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 | Lu-Hf 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. Four 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 by cathodoluminescence imaging.

Zircon Analysis by TIMS

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

Zircons are processed employing the chemical abrasion (CA-TIMS) single-grain U-Pb 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 VG354S or VG54R mass spectrometer. For this work, 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 (K.R.Ludwig, 2003 ) is used for final plotting of data.

Zircon Analysis by Laser Ablation ICPMS

PCIGR also 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-50-LR; Australian Scientific Instruments) coupled to a quadrupole ICPMS (7700 Series; Agilent Technologies) or a HR-SF-ICPMS (AttoM; Nu Instruments);
(2) an ArF excimer (λ =193 nm) laser ablation system (NWR193UC; Electro Scientific Industries/New Wave Research) coupled to a HR-SF-ICPMS (Element2; ThermoFinnigan).

Zircons are routinely analyzed from igneous rocks as well as detrital zircon or stream sediment samples. Both U-Pb data for geochronology and about thirty rare earth and trace elements are acquired from a single laser shot. Data are reduced using Iolite software (Patton et al, 2011). Final 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 (K.R. 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)


Lu-Hf Dating

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 and from Archean to Miocene, 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 multi-collector ICPMS instrument. Hafnium analyses are normalized to JMC-475 and ATI-475, which is a in-house developed, fully calibrated reference solution made from “JMC-475” Hf ingots (0.282160 ± 2; 2SD).

 

References

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.

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.