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

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 (K.R. Ludwig, 2003 ) is used for final interpretation and plotting of the analytical results.

 

Further Reading

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

Mattinson J.M. 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 (2005).

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. 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 (2008).

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.