High spatial resolution geochronology

Detrital zircon

We produce a large volume of laser ablation U-Pb data on zircons (Figure 1). These are typically used to constrain the timing of magmatic and metamorphic events, and, combined with other radiogenic isotope systems, allow for detailed petrogenetic studies of crustal rocks. Recently, more emphasis has been directed to dating other accessory phases (monazite, titanite, allanite, xenotime, etc.). The use of multiple minerals is particularly useful to solve problems in metamorphic petrology, and dating of detrital U-bearing minerals (zircon, rutile) to inform studies of erosion and tectonics.

We employ three laser ablation systems (one solid state (Nd:YAG) and two excimer gas systems) and couple these to either a MC-ICP-MS, or SC-SF-ICP-MS. We can accurately determine the age of various U-(Th)-Pb mineral phases on the 10–50 μm scale. Ablation pit depths average 15 μm, using 6–140 ng of zircon respectively for this spot size range. For a 25 μm ablation spot, 35 ng of material are utilised compared to 1–2 ng for a SIMS analysis of equivalent spot size. Total combined uncertainties of approximately one per cent 2SD on the 206Pb/238U age are directly comparable to that produced by SIMS. Recently we described a new approach to laser ablation geochronology, which involves single shot ablation so that thin (approximately 1 μm) mineral overgrowths can be dated.

We take a comprehensive approach to such work, employing laser ablation ICP-MS and TIMS analyses where appropriate. We are constantly striving to improve our multiple mineral reference materials to be able to date almost any mineral that contains uranium, even at low concentration and young age.

Key references

Cottle, J M, Horstwood, M S A, and Parrish, R R.  2009.  A new approach to single shot laser ablation analysis and its application to in situ Pb/U geochronology J.  Anal. At. Spectrom.  DOI: 10.1039/b821899d.

Horstwood, M S A.  2008.  Data reduction strategies, uncertainty assessment and resolution of LA–(MC–)ICP–MS isotope data.  In: Laser ablation ICP-MS in the Earth Sciences: current practices and outstanding issues. Ed. Paul Sylvester  Mineralogical Association of Canada Short Course Series Volume, 40, 283–300.

Contact

Please contact Béatrice Bullock-von Moos for further information.