Highly accurate dating of micrometre-scale baddeleyite domains through combined focused ion beam extraction and U-Pb thermal ionisation mass spectrometry (FIB-TIMS)
Abstract. Baddeleyite is a powerful chronometer of mafic magmatic and meteorite impact processes. High precision and accuracy U-Pb ages can be measured from single grains by isotope dilution thermal ionisation mass spectrometry (ID-TIMS), but this requires destruction of the host rock for highly challenging grain isolation and dissolution. As a result, the technique is rarely applied to precious samples with very 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, utilising both Xe+ plasma and Ga+ ion sources, to liberate baddeleyite subdomains in-situ, allowing their extraction for ID-TIMS dating. We have analysed the U-Pb isotope systematics of domains ranging between 200 um and 10 um in length and 5 ug to 0.1 ug in mass. In total, seven domains of Phalaborwa baddeleyite extracted using a Xe+-pFIB yield a weighted mean 207 Pb/206 Pb age of 2060.1 ± 2.4 Ma (0.12 %; all uncertainties 2 sigma), within uncertainty of reference values. The smallest extracted domain (ca. 10 × 15 times; 10 um) yields an internal 207 Pb/206 Pb uncertainty of ±0.15 %. Comparable levels of precision are achieved using a Ga+-source FIB instrument (±0.20 %), though the slower cutting speed limits potential application to larger grains. While the U-Pb data are between 0.5 and 13.6 % discordant, the results generate a precise upper intercept age in U-Pb concordia space of 2061.1 × 7.4 Ma; (0.72 %). Importantly, the extent of discordance does not correlate with the ratio of material to ion-milled surface area, showing that the FIB extraction does not induce disturbance of U-Pb systematics even the smallest extracted domains. Instead, 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 high-accuracy in-situ U-Pb dating, which makes a wide range of targets and processes newly accessible to geochronology.
