Annealing plus chemical abrasion: A remarkably effective tool for eliminating the effects of lead loss from volcanic zircons
Abstract
Understanding short-term events and processes in Earth's history is highly dependent on the precision and accuracy of underlying radio-isotopic ages, provided in many cases by high-precision U/Pb ages of zircons in volcanic ash falls. Although low procedural blanks combined with modern mass spectrometry allow permil-precision ages to be determined on individual zircon crystals, such analytical precision does not necessarily translate into accuracy in a geological sense. Extracting robust ages from complex zircon populations is often compromised by apparent-age scatter arising both from anomalously young ages (a result of Pb loss) and by anomalously old ages (from xenocrystic contamination and/or inheritance). Such complications arise in particular for relatively young rocks (say <300 Ma, where accuracy of the more-robust 207Pb/206Pb ages is unacceptable and even the precision is worse than for 206Pb/238U), wherein reliable U/Pb dating requires the production of a coherent cluster of internally concordant 206Pb/238U ages. Air abrasion (the main "traditional" technique for minimizing Pb loss) as well as leaching the zircons in aggressive reagents (HF or NaOH) have been shown to be only moderately successful in general, as demonstrated by single crystal analyses with excess U/Pb age-scatter. Multi-crystal samples aimed at improving the precision of individual analyses impose the demonstrable likelihood of averaging Pb-loss effects, and thus are prone to yield slightly younger and inaccurate age (Mundil et al., 2001). An approach pioneered and refined by Mattinson (2001), however, dramatically improves the quality of complex zircon populations by entirely eliminating (at least in all of our tests so far) the effects of post-depositional Pb loss, even for zircon populations that are profoundly affected. In the Mattinson procedure, zircons are first annealed at a temperature of 800° C, resulting in crystals devoid of radiation damage. Following annealing, the zircons are aggressively leached by HF vapor at 220° C in pressurized Teflon capsules, resulting in "chemically abraded" crystals (Mattinson, 2001). A comparison among untreated crystals, physically abraded crystals, and annealed + chemically abraded crystals from the same ash-fall unit show that only the last technique yields statistically coherent U/Pb ages, whereas the first two approaches yield U/Pb ages with significant scatter arising from variable degrees of Pb loss. We present data-sets for zircons from ash-falls (late Paleozoic and early Mesozoic) that demonstrate the dramatic improvement in statistical quality resulting from annealing plus chemical abrasion. The annealing + chemical abrasion method apparently permits, for the first time, the determination of precise, robust, and geologically meaningful U/Pb ages from complex zircon populations affected by significant post-depositional Pb loss. Mattinson, J.M., 2001, Eos Trans. AGU, 82(47), Fall Meet. Suppl., Abstract V22C-1056. Mundil, R. et al. 2001, EPSL, 187, 131-145.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.V22E..07M
- Keywords:
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- 1035 Geochronology;
- 1094 Instruments and techniques;
- 8404 Ash deposits