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1. 40Ar/39Ar analytical procedures
Sample T0 (Table 1) was irradiated for a duration of 40 hours in the position 5C in the nuclear reactor at the McMaster University in Hamilton, Canada. Groups of flux monitors (15 in total) were located at intervals along the length of the irradiation capsule and J-values for individual samples were determined by second-order polynomial interpolation between replicate analyses of splits for each position in the capsule. Typically, J-values vary by <10% over the length of the capsule. No attempt is made to monitor horizontal flux gradients as these are considered to be minor in the core of the reactor. The samples and flux monitors were loaded into pits in a copper sample-holder, beneath a differentially-pumped, ZnS view-port on a small, bakeable, stainless-steel chamber connected to an ultra-high vacuum purification system. A New Wave Research MIR10-30 CO2 laser was used for step-heating and the laser beam was defocused to cover the entire sample.
Measured argon-isotope peak heights were extrapolated to zero-time, normalized to the 40Ar/36Ar atmospheric ratio (295.5) using measured values of atmospheric argon, and corrected for neutron-induced 40Ar from potassium, 39Ar and 36Ar from calcium and 36Ar from chlorine (Roddick, 1983). Dates and errors were calculated using formulae given by Dalrymple et al. (1981), and the constants recommended by Steiger and Jäger (1977). Isotope correlation analysis used the formulae and error propagation of Hall (1981) and the regression of York (1969). Errors shown on the age spectrum and isotope correlation diagram represent the analytical precision at 1σ, assuming that the error in the age of the flux monitor is zero. This is suitable for comparing within-spectrum variation and determining which steps form a plateau (McDougall and Harrison, 1988). A conservative estimate of this error in the J-value is 0.5% and can be added for inter-sample comparison. The dates and J-values are referenced to Hb3Gr hornblende at 1072 Ma (Roddick, 1983).
Sample T1-A (Table 2) was irradiated at the USGS TRIGA Reactor, Denver, Colorado along with GA1550 biotite flux monitors (98.79 ± 0.54 Ma, (Renne et al., 1998) to calculate J-factors and K2SO4 and CaF2 salts to calculate correction factors for interfering neutron reactions. Following a 2 to 3 week cooling period to allow for the decay of short-lived isotopes, samples were loaded into the arms of a glass storage tree above a double-vacuum, resistance-heated furnace and heated to 120° at the same time that the entire extraction line was baked for 48 hours at 220°C. Getters and furnace were independently degassed near the end of the bake-out. Samples were then dropped into the furnace and argon was extracted from each sample using a computer controlled step-heating routine. The temperature of the furnace is estimated to be accurate to ± 20°C. Each heating step had a duration of 12 minutes followed by a cool down to 500°C prior to advancing the gas into two successive gettering stages for argon purification. The argon was then admitted into a VG 5400 mass spectrometer, where it was ionized and detected by a VG electron multiplier and digitized with a Keithley 617 Electrometer. Data collection and processing were accomplished using the computer program Mass Spec (Deino, 2001). The decay constants used were those recommended by Steiger and Jäger (1977). Baseline values were subtracted and the isotopic measurements then were regressed to time zero using standard linear regression techniques. Additional corrections and associated uncertainties were applied to account for blanks, machine discrimination, atmospheric contribution, and interfering isotopes produced in the reactor from Ca, K and Cl present in the samples.
