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

White, Lee F.; Tait, Kimberly T.; Kamo, Sandra L.; Moser, Desmond E.; Darling, James R.

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 (ID-TIMS), 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 inline-formulaXe+ plasma and inline-formulaGa+ 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 inline-formulaµm in length and from 5 to inline-formula≤0.1inline-formulaµg in mass. In total, six domains of Phalaborwa baddeleyite extracted using a inline-formulaXe+ plasma FIB (pFIB) yield a weighted mean inline-formula207Pb∕206Pb age of inline-formula2060.1±2.5 Ma (0.12 %; all uncertainties inline-formula2σ), within uncertainty of reference values. The smallest extracted domain (ca. inline-formula M10inlinescrollmathml normal 10 × normal 15 × normal 10 61pt10ptsvg-formulamathimg84f4b31fb299567abff6b1714c00e33a gchron-2-177-2020-ie00001.svg61pt10ptgchron-2-177-2020-ie00001.png inline-formulaµm) yields an internal inline-formula207Pb∕206Pb age uncertainty of inline-formula±0.37 %. Comparable control on cutting is achieved using a inline-formulaGa+-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 area, and results generate an accurate upper-intercept age in U–Pb concordia space of inline-formula2060.20±0.91 Ma (0.044 %). Thus, we confirm the natural U–Pb variation and discordance within the Phalaborwa baddeleyite population observed with other geochronological techniques. Our results demonstrate the FIB-TIMS technique to be a powerful tool for highly accurate in situ inline-formula207Pb∕206Pb (and potentially U–Pb in concordant materials) age analysis, allowing dating of a wide variety of targets and processes newly accessible to geochronology.

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White, Lee F. / Tait, Kimberly T. / Kamo, Sandra L. / et al: Highly accurate dating of micrometre-scale baddeleyite domains through combined focused ion beam extraction and U–Pb thermal ionization mass spectrometry (FIB-TIMS). 2020. Copernicus Publications.

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