U-Pb geochronology of zircons was conducted by laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) at the Arizona LaserChron Center . The analyses involve ablation of zircon with a New Wave/Lambda Physik DUV193 Excimer laser (operating at a wavelength of 193 nm) using a spot diameter of ____ microns. The ablated material is carried in helium into the plasma source of a GVI Isoprobe, which is equipped with a flight tube of sufficient width that U, Th, and Pb isotopes are measured simultaneously. All measurements are made in static mode, using 10e11 ohm Faraday detectors for 238U, 232Th, 208Pb, and 206Pb, a 10e12 ohm faraday collector for 207Pb, and an ion-counting channel for 204Pb. Ion yields are ~1.0 mv per ppm. Each analysis consists of one 12-second integration on peaks with the laser off (for backgrounds), 12 one-second integrations with the laser firing, and a 30 second delay to purge the previous sample and prepare for the next analysis. The ablation pit is ~12 microns in depth.
For each analysis, the errors in determining 206Pb/ 238U and 206Pb/ 204Pb result in a measurement error of ~1-2% (at 2-sigma level) in the 206Pb/ 238U age. The errors in measurement of 206Pb/ 207Pb and 206Pb/ 204Pb also result in ~1-2% (at 2-sigma level) uncertainty in age for grains that are >1.0 Ga, but are substantially larger for younger grains due to low intensity of the 207Pb signal. For most analyses, the cross-over in precision of 206Pb/ 238U and 206Pb/ 207Pb ages occurs at 0.8-1.0 Ga.
Common Pb correction is accomplished by using the measured 204Pb and assuming an initial Pb composition from Stacey and Kramers (1975) (with uncertainties of 1.0 for 206Pb/ 204Pb and 0.3 for 207Pb/ 204Pb). Our measurement of 204Pb is unaffected by the presence of 204Hg because backgrounds are measured on peaks (thereby subtracting any background 204Hg and 204Pb), and because very little Hg is present in the argon gas.
Inter-element fractionation of Pb/U is generally ~20%, whereas apparent fractionation of Pb isotopes is generally ~2%. In-run analysis of fragments of a large zircon crystal (generally every fifth measurement) with known age of 564 ± 4 Ma (2-sigma error) is used to correct for this fractionation. The uncertainty resulting from the calibration correction is generally 1-2% (2-sigma) for both 206Pb/ 207Pb and 206Pb/ 238U ages.
The analytical data are reported in Table ___. Uncertainties shown in these tables are at the 1-sigma level, and include only measurement errors.
Interpreted ages are based on 206Pb/ 238U for <1000 Ma grains and on 206Pb/ 207Pb for >1000 Ma grains. This division at 1000 Ma results from the increasing uncertainty of 206Pb/ 238U ages and the decreasing uncertainty of 206Pb/ 207Pb ages as a function of age. Analyses that are >30% discordant (by comparison of 206Pb/ 238U and 206Pb/ 207Pb ages) or >5% reverse discordant (in italics in Table ___) are not considered further.
The resulting interpreted ages are shown on relative age-probability diagrams (from Ludwig, 2003). These diagrams show each age and its uncertainty (for measurement error only) as a normal distribution, and sum all ages from a sample into a single curve.
Ludwig, K.R., 2003, Isoplot 3.00. Berkeley Geochronology Center, Special Publication No. 4, 70 p.
Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two‑stage model: Earth and Planetary Science Letters, v. 26, p. 207‑221.
Notes inserted below data table: