Geochronology

U-Pb 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 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);

(2) An ArF excimer (λ =193 nm) laser ablation system (NWR193UC; Electro Scientific Industries/New Wave Research) coupled to a HR-SF-ICP-MS (Element 2; Thermo Scientific).

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.

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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 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).

For questions about Lu-Hf dating, contact Dr. Matthijs Smit.
Consult the Fees page for a complete list of sample preparation options and analytical costs.

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References

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.

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.