Samples for U-Pb dating are processed using a Rhino jaw crusher, a Bico disk grinder equipped with ceramic grinding plates, and a Wilfley wet shaker table equipped with a machined Plexiglass top, followed by conventional heavy liquid and magnetic separation using a Frantz magnetic separator. Four binocular microscope workstations are available for sample picking. The external morphology of mineral grains for analysis can be documented by SEM, and internal structure can be examined in polished grain mounts by cathodoluminescence imaging.
Zircon Analysis by TIMS
TIMS U-Pb geochronology is widely recognized as one of the most robust and precise dating techniques. We have dated rocks from Pliocene to Archean in age, for clients from universities, government and industry.
Zircons are processed with the chemical abrasion (CA-TIMS) single-grain U-Pb technique that uses EARTHTIME ET535 or a UBC 205Pb-233-235U isotopic tracer. A variety of accessory phases, including titanite, monazite, rutile, apatite, allanite and garnet, are prepared using single- or multi-grain fractions and ion exchange chromatography, and a mixed EARTHTIME or UBC spike (Scoates and Friedman, 2008; Wall et al., 2016).
At PCIGR, U and Pb analysis of zircons is done on either a VG354S or VG54R TIMS, where both instruments employ an analogue single Daly collector. U and Pb are loaded together on an outgassed zone-refined Re filament, and run separately in peak-hopping mode. Data reduction is done with U-Pbr, an Excel-based routine based on the error estimate algorithms published by Schmitz and Shoene (2007). The ISOPLOT software package (Ludwig, 2003) is used for final plotting of data.
Zircon Analysis by Laser Ablation ICP-MS
PCIGR performs U-Pb dating of zircons by laser ablation, using two set-ups:
(1) An ArF excimer (λ =193 nm) laser ablation system (RESOlution M-50LR; Australian Scientific Instruments) coupled to a quadrupole ICP-MS (7700 Series; Agilent Technologies) or a HR-SF-ICP-MS (AttoM; Nu Instruments) (Wall et al., 2016; VerHoeve et al., 2018);
Zircons are routinely analyzed from igneous rocks as well as detrital zircon or stream sediment samples. Both U-Pb data for geochronology and up to 30 user-selected trace elements, including rare earth elements, are acquired from a single laser shot. Data are reduced using Iolite software (Paton et al., 2011). Reported dates for igneous rock samples are based on the weighted mean of the calculated 206Pb/238U dates for relatively young zircons (Phanerozoic), and on the weighted mean of the calculated 207Pb/206Pb dates for zircons older than 1.5 Ga. The ISOPLOT software (Ludwig, 2003) is used for reporting, interpretation and plotting of the analytical results.
For U-Pb TIMS questions and/or sample submission, contact Dr. Corey Wall.
For questions about zircon LA-ICP-MS, contact Dr. Marg Amini.
Consult the Fees page for a complete list of sample preparation options and analytical costs.
Samples are crushed, washed in distilled water and ethanol, dried and sieved with 40–60 mesh. Appropriate mineral grains are picked out of the bulk fraction. The mineral separates are wrapped in aluminum foil and stacked in an irradiation capsule with similar-aged samples and neutron flux monitors (Fish Canyon Tuff sanidine (FCs), 28.201 ± 0.036 Ma) (Kuiper et al., 2008), and irradiated at the McMaster Nuclear Reactor in Hamilton, Ontario.
Samples are loaded into pits in a copper disk in an evacuated sample chamber. They are step-heated at incrementally higher powers in the defocused beam of a 10W CO2 laser (New Wave Research MIR10) until fused. The gas evolved from each step is analyzed by a VG5400 mass spectrometer equipped with an ion-counting electron multiplier.
All measurements are corrected for total system blank, mass spectrometer sensitivity, mass discrimination, radioactive decay during and subsequent to irradiation, as well as interfering Ar from atmospheric contamination and the irradiation of Ca, Cl and K (isotope production ratios: (40Ar/39Ar)K = 0.0302 ± 0.00006, (37Ar/39Ar)Ca = 1416.4 ± 0.5, (36Ar/39Ar)Ca = 0.3952 ± 0.0004, Ca/K = 1.83 ± 0.01 (37ArCa/39ArK)).
