Conventional geochronometers with negligible common-Pb

zircon, baddeleyite, monazite & xenotime

Standards we use:

1. Zircon: FC1 (1099), 91500 (1065 Ma), Plesovice (337 Ma), BB (565 Ma), Kara-18 (2635 Ma), GSCz1242 (2679 Ma), , KVT (3226 Ma),
2. Baddeleyite: FC1 (1099), Phalabowra (2059 Ma)
3. Monazite: GSC8153, 44069, Diamantina, Thompson Mine, Trebilcock
4. Xenotime: GSC6413, Datas

General U-(Th)-Pb dating

Routine laser ablation geochronology of zircon, baddeleyite, monazite and xenotime is now carried out on the Agilent 8900 instrument using ‘single-quad’ mode: Q1 and the ORS act as simple ion guides and Q2 is used for mass filtering. Crater sizes and other operating conditions vary depending on the size of mineral targets, internal zoning features, U concentration and expected age. For zircon and baddeleyite we typically analyze with crater diameter between 15-30 µm. In extreme cases we can push this to 10 µm if we can obtain sufficient signal (ideally >1000 cps) for 207Pb. Monazite and xenotime can be analyzed with craters down to 5 µm diameter because of their higher U, Th, and Pb* concentrations.

High quality LA ICP-MS geochronology results are best achieved by guiding the selection of target locations using combinations of µ-XRF, optical (TL and RL), BSE, and CL images or LAICPMS trace element maps. For polished thin sections, wide area µ-XRF maps place target minerals in broader textural context whereas the other techniques reveal details of internal zoning. The spatial resolution now achieved by LA QQQ-ICP-MS demands rigorous imaging and interpretation of internal zoning domains.

Removal of ²⁰⁴Hg interference for accurate ²⁰⁴Pb correction

For young zircon targets, even small amounts of 204Pb contamination can push isotope ratios (mostly ²⁰⁷Pb/²³⁵U) off Concordia. In cases where accurate measurement of 204Pb contamination is critical, the Agilent 8900 is operated in MS/MS mode and a small flow of NH3 in the reaction/collision cell can be used to effectively eliminate ²⁰⁴Hg interfering on ²⁰⁴Pb: Hg reacts strongly with NH₃ whereas Pb intensities are preserved at >85% levels. The combined He + NH₃ in the cell does cause some reaction with U and care needs to be taken to adjust ORS settings to ensure appropriate sensitivity for ²³⁸U and ²³²U. This setup is most appropriate for crater sizes >20 µm. A recent run in this mode using a 26 µm crater achieved ²⁰⁴Pb average detection limit of 0.038 ppm and accurately corrected data points with between 2 and 5 cps net ²⁰⁴Pb.

2022 G-chron proficiency testing results

The UNB Mantis lab participates in the International Association of Geoanalysis (IAG) proficiency testing program for geochronology (http://www.gchron.info/default.asp). An excerpt of results of the most recent round of testing on Kara-18 Archean zircon material are shown below. Our lab (randomized as lab B60) was one of the 14 out of 50 labs worldwide that achieved a perfect z-score for both reported isotopic ages and ratios.

Split-stream zircon dating

Split-stream (SS) zircon dating is carried out with the 7700x ICP-MS dedicated to trace-element quantification and the 8900 QQQ-ICP-MS dedicated to U-Th-Pb ratio measurement. Despite halving the ablated material travelling to each instrument, the 8900 is still able to achieve sensitivity on 207Pb that is on par with what we historically measure on the 7700x on its own.  For Proterozoic and older zircon we can operate in SS mode with laser crater diameters ranging from 10-20 micron and ablation times of 30sec. Younger zircons require commensurably larger craters to achieve suitable internal precision. A typical TE element list on the 7700x includes: Si, P, Ca, Sc, Ti, Y, Zr, Nb, Hf, and REE. The 8900 simultaneously measures: Zr (guide mass), ²⁰²Hg, ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th and ²³⁸U.