Small-volume U-Pb zircon geochronology

U–Pb zircon geochronology was conducted by small-volume laser ablation multicollector inductively coupled plasma mass spectrometry (LA–MC–ICP–MS) at the University of Arizona LaserChron Center (Gehrels et al., 2006; Gehrels et al., 2008; Johnston et al., in review). The analyses involve ablation of zircon with a New Wave DUV193 Excimer laser (operating at a wavelength of 193 nm) using a spot diameter of 10 µm and yielding a pit depth of 5 µm. 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 Faraday detectors for ²³⁸U, ²³²Th, and Channeltron detectors for ^208-204Pb. Each 30-second analysis consists of a 10-second background acquisition followed by a 20-second total count integration of the cumulative signal on each peak collected during laser sampling. Measurement errors are calculated as a function of the total number of counts, with Pb isotopes on Channeltron detectors precise to ~2% before increasing exponentially at count rates <10,000 cps, and U–Th isotopes on faraday detectors errors precise to ~1.5% before exponentially increasing at count rates <350,000 cps.

Inter-elemental fractionation of ²⁰⁶Pb/²³⁸U is generally ~70%, whereas fractionation of ²⁰⁶Pb/²⁰⁷Pb is generally ~20%. In-run analysis of fragments of a large zircon crystal (generally every fifth measurement) with known age of 563.5 ± 3.2, (2σ error, Gehrels et al., 2008) is used to correct for this fractionation. Uncertainty resulting from this calibration correction generally leads to a systematic error of ~1% (2σ standard error) for both ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁶Pb/²³⁸U ages, that is then added in quadrature to the decay constant errors and the error on the external standard, and yields a total systematic error of 1.0–1.5%  (2σ standard error). Ultimately, this systematic error is added in quadrature to analytical errors to calculate the total age uncertainty on the unknown grain.

Low common Pb concentrations in zircon in combination with small-volume analysis typically yields count rates <100 cps above background, correspondingly high errors, and values that are not significantly different than zero. These difficulties prohibit accurate calculation of the ²⁰⁶Pb/²⁰⁴Pb fraction factor and yield unreliable ²⁰⁶Pb/²⁰⁴Pb values. As such, common Pb corrections have not been applied to any of the data presented here. Ages for samples that are strongly influenced by common Pb are calculated using Isoplot (Ludwig, 2003) to regress the data through common Pb (using Stacey and Kramers, 1975, to estimate initial ²⁰⁷Pb/²⁰⁶Pb) on a Terra–Wasserburg diagram.

The analytical data are reported in Table ___. Uncertainties shown in these tables are at the 1σ level, and include only measurement errors.

References

Gehrels, G., Valencia, V. A., and Pullen, A., 2006, Detrital Zircon Geochronology by Laser-Abation Multicollector ICPMS at the Arizona Laserchron Center, in Emergin Opportunities, Paleontological Society Short Course, Philadelphia, PA, p. 67–76.

Gehrels, G., Valencia, V. A., and Ruiz, J., 2008, Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry: Geochemistry, Geophysics, and Geosystems, v. 9, no. 3, p. doi: 10.1029/2007GC001805.

Johnston, S. M., Gehrels, G., Valencia, V. A., and Ruiz, J., in review, Small-Volume U–Pb Zircon Geochronology by Laser Ablation–Multicollector–ICP–MS: Chemical Geology.

Ludwig, K., 2003, User's Manual for Isoplot 3.00, A Geochronological Toolkit for Microsoft Excel: Berkeley Geochronology Center Special Publication, v. 4.

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:

¹GX-A notation refers to Grain X, analysis Z.

²Uncertainties for individual analyses are reported at the 1σ level, and include only measurement errors.

³U concentration and U/Th are calibrated relative to Sri Lanka Zircon standard, and accurate to ~20%.

⁴U/Pb and ²⁰⁶Pb/²⁰⁷Pb fractionation is calibrated relative to fragments of a large Sri Lanka Zircon of 563.5 ± 3.2 Ma (2σ).

⁵U decay constants and composition as follows: ²³⁸U = 9.8485 x 10^-10, ²³⁵U = 1.55125 x 10^-10, ²³⁸U/²³⁵U = 137.88.

⁶Cumulative systematic errors are ____ (2σ standard error on the mean).

Gehrels, G., Valencia, V. A., and Pullen, A., 2006, Detrital Zircon Geochronology by Laser-Abation Multicollector ICPMS at the Arizona Laserchron Center, in Emergin Opportunities, Paleontological Society Short Course, Philadelphia, PA, p. 67–76.

Gehrels, G., Valencia, V. A., and Ruiz, J., 2008, Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry: Geochemistry, Geophysics, and Geosystems, v. 9, no. 3, p. doi: 10.1029/2007GC001805.

Johnston, S. M., Gehrels, G., Valencia, V. A., and Ruiz, J., in review, Small-Volume U–Pb Zircon Geochronology by Laser Ablation–Multicollector–ICP–MS: Chemical Geology.

Ludwig, K., 2003, User's Manual for Isoplot 3.00, A Geochronological Toolkit for Microsoft Excel: Berkeley Geochronology Center Special Publication, v. 4.

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.