U-Pb Isotopic Data and Ages of Detrital Zircon from Selected Rocks from Southwestern Alaska and the Yukon-Tanana Upland and Environs of Eastern Interior Alaska and Eastern Yukon, Canada

(University of British Columbia) Zircons were separated from their host rocks using conventional mineral separation methods and sectioned in an epoxy grain mount along with grains of internationally accepted standard zircon (FC-1, a ~1100-Ma zircon standard), and brought to a very high polish. The grains were examined using a stage- mounted cathodoluminescence imaging setup that makes it possible to detect the presence of altered zones or inherited cores within the zircon. The highest quality portions of each grain, free of alteration, inclusion, or cores, were selected for analysis. The surface of the mount was then washed for ~10 minutes with dilute nitric acid and rinsed in ultraclean water. Analyses were carried out using a New Wave 213nm Nd-YAG laser coupled to a Thermo Finnigan Element2 high resolution ICP-MS (inductively coupled plasma mass spectrometer). Ablation took place within a New Wave "Supercell" ablation chamber, which was designed to achieve very high efficiency entrainment of aerosols into the carrier gas. Helium was used as the carrier gas for all experiments, and gas flow rates, together with other parameters such as torch position, were optimized before beginning a series of analyses. We typically used a 25-micron spot with 60 percent laser power and did line scans rather than spot analyses to avoid within-run elemental fractions. Each analysis consisted of a 7-second background measurement (laser off) followed by a ~28-second data acquisition period with the laser firing. A typical analytical session consisted of four analyses of the standard zircon, followed by four analyses of unknown zircons, two standards, four unknowns, and so forth, and finally four standard analyses. Data were reduced using the GLITTER software package developed by the GEMOC group at Macquarrie University, which subtracts background measurements, propagates analytical errors, and calculates isotopic ratios and ages. This application generated a time-resolved record of each laser shot. For detrital zircon samples, 60-100 grains were analysed.

(Apatite to Zircon, Inc.) Mineral separates were obtained at the laboratories of Apatite to Zircon, Inc., in Viola, Idaho. Lithium polytungstate and a centrifuge were used in place of the conventional Wilfley table, thus guarding against loss of zircon grains that might inadvertently be washed away, undetected, in the conventional method. Zircons (both standards and unknowns) were then mounted in 1-cm2 epoxy wafers and ground down to expose internal grain surfaces before final polishing. Grains, and the locations for laser spots on these grains, were selected for analysis from all sizes and morphologies present using transmitted light with an optical microscope at a magnification of x 2000. This approach is used because it allows the recognition and characterization of features below the surface of individual grains, including the presence of inclusions and the orientation of cracks, which could otherwise result in spurious isotopic counts.

(Apatite to Zircon, Inc.) Isotopic analyses were performed with a New Wave UP-213 laser ablation system in conjunction with a ThermoFinnigan Element2 single collector double-focusing magnetic sector inductively coupled plasma- mass spectrometer (LAICP-MS) in the GeoAnalytical Lab at Washington State University. In comparison to a quadropole ICP-MS, the Element2 has flat-top peaks and higher sensitivity, resulting in larger Pb signals, better counting statistics, and more precise and accurate measurement of isotopic ratios. For all analyses (both standard and unknown), the diameter of the laser beam was set at 20 µm and the laser frequency was set at 5 Hz, yielding ablation pits ~10-15 µm deep. He and Ar gas were used to deliver the ablated material into the plasma source of the mass spectrometer. Each analysis of 250 cycles took approximately 30 seconds to complete and consisted of a 6-second integration on peaks with the laser turned off (for background measurements) followed by a 25-second integration with the laser firing. A delay of as much as 30 seconds occurred between analyses in order to purge the previous analysis and prepare for the next. The isotopes measured included 202Hg, 204(Hg + Pb), 206Pb, 207Pb, 208Pb, 232Th, 235U, and 238U. The Element2 detector was set at analog mode for 232Th and 238U and at pulse counting mode for all other isotopes. Common Pb correction was made by using the measured 204Pb content and assuming an initial Pb composition from Stacey and Kramers (1975). Interelement fractionation of Pb/U is generally <20 percent, whereas fractionation of Pb isotopes is generally <5 percent. At the beginning of each LA-ICP-MS session, zircon reference materials (Peixe and FC1) were analyzed until fractionation was stable and the variance in the measured 206Pb/238U and 207Pb/206Pb ratios was at or near 1 percent. In order to correct for interelement fractionation during the session, these standards were generally reanalyzed after each 15-25 unknowns. Fractionation also increases with depth into the laser pit. The accepted isotopic ratios were accordingly determined by least-squares projection through the measured values back to the initial determination.

(Apatite to Zircon, Inc. and Geosep Services) Mineral separates were obtained at the laboratories of Geosep Services in Moscow, Idaho. Lithium polytungstate and a centrifuge were used in place of the conventional Wilfley table, thus guarding against loss of zircon grains that might inadvertently be washed away, undetected, in the conventional method. Zircons (both standards and unknowns) were then mounted in 1-cm2 epoxy wafers and ground down to expose internal grain surfaces before final polishing. Grains, and the locations for laser spots on these grains, were selected for analysis from all sizes and morphologies present using transmitted light with an optical microscope at a magnification of x 2000. This approach is used because it allows the recognition and characterization of features below the surface of individual grains, including the presence of inclusions and the orientation of cracks, which could otherwise result in spurious isotopic counts. Data were collected for the following isotopic masses: 202Hg, 204Hg+204Pb, 206Pb, 207Pb, 208Pb, 232Th, 235U, and 238U (250 data scans over 30 s) followed by 28Si and 91Zr (5 data scans over 4 s). The instruments used were a New Wave YAG 213 nm laser ablation (LA) system in line with a Finnigan Element2 magnetic sector, inductively coupled plasma, mass spectrometer (ICP-MS) at the Washington State University Geoanalytical Laboratory in Pullman, Washington, U.S.A. (e.g., Chang et al., 2006). All analyses were performed using a 20 µm spot. Following approximately 6 s of background data collection, laser ablation commenced and data were collected for the ablated material. Ablated material was transported to the plasma line using He; Ar was the plasma gas.

(USGS Denver) Samples were collected from the field and sent to U.S. Geological Survey-Geology, Geophysics and Geochemistry Science Center, Denver, CO. Zircon was separated from the rock using standard rock crushing and separation procedures. Hand-picked zircon grains were mounted in epoxy and subjected to cathodoluminescence (CL) imaging on the FEI Quanta 450 FEG scanning electron microscope at the USGS Microbeam Laboratory in Denver, Colorado. LA-ICPMS U-Th-Pb data was obtained for the zircon grains using a Teledyne-Photon Machines Excite-Analyte(TM) 193 nm excimer laser and a Nu Instruments Attom(TM) high-resolution sector field ICPMS at the USGS-Denver G3 Plasma Lab. Laser ablation spot sizes were 25 um and the locations were chosen based on the zoning pattern of each grain as shown in the CL imaging as well as the absence of cracks, veins, and inclusions. Prior to analysis, the Laser Ablation: ICPMS system was tuned using National Institute of Standards and Technology (NIST) 610 glass to achieve optimal signal intensity, peak shape, and Th/U ratio. Primary reference material Temora2 (Black and others, 2004) as well as secondary reference materials Plesovice (Slama and others, 2008) and FC1 (Mattinson, 2010) were analyzed during the run.