Journal cover Journal topic
Geochronology Advances in geochronological science
Journal topic
Discussion papers
https://doi.org/10.5194/gchron-2019-17
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gchron-2019-17
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 29 Nov 2019

Submitted as: research article | 29 Nov 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Geochronology (GChron).

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

Lee F. White1,2, Kimberly T. Tait1,2, Sandra L. Kamo1,3, Desmond E. Moser