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

Abstract

<jats:p>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 (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 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 area, and results generate an accurate upper-intercept age in U–Pb concordia space of 2060.20±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 207Pb∕206Pb (and potentially U–Pb in concordant materials) age analysis, allowing dating of a wide variety of targets and processes newly accessible to geochronology. </jats:p>

Applying U-Pb chronometry and trace element geochemistry of apatite to carbonatite-phoscorite complexes – as exemplified by the 2.06 Ga Phalaborwa Complex, South Africa

“…Therefore, we consider this age to reliably determine the emplacement timing of the Loolekop carbonatite-phoscorite complex. This age falls within the range of ages reported from U-Pb baddeleyite (2 060.0 ± 2.5 Ma, combined from Reischmann, 1995;Horn et al, 2000;Wingate and Compston, 2000;French et al, 2002;Heaman, 2009;Wu et al, 2011;White et al, 2020) and U-Pb zircon (2 053.5 ± 1.2 Ma, combined from Reischmann, 1995;Chen et al, 2002;Heaman, 2009;Wu et al, 2011) in the same rock types (Figure 6). The close agreement between all three mineral-isotope systems confirms that apatite can be used as geochronometer in the absence of other accessory minerals (zircon and baddeleyite), to determine the age of magmatic systems, which were not affected by post-crystallisation thermal perturbation.…”

“…Chen et al (2002), Heaman (2009): zircon and baddeleyite Pb-Pb TIMS. Wu et al (2011), White et al (2020): SIMS U-Pb and Pb-Pb. Reischmann (1995), Horn et al (2000), Chen et al (2002) and White (2020) do not mention a specific sample location, only the Phalaborwa Igneous Complex.…”

“…Wu et al (2011), White et al (2020): SIMS U-Pb and Pb-Pb. Reischmann (1995), Horn et al (2000), Chen et al (2002) and White (2020) do not mention a specific sample location, only the Phalaborwa Igneous Complex. Heaman and LeCheminant (1993), Kröner and Willner (1998), Wingate and Compston (2000), Scherer et al (2001) and Heaman (2009) indicate a carbonatite origin for the analysed zircons and baddeleyites.…”

Petrographic controls on baddeleyite occurrence in a suite of eight basaltic shergottites

“…2017; Wu and Hsu 2020), while White et al. (2020) identify a minimum volume of 15 × 5 × 5 μm for focused ion beam baddeleyite extraction for TIMS U‐Pb analysis. In addition, the variable response of baddeleyite to heterogeneously distributed shock experienced by shergottites may result in the formation of microstructural subdomains exhibiting variable U‐Pb isotopic disturbance (Moser et al.…”

Supplemental Material: Recommendations for the reporting and interpretation of isotope dilution U-Pb geochronological information

“…As analytical blanks decrease and the sensitivity of mass spectrometers increases, dating of increasingly small domains of minerals has become possible, whereby it is no longer uncommon to fracture and analyze fragments of individual grains to explore age heterogeneity within single grains, which can be used to investigate dispersion within a larger sample (Gordon et al, 2010;Hawkins and Bowring, 1997;Samperton et al, 2015;Steiger et al, 1993;Tapster et al, 2016;White et al, 2020). Though there are practical limitations such as analytical imprecision due to reduced Pb* and U in sub-grain samples, we recommend the practice of dating individual crystal fragments to understand intra-grain variation and minimize averaging domains of varying geological age.…”