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Supplementary material

Apatite fission-track (AFT) method

Fission tracks form within an apatite crystal as the result of the spontaneous fission of ²³⁸U. During fission, the nucleus splits into two particles that are repelled by each other, thereby creating a linear zone of damage in the crystal lattice that is revealed by chemical etching (Price and Walker 1962). Once cooled below ~60°C, and if unaffected by subsequent higher temperatures, fission tracks at surface temperatures retain a relatively constant mean length of about 14 μm over geologic time. However, with increased temperature, the damaged crystal lattice repairs itself through the process of annealing, which is manifested as track shortening (e.g., Naeser 1979). Tracks in apatite whose composition is similar to the Durango fluorine-rich apatite standard are partially annealed above ~60°C and totally annealed above ~110°C (Gleadow et al. 1986; Dumitru 2000; Tagami and O’Sullivan 2005; Donelick et al. 2005). This temperature range is referred to as the "partial annealing zone" (PAZ). In addition to yielding shorter fission tracks, increased annealing also results in lower track density and reduced fission-track age. Apatite fission-track ages of samples that exceeded 110°C prior to rapid cooling—the case with all the igneous samples that constitute our study—typically show little variation between single-grain ages and have narrow symmetrical age distributions and mean track-length distributions> 14 µm with low standard deviations, in the case of volcanic samples cooled at ambient temperatures, and >13 µm for intrusive samples that cooled rapidly through the PAZ (Donelick et al. 1990).

Apatite Fission-Track Laboratory Procedures

Apatite separates for all but one sample were obtained from crushed and separated material using standard gravimetric and magnetic mineral separation techniques prepared during previous thermochronologic or geochronologic studies. Spontaneous track counts and confined track length measurements were performed using nonpolarized light at 2000x magnification. Laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) analyses of samples used in age determinations for Apatite to Zircon, Inc. (A2Z) reports 931 through 1198 were performed by Paul B. O’Sullivan using the Element2 mass spectrometer located at the Washington State University School of Earth and Environmental Sciences GeoAnalytical Laboratory in Pullman, Washington. LA-ICP-MS analyses for A2Z report 1396 were carried out at the Donelick Properties office in Viola, Idaho.

A general discussion of the methods undertaken to process and analyze samples is presented below. For each sample subjected to apatite fission-track analysis (AFT), at least one 1cm² grain mount, consisting of apatite grains immersed in epoxy resin, was prepared, cured at 90C for 1 hour, and polished to expose internal surfaces of the apatite grains. After polishing, mounts were immersed in 5.5N HNO₃ for 20.0 seconds ( 0.5 seconds) at 21C ( 1C) to reveal all natural fission tracks that intersected the polished grain surfaces.

The feasibility of measurement of apatite fission-track grain ages and track lengths was assessed by scanning the polished and etched grain mount to determine if any dateable apatite grains were present. Measurement of fission-track parameters was considered feasible if more than one dateable grain was observed.

Representative kinetic parameters (Dpar—the maximum diameter of fission track etch pits at their intersection with the polished and etched, c-axis-parallel apatite surface, which is used as a proxy for the solubility of fission tracks in their host apatite grains) were measured and spontaneous (natural) fission-track densities were counted for each grain considered suitable for dating. Between one and four etch pit diameters were measured and an arithmetic mean Dpar value was calculated for each datable grain.

LA-ICP-MS Analysis

Grains were then revisited using the LA-ICP-MS to make spot analyses to determine U, Th, and Sm concentrations of each grain for which natural fission-track densities had been previously determined. A single stationary spot of 16 µm diameter was used for each grain, centered in the approximate center of the area where tracks had been counted. The depth of each pit is listed in Supplementary Table S1. Note that if optical examination suggested that natural track densities were even moderately inconsistent within a grain, which is evidence of U zoning, that grain was not dated..

For apatite, the fundamental assumption is made that Ca occurs in stoichiometric amounts in all grains analyzed. The isotope ⁴³Ca is used as the indicator of the volume of apatite ablated. Samples were ablated in a helium atmosphere to reduce condensation and elemental fractionation. A total of 30 scans for ²³⁸U, ²³²Th, ¹⁴⁷Sm, and ⁴³Ca were performed for each spot analyzed. Of these scans, approximately 10 were performed while the laser was warming up and blocked from contacting the grain surface, during which time background counts were collected. Once the laser was permitted to hit the grain surface, a cylindrical pit was excavated to a depth beyond which uranium did not contribute fission tracks to the etched grain surface. Between 15 and 20 scans performed during pit excavation were required to reach this depth. The depths of a representative number of laser pits were measured and the ²³⁸U/⁴³Ca value for each pit as a whole was determined based on the weighted mean of the ²³⁸U/⁴³Ca value for individual scans relative to the depths from which the ablated material was derived (see Hasebe et al. 2004; Donelick et al. 2005).

Fission-Track Age Measurement

Fission-track ages and errors were calculated using: (a) the ratio of the density of natural fission tracks present in the grain to the amount of ²³⁸U present and (b) a modified version of the radioactive decay equation that includes a LA-ICP-MS zeta calibration factor (see equations 1b for age equation and 2b for error calculation in Donelick et al. 2005). The zeta calibration factor is determined for each sample analyzed during each LA-ICP-MS session by analyzing the U:Ca ratio of apatite calibration standards with known ages at the beginning and end of each LA-ICP-MS session. The standards used are Durango apatite, 30.6 ± 0.3 Ma.

