Analytical Methods

Analytical Methods

U-Pb age determinations

Zircons were prepared by standard mineral separation and purification methods (crushing and milling, concentration via hand washing, magnetic separation, and heavy liquids). In order to minimize the effects of secondary lead loss, chemical abrasion techniques were used, involving high-temperature annealing followed by a HF leaching step (Mattinson 2003, 2005). This technique has been shown to be more effective in removing strongly radiation-damaged zircon domains, which underwent lead-loss during post-crystallization fluid processes (Mundil et al. 2004) and is described in detail in Ovtcharova et al. (2006). After adding a mixed 205Pb-235U spike, zircons were dissolved in 63 ml concentrated HF with a trace of 7N HNO3 at 180°C for 5 days, evaporated and re-dissolved overnight in 36 ml 3N HCl at 180°C. Titanites were handpicked under the binocular microscope and microfractions consisting of 3 to 6 grains were dissolved using HF-HNO3 in Savillex vials at 120°C on a hotplate for 8 days. Pb and U were separated by anion exchange chromatography using 40 ml micro-columns and minimal amounts of ultra-pure HCl for zircon, and in 400 µl columns, including a HBr cleaning step, for titanite; eluants were dried down with 3 ml 0.06N H3PO4.

Isotopic analysis of DTR64 and DTR184 titanite was performed in ETH-Zurich (Switzerland) on a MAT262 mass spectrometer equipped with an ETP electron multiplier backed by a digital ion counting system. Calibration and non-linearity correction of the ETP electron multiplier are detailed in Ovtcharova et al. (2006). Analysis of DTR69 titanite and of zircon samples was done on a Triton mass spectrometer at the University of Geneva (Switzerland), using a linear MasCom multiplier, which was calibrated using Sr SRM987, Pb SRM982, and U500 solutions. Both Pb and U were loaded with 1 ml of silica gel-phosphoric acid mixture on outgassed single Re filaments, and Pb as well as U (as UO2) isotopes were measured sequentially on the electron multiplier. Total procedural common Pb blanks are 0.5 ± 0.2 pg for zircon and 8 pg for titanite. Common lead concentrations in excess of the blank were corrected using the model of Stacey and Kramers (1975). The international R33 standard zircon (Black et al. 2004) has been dated at an age of 419.3 ± 0.3 Ma for 18 determinations on both MAT262 (ETH Zurich) and Triton (University of Geneva) mass spectrometers, and at 419.19 ± 0.27 Ma for 13 Triton measurements only. Calculation of concordant ages and averages was done with the Isoplot/Ex v.3 program of Ludwig (2003). Ellipses of concordia diagrams (Figure 3) represent 2s uncertainties. Errors of the mean 206Pb/238U ages are all reported at 95% confidence limits with and without decay constant uncertainty, and include non-systematic errors from the analysis plus the systematic errors of spike and blank isotopic composition propagated to each individual data point. Isotopic data of the U-Pb age determinations on zircon and titanite of the Nambija district are reported in Table 1.

Re-Os age determinations

Molybdenite was extracted from hand samples using a small hand-held drill. The molybdenite targeted for Re-Os analysis was fresh, and to the extent possible relatively pure, and occurred in a clear paragenetic relationship with other features and veins in the hand specimen. Dilution of the mineral separate by quartz (whose incorporation in the analyzed fraction has no effect on the calculated Re-Os age) or other sulfides was nevertheless noted under a high-power binocular microscope before sample dissolution.

Samples were digested using the Carius tube method whereby the sample is dissolved and equilibrated with isotopically-enriched Re and Os spikes in HNO3-HCl acid. The mixture was sealed in a thick-walled glass ampoule and heated for 12 hours at 275°C (Shirey and Walker, 1995). Os was recovered by solvent extraction using either CCl4 (Pangui district molybdenite) or CHCl3 (Nambija district molybdenite), followed by micro-distillation for further purification. The Re was separated and purified using anion exchange columns. Analytical details can be found in Markey et al. (1998, 2003) and Zimmerman et al. (2008).