Sample T1-N (Table 3) was irradiated at the U.S. Geological Survey TRIGA Reactor, Denver, Colorado. The sample was in-core for 7 hours in the In-Core Irradiation Tube (ICIT) of the 1 MW TRIGA type reactor. Correction factors for interfering neutron reactions on K and Ca were determined by repeated analysis of K-glass and CaF2 fragments. Measured (40Ar/39Ar)K values were 1.74 (± 67.07%) x 10-2. Ca correction factors were (36Ar/37Ar)Ca = 2.16 (± 8.78%) x 10-4 and (39Ar/37Ar)Ca = 6.70 (± 1.60%) x 10-4. J factors were determined by fusion of 4-8 individual crystals of neutron fluence monitors which gave reproducibility’s of 0.25% to 0.48% at each standard position. Variation in neutron fluence along the 100 mm length of the irradiation tubes was <4%. Matlab curve fit was used to determine J and uncertainty in J at each standard position. No significant neutron fluence gradients were present within individual packets of crystals as indicated by the excellent reproducibility of the single crystal fluence monitor fusions. Irradiated FC-2 sanidine standards together with CaF2 and K-glass fragments were placed in a Cu sample tray in a high vacuum extraction line and were fused using a 20 W CO2 laser. Sample viewing during laser fusion was by a video camera system and positioning was via a motorized sample stage. Samples analyzed by the furnace step heating method utilized a double vacuum resistance furnace similar to the Staudacher et al. (1978) design. Reactive gases were removed by three GP-50 SAES getters prior to being admitted to a MAP 215-50 mass spectrometer by expansion. The relative volumes of the extraction line and mass spectrometer allow 80% of the gas to be admitted to the mass spectrometer for laser fusion analyses and 76% for furnace heating analyses. Peak intensities were measured using a Balzers electron multiplier by peak hopping through 7 cycles; initial peak heights were determined by linear regression to the time of gas admission. Mass spectrometer discrimination and sensitivity was monitored by repeated analysis of atmospheric argon aliquots from an on-line pipette system. Measured 40Ar/36Ar ratios were 279.72 ± 0.43% during this work, thus a discrimination correction of 1.0565 (4 AMU) was applied to measured isotope ratios. The sensitivity of the mass spectrometer was ~6 x 10-17 mol mV-1 with the multiplier operated at a gain of 36 over the Faraday. Line blanks averaged 26.00 mV for mass 40 and 0.01 mV for mass 36 for laser fusion analyses and 18.97 mV for mass 40 and 0.07 mV for mass 36 for furnace heating analyses. Discrimination, sensitivity, and blanks were relatively constant over the period of data collection. Computer automated operation of the sample stage, laser, extraction line and mass spectrometer as well as final data reduction and age calculations were done using LabSPEC software written by B. Idleman (Lehigh University). An age of 28.02 Ma (Renne et al., 1998) was used for the Fish Canyon Tuff sanidine fluence monitor in calculating ages for samples.
2. Rock magnetism
Magnetic hysteresis loops and IRM curves were acquired in order to assess the coercivity distributions of the magnetic assemblages. Eight crushed representative samples were measured with a Lakeshore 7404 Vibrating Sample Magnetometer at the Laboratorio de Efecto Mössbauer y Magnetismo from the Universidad Nacional de La Plata, Argentina. Normalized hysteresis loops from bulk samples yield a signal dominated by magnetite (Fig. 1 a, f,), magnetite and paramagnetic minerals (Fig. 1 c) and by mixture of magnetite and hematite resulting in wasp-waisted hysteresis loops (Tauxe et al. 1998) (Fig. 1 b, e, g, h and i). The presence of a mixture of hematite and magnetite results in IRM curves are consistent with the hysteresis measurements, with samples showing low field saturation typical of almost exclusive presence of magnetite (Fig. 2 a, c, e,) and samples showing a combination of magnetite plus hematite, as denoted by lack of saturation at high induced fields (Fig. 6 b, d, f, g).
Dalrymple, G. B., Alexander Jr., E. C., Lanphere, M. A. & Kraker, G. P., 1981. Irradiation of samples for 40Ar/39Ar dating using the Geological Survey TRIGA Reactor, U.S. Geological Survey, Professional Paper, 1176
Deino, A.L., 2001, Users manual for Mass Spec v. 5.02, Berkeley Geochronology Center Special Publications, 1a
Hall, C.M., 1981. The application of K–Ar and 40Ar/39Ar methods to the dating of recent volcanics and the Laschamp event. Ph.D. thesis, University of Toronto.
McDougall, I., & Harrison, T.M., 1988. Geochronology and Thermochronology by the 40Ar/39Ar Method, 212 pp., Oxford University Press, New York.
Renne, P. R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., & DePaolo, D.J., 998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating, Chemical Geology, 145, 117-152.
Roddick, J. C., 1983. High precision intercalibration of 40Ar/39Ar standards, Geochimica et Cosmochimica Acta, 47, 887-898.
Tauxe, L., 1998. Paleomagnetic principles and practice. Kluwer Academic Publishers, pp. 299.
Steiger, R. H., & Jäger, E., 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmo-chronology, Earth and Planetary Science Letters, 36, 359-362.
Staudacher, T. H., Jessberger, E. K., Dorflinger, D. & Kiko, J., 1978. A refined ultrahigh-vacuum furnace for rare gas analysis, Journal of Physics E: Scence. Instruments, 11, 781-784.
York, D., 1969. Least squares fitting of a straight line with correlated errors. Earth and Planetary Science Letters, 5, 320–324
Figure 1. Hysteresis loops from selected samples
Figure 2. Acquisition of isothermal remanent magnetization (IRM) in selected samples
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|Supplementary Methods||1 Supplementary Lecture Material|