the apatite unknowns is fixed at 10% based on repeated (U+Th)/He measurements of the Durango apatite standard.

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