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"linking zircon morphology, chemistry and dating - a LA-ICPMS approach"

English title "linking zircon morphology, chemistry and dating - a LA-ICPMS approach"
Applicant Bussy François
Number 109587
Funding scheme Project funding (Div. I-III)
Research institution Institut de Minéralogie et Géochimie Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Mineralogy
Start/End 01.11.2005 - 30.09.2007
Approved amount 94'910.00
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Keywords (6)

zircon; U-Pb dating; La-ICPMS; REE; trace-element; oxygen

Lay Summary (English)

Lead
Lay summary
The aim of this project (refocused since the original application) is to improve the In Situ isotopic U-Pb dating of zircon by laser ablation ICPMS in our laboratory equipped with a quadrupole Elan 6100 DRC ICPMS machine coupled to a Lambda Physik 193 nm Excimer laser. Preliminary results have shown that isotopic fractionation occurs both on the ablation site and in the mass spectrometer, resulting in unreliable accuracy, whereas precision is on the order of 1-3% (95% confidence).
In Situ zircon dating is an increasingly important tool in many fields of igneous, metamorphic and sedimentary geology. Indeed, the need for fast, reliable and spatially discriminating ages is steadily growing, in particular for provenance studies of sedimentary rocks. This demand cannot be satisfied by the conventional, isotope-dilution TIMS technique, which is certainly much more precise and accurate than In Situ techniques, but has a very low productivity at relatively high costs and lacks spatial resolution (core-rim discrimination in zoned grains). The TIMS technique is now dedicated to the very-high precision (few ‰) dating of specific events (e.g. calibration of the geological time scale, individual pulses in magmatic intrusions). Alternatively, SHRIMP and SIMS machines offer good quality ages, but their access is strongly limited and their cost rather high. The most promising technique by far is LA-ICPMS, as shown by several publications in the few past years. It allows dating of 60 to 80 zircon grains per day at very low cost in terms of maintenance, dedicated staff and consumables.
In order to improve the accuracy of the data, we plan to make and inject a spike solution with isotopic masses close to those of interest (i.e.206Pb, 207Pb, 238U) simultaneously to sample ablation in order to correct for ICPMS fractionation. A desolvating devise will be added to keep analytical conditions dry.
Ablation-related fractionation will be corrected for by "back to time zero" extrapolation of the time-dependent measured Pb/U ratios. Various external zircon standards will be regularly used as monitors during analytical sessions.
Direct link to Lay Summary Last update: 21.02.2013

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