Pacific Centre for Isotopic and Geochemical Research
Department of Earth, Ocean and Atmospheric Sciences,
The University of British Columbia
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Geochronology

U-Pb Dating
Ar-Ar Dating


U-Pb Dating

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 shaking table equipped with a machined Plexiglass top, followed by conventional heavy liquids and magnetic separation using a Frantz magnetic separator. A number of binocular microscope work stations 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 using either BSE imaging or cathodoluminescence on an SEM.

Zircon Analysis by TIMS

TIMS U-Pb is recognised as one of the most robust dating techniques. We have dated rocks from Pliocene to Archean in age, for clients including geologists from universities, government and industry.

Zircons are processed employing the chemical abrasion (CA-TIMS) single-grain U-Pb no-chemistry technique using EARTHTIME ET535 or 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 mixed EARTHTIME or UBC spike.

U and Pb analysis of zircons is done on either a VG54R or a VG354S mass spectrometer. For this work both instruments employ an analogue single Daly collector. U and Pb are loaded together on the 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 (K.R.Ludwig, 2003 ) is used for final plotting of data.

Zircon Analysis by Laser Ablation ICPMS

We also perform U-Pb dating of zircons by laser ablation with a New Wave 213nm Nd-YAG laser fitted with a New Wave “Supercell” ablation chamber, coupled to the Thermo Finnigan Element2 high resolution ICP-MS.

Selected zircons are mounted in an epoxy puck along with several crystals of internationally accepted standard zircon (Plešovice, Temora2), and brought to a very high polish. Line scans are run on 16-20 crystals for igneous rocks, and a minimum of 60 randomly chosen crystals for detrital samples.

Data are reduced using Iolite software (Patton et al, 2011). Final dates are based on the weighted mean of the calculated 206 Pb/ 238 U dates for relatively young zircons (Phanerozoic), and on the weighted mean of the calculated 207 Pb/ 206 Pb dates for zircons older than 1000Ga. The ISOPLOT software (Ludwig, 2003) is used for final interpretation and plotting of the analytical results.


Argon-Argon Dating

Samples are crushed, washed in distilled water and ethanol, and sieved when dry to -40+60mesh. 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 were 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 ver.3.09 (Ludwig, 2003). Errors are quoted at the 2-sigma (95% confidence) level and are propagated from all sources except 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 comprising more than 60% of the 39Ar;
2. Probability of fit of the weighted mean age 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 nonzero slopes with the same sign (at 1.8σ nine or more steps only)

 

Further Reading

Koppers, Anthony A.P. (2002) ArArCALC – software for 40Ar/39Ar age calculations. Computers and Geosciences 28, 605-619.

Kuiper, K.F., Deino, A, Hilgen, F.J., Krijgsman, W., Renne, P.R., Wijbrans, J.R. (2008) Synchronizing rock clocks of earth history. Science 320, 500-504.

Ludwig, K. (2003) Isoplot/Ex, version 3: A geochronological toolkit for Microsoft Excel:Berkeley, California, Geochronology Center, Berkeley.

Patton, C., Hellstrom, J., Paul, B., Woodhead, J. Hergt, J. (2011) Iolite: freeware for the visualization and processing of mass spectrometry data; Journal of Analytical Atomic Spectroscopy, 26, pp. 2508-2518.

Mattinson J.M. (2005) Zircon U–Pb chemical abrasion (“CA-TIMS”) method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220, 47-66.

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, Geochem. Geophys. Geosyst., 8, Q08006, doi:10.1029/2006GC001492.

Scoates J.S. and Friedman R.M. (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.