Highly accurate dating of micrometre-scale baddeleyite domains through combined focused ion beam extraction and U–Pb thermal ionization mass spectrometry (FIB-TIMS)

Lee Francis White, Kimberly Tait, Sandra L. Kamo, Desmond E. Moser, James Darling

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Abstract

Baddeleyite is a powerful chronometer of mafic magmatic and meteorite impact processes. Precise and accurate U–Pb ages can be determined from single grains by isotope dilution thermal ionization mass spectrometry (IDTIMS), but this requires disaggregation of the host rock for grain isolation and dissolution. As a result, the technique is rarely applied to precious samples with limited availability (such as lunar, Martian, and asteroidal meteorites and returned samples) or samples containing small baddeleyite grains that cannot readily be isolated by conventional mineral separation techniques. Here, we use focused ion beam (FIB) techniques, utilizing both Xe+ plasma and Ga+ ion sources, to liberate baddeleyite subdomains directly, allowing their extraction for ID-TIMS dating. We have analysed the U–Pb isotope systematics of domains ranging between 200 and 10 µm in length and from 5 to ≤ 0.1 µg in mass. In total, six domains of Phalaborwa baddeleyite extracted using a Xe+ plasma FIB (pFIB) yield a weighted mean 207Pb/206Pb age of 2060.1 ± 2.5 Ma (0.12 %; all uncertainties 2σ), within uncertainty of reference values. The smallest extracted domain (ca. 10×15×10 µm) yields an internal 207Pb/206Pb age uncertainty of ±0.37 %. Comparable control on cutting is achieved using a Ga+-source FIB instrument, though the slower speed of cutting limits potential application to larger grains. While the U–Pb data are between 0.5 % and 13.6 % discordant, the extent of discordance does not correlate with the ratio of material to ion-milled surface
Original languageEnglish
Pages (from-to)177-186
JournalGeochronology
Volume2
DOIs
Publication statusPublished - 7 Jul 2020

Keywords

  • Geochronology
  • FIB
  • FIB-SEM
  • Baddeleyite
  • isotope geochemistry
  • geology
  • geological materials
  • RCUK
  • STFC
  • ST/S000291/1

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