Highly accurate dating of micrometre-scale baddeleyite domains through combined focused ion beam extraction and U–Pb thermal ionization mass spectrometry (FIB-TIMS)
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Published:2020-07-07
Issue:2
Volume:2
Page:177-186
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ISSN:2628-3719
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Container-title:Geochronology
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language:en
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Short-container-title:Geochronology
Author:
White Lee F., Tait Kimberly T., Kamo Sandra L., Moser Desmond E.ORCID, Darling James R.
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.
Funder
Science and Technology Facilities Council
Publisher
Copernicus GmbH
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