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

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

doi:10.5194/gchron-2-177-2020

Abstract

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.

Highlights

The generation of high-precision chronological data is a cornerstone of the Earth and planetary sciences, providing an absolute measurement on which to anchor relative observations of geological time (e.g. Gradstein et al, 2004)
11 thermal ionization mass spectrometry (TIMS) analyses were conducted on grains and subdomains of the Phalaborwa baddeleyite, with one sample extracted by Ga+ Focused ion beam (FIB), six using the Xe+ plasma FIB (pFIB), two with no exposure to the FIB instruments, and two entirely separate whole grains
3.1 FIB extraction for U–Pb isotopic analysis. The effects of both the Ga+ and Xe+ FIB instruments on the U–Pb isotope systematics in accessory phase geochronometers has never been explored, and the potential for the ion beam to induce Pb diffusion and loss in exposed surface areas must be addressed for the FIB-TIMS technique

Summary

Introduction

The generation of high-precision chronological data is a cornerstone of the Earth and planetary sciences, providing an absolute measurement on which to anchor relative observations of geological time (e.g. Gradstein et al, 2004). The generation of high-precision chronological data is a cornerstone of the Earth and planetary sciences, providing an absolute measurement on which to anchor relative observations of geological time The most precise radiogenic isotopic ratios (e.g. U–Th–Pb, Sm–Nd, Rb–Sr) are generated using isotope dilution thermal ionization mass spectrometry (ID-TIMS; Parrish and Noble, 2003), which has been used to measure the timing of solar system formation (Amelin et al, 2002), initial differentiation of the Moon (Barboni et al, 2017), and the timing of crustal formation on Mars (Bouvier et al, 2018), often with internal age uncertainties on the order of ∼ 0.1 % 2σ. White et al.: FIB-TIMS U–Pb geochronology direct, high-precision dating of magmatic, metamorphic, and shock metamorphic events White et al.: FIB-TIMS U–Pb geochronology direct, high-precision dating of magmatic, metamorphic, and shock metamorphic events (e.g. Krogh et al, 1987; Parrish and Noble, 2002; Bouvier et al, 2018)

Results

Discussion

Conclusion