Details of the analyses, including plateau (spectrum) and inverse correlation plots, are presented in Excel spreadsheets. Initial data entry and calculations are carried out using the software ArArCalc (Koppers, 2002). The plateau and correlation ages are calculated using Isoplot version 3.09 (Ludwig, 2003). Errors are quoted at the 2-sigma (95% confidence) level and are propagated from all sources except for mass spectrometer sensitivity and age of the flux monitor.
The best statistically justified plateau and plateau age are picked based on the following criteria:
(1) Three or more contiguous steps comprise more than 60% of the 39Ar;
(2) Probability of fit of the weighted mean age is greater than 5%;
(3) Slope of the error-weighted line through the plateau ages equals zero at 5% confidence;
(4) Ages of the two outermost steps on a plateau are not significantly different from the weighted-mean plateau age (at 1.8σ, six or more steps only);
(5) Outermost two steps on either side of a plateau must not have non-zero slopes with the same sign (at 1.8σ, nine or more steps only).
Garnet Lu-Hf analysis is an emerging technique in chronology, which enables dating of garnet — the main petrogenetic indicator mineral in metamorphosed rocks. The technique has been used on rocks from (ultra-)mafic to felsic composition, and from the Archean to Miocene in age, providing reliable high-precision (0.20–1.5%; 2SD) age constraints on garnet (re-)crystallization in the crust and mantle.
Garnet grain populations, or even individual grains or sections thereof, are washed and admixed with a calibrated and routinely tested UBC 176Lu-180Hf isotope tracer. The mixture is then subjected to step-wise table-top dissolution and cation-exchange chromatography. Lutetium and Hf isotope compositions are measured on the Nu Plasma MC-ICP-MS. Hafnium analyses are normalized to JMC-475 and ATI-475, which is an in-house developed, fully calibrated reference solution made from “JMC-475” Hf ingots (0.282160 ± 2; 2SD).
Baxter, E. and Scherer, E. 2013. Garnet geochronology: Timekeeper of tectonometamorphic processes. Elements, 9(6): 433-438, doi:10.2113/gselements.9.6.433.
Ludwig, K.R. 2003. User’s manual for Isoplot 3.00: A geochronological toolkit for Microsoft Excel. Geochronology Center, Berkeley, California. https://searchworks.stanford.edu/view/6739593.
Koppers, A.A.P. 2002. ArArCALC—software for 40Ar/39Ar age calculations. Computers and Geosciences, 28(5): 605-619, doi:10.1016/S0098-3004(01)00095-4.
Paton, C., Hellstrom, J., Paul, B., Woodhead and J. Hergt, J. 2011. Iolite: Freeware for the visualization and processing of mass spectrometry data. Journal of Analytical Atomic Spectroscopy, 26: 2508-2518, doi:10.1039/C1JA10172B.
Schmitz, M.D. and Schoene, B. 2007. Derivation of isotope ratios, errors, and error correlations for U-Pb geochronology using 205Pb-235U-(233U)-spiked isotope dilution thermal ionization mass spectrometric data. Geochemistry, Geophysics, Geosystems, 8: Q08006, doi:10.1029/2006GC001492.
Scoates, J. and Friedman, R. 2008. Precise age of the platiniferous Merensky Reef, Bushveld Complex, South Africa by the U-Pb zircon chemical abrasion ID-TIMS technique. Economic Geology, 103: 465-471, doi:10.2113/gsecongeo.103.3.465.
Steiger, R.H. and Jäger, E. 1977. Subcommission on geochronology: Convention on the use of delay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36(3): 359-362, doi:10.1016/0012-821X(77)90060-7.
VerHoeve, T., Scoates, J., Wall, C., Weis, D. and Amini, M. 2018. Evaluating downhole fractionation corrections in LA-ICP-MS U-Pb zircon geochronology. Chemical Geology, 483: 201–217, doi:10.1016/j.chemgeo.2017.12.014.
Wall, C., Scoates, J. and Weis, D. 2016. Zircon from the Anorthosite zone II of the Stillwater Complex as a U-Pb geochronological reference material for Archean rocks. Chemical Geology, 436: 54-71, doi:10.1016/j.chemgeo.2016.04.027.