Purified Re and Os were loaded on Pt filaments, and isotopic compositions were determined using negative thermal ion mass spectrometry (NTIMS: Creaser et al. 1991; Völkening et al. 1991). All Re-Os analyses were performed in the high-level laboratory at the AIRIE Program, Colorado State University, using Faraday cup measurements taken on a NBS 12-inch radius, 90° sector mass spectrometer.

Complete analytical data are presented in Table 2, and details of associated uncertainties are provided in footnotes. For molybdenites with high Re, such as the data reported in this study, the selection of the initial Os isotopic ratio does not significantly influence age calculations. The results are also insensitive to the amount of common Os introduced by minor chalcopyrite and/or pyrite in several of the molybdenite separates. Nonetheless, a correction for any common Os was made. Similarly, blank corrections are insignificant in this study, but were nevertheless undertaken. AIRIE uses several in-house molybdenites as reference materials to track reproducibility in the lab.

40Ar/39Ar age determinations

Selected samples were processed for 40Ar/39Ar analysis of biotite or hornblende by standard mineral separation techniques, including hand-picking of grains in the size range 0.5 to 1 mm. Individual mineral separates were loaded into aluminum foil packets along with a single grain of Fish Canyon Tuff Sanidine (FCT-SAN) to act as flux monitor (apparent age = 28.02 ± 0.28 Ma; Renne et al. 1998). The sample packets were arranged radially inside an aluminum can. The samples were then irradiated for 12 hours at the research reactor of McMaster University in a fast neutron flux of approximately 3x1016 neutrons/cm2.

Laser 40Ar/39Ar step-heating analysis was carried out at the Geological Survey of Canada laboratories in Ottawa, Ontario. Upon return from the reactor, samples were split into one or two aliquots of one to four grains each, and loaded into individual 1.5 mm-diameter holes in a copper planchet. The planchet was then placed in the extraction line and the system evacuated. Heating of individual sample aliquots in steps of increasing temperature was achieved using a 45W CO2 laser equipped with slightly defocused beam. The released Ar gas was cleaned over getters for ten minutes, and then analyzed isotopically using the secondary electron multiplier system of a VG3600 gas source mass spectrometer; details of data collection protocols can be found in Villeneuve and MacIntyre (1997), and Villeneuve et al. (2000). Error analysis on individual steps follows numerical error analysis routines outlined in Scaillet (2000); error analysis on grouped data follows algebraic methods of Roddick (1988).

Corrected argon isotopic data are listed in Table 3, and presented as spectra of gas release plots (Figure 4). Each gas-release spectrum plotted contains step-heating data from one aliquot. Such plots provide a visual image of reproducibility of heating profiles, evidence for Ar-loss in the low temperature steps, and the error and apparent age of each step.

Neutron flux gradients throughout the sample canister were evaluated by analyzing the sanidine flux monitors, included with each sample packet, and interpolating a linear fit against calculated J-factor and sample position. The error on individual J-factor values is conservatively estimated at ±0.6% (2s). Because the error associated with the J-factor is systematic and not related to individual analyses, correction for this uncertainty is not applied until calculation of dates from isotopic correlation diagrams (Roddick, 1988). If there is no evidence for excess 40Ar the regressions are assumed to pass through the 40Ar/36Ar value for atmospheric air (295.5) and are plotted on gas release spectra. Blanks were estimated at between 40Ar = 2.5–3.6x10-7 nm, 39Ar = 4.2–13.3x10-9 nm, 38Ar = 0.4–1.7x10-9 nm, 37Ar = 0.4–1.7x10-9 nm, 36Ar = 0.7–1.3x10-9 nm, all at ±20% uncertainty. Nucleogenic interference corrections are (40Ar/39Ar)K = 0.025 ± 0.005, (38Ar/39Ar)K = 0.011 ± 0.010, (40Ar/37Ar)Ca = 0.002 ± 0.002, (39Ar/37Ar)Ca = 0.00068 ± 0.00004, (38Ar/37Ar)Ca = 0.00003 ± 0.00003, (36Ar/37Ar)Ca = 0.00028 ± 0.00016. All errors are quoted at the 2s level of uncertainty.

References

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