Apatite Fission-Track Length Measurement

In order to enhance the number of confined tracks available for length measurement (e.g., Donelick and Miller 1991; Donelick et al. 2005), subsequent to fission-track age determination the grain mounts were irradiated with approximately 10⁷ tracks/cm² fission fragments from a ²⁵²Cf source in a vacuum chamber. Donelick and Miller (1991) demonstrated that irradiating apatite grains with ²⁵²Cf-derived fission fragments could yield a 20-fold increase in the number of available fission tracks for length measurement. The ²⁵²Cf-irradiated apatite mounts were re-etched using the same formula as before in order to reveal horizontal, confined fission tracks within the apatite grains. Only natural, horizontal, confined fission tracks in apatite with clearly visible ends were considered candidates for length measurement. The length and crystallographic orientation of each fission track were determined using a digitizing tablet interfaced with a personal computer. The precision of each track length is estimated to be ±0.20 m; the precision of each track angle to the crystallographic c-axis is estimated to be ±2 degrees.

Summary of HeFTY Data Modeling Parameters

Interpretative t-T modeling was completed for each sample using version 1.8.0 of the kinetic modeling program HeFTy (Ketcham 2005; Ketcham et al. 2007). All age calculations completed within HeFTy were done using the age equations presented in Donelick et al. (2005). All apatite grains analyzed for this study were etched for 20 seconds at 21°C using 5.5 N HNO₃ as required to utilize the annealing equations used in HeFTy (Ketcham et al. 1999). The following parameters were used for all kinetic modeling within HeFTy:

1) C-axis projected track lengths calculated using the measured angle between each track’s orientation relative to the c-axis of the grain

2) ²⁵²Cf-irradiation used for enhanced confined-track measurements

3) Mean Dpar values measured for every age and track-length grain

4) Original track counts used for grain-age calculations

5) Calculations used annealing model of Ketcham et al. (1999)

6) A default value of 0.893 used for the length reduction in age standard

7) the default zeta mode used the LA–ICP–MS ratios

8) Lom=(0.283)(Dpar)+15.63 μm and Loc=(0.35)(Dpar)+15.72 μm

9) The range in the kinetic variable Dpar for all age and length grains within each sample was small (<0.5 µm), so a single population using the midpoint Dpar value was modeled.

10) Other than default values for the starting (100 Ma, 200°C; 250°C for three samples), and ending (0 Ma, 0°C), time and temperature, few additional t-T constraints were introduced to each model that might have restricted the possible results.

11) All modeled results were generated using 10,000 model paths with Monte Carlo scheme, with each path segment constrained by monotonic cooling, each segment halved one time, and without enforcing maximum heating or cooling rates.

12) Only pooled ages are presented; these incorporate the original track counts and isotopic values for each grain, and therefore are most representative of the original generated data.

REFERENCES

Donelick, R.A., and Miller, D.S. 1991. Enhanced TINT fission track densities in low spontaneous track density apatites using ²⁵²Cf-derived fission fragment tracks: A model and experimental observations. Nuclear Tracks and Radiation Measurements, 18: 301–307.

Donelick, R.A., Roden, M.K., Mooers, J.D., Carpenter, B.S., Miller, D.S. 1990. Etchable length reduction of induced fission tracks in apatite at room temperature (~23° C): crystallographic orientation effects and “initial" mean lengths. Nuclear Tracks and Radiation Measurements, 17: 261–265.

Donelick, R.A, O’Sullivan, P.B., and Ketcham, R.A. 2005. Apatite fission-track analysis. Reviews in Mineralogy and Geochemistry, 58: 49–94.

Dumitru, T.A. 2000. Fission-track geochronology. In Quaternary geochronology: Methods and applications, 4. Edited by J.S. Noller, J.M. Sowers, and W.R. Lettis. American Geophysical Union Reference Shelf: 131–155. doi: 10.1029/RF004p0131

Gleadow, A.J.W., Duddy, I.R., Green, P.F., and Lovering, J.F. 1986. Confined fission track lengths in apatite: A diagnostic tool for thermal history analysis. Contributions to Mineral Petrology, 94: 405–415.

Hasebe, N., Barbarand, J., Jarvis, K., Carter, A., and Hurford, A.J. 2004. Apatite fission-track chronometry using laser ablation ICP-MS. Chemical Geology, 207: 135–145.

Ketcham, R.A. 2005. Forward and inverse modeling of low-temperature thermochronometry data. Reviews in Mineralogy and Geochemistry, 58: 275–314.

Ketcham, R.A., Donelick, R.A., and Carlson, W.D. 1999. Variability of apatite fission-track annealing kinetics: III. Extrapolation to geological time scales. American Mineralogist, 84: 1235–1255.

Ketcham, R.A., Carter, A., Donelick, R.A., Barbarand, J., and Hurford, A.J. 2007. Improved modeling of fission-track annealing in apatite. American Mineralogist, 92: 799–810.

Naeser, C.W 1979. Fission-track dating and geologic annealing of fission tracks. In Lectures in Isotope Geology. Edited by E. Jäger and J.C. Hunziker. Springer Publishing Co., Inc., New York, pp. 154–169.

Price, P.B., and Walker, R.M. 1962. Chemical etching of charged particle tracks. Journal of Applied Physics, 33: 3407–3412.

Tagami, T., and O’Sullivan, P.B. 2005. Fundamentals of fission-track thermochronology. Reviews in Mineralogy and Geochemistry, 58: 19–47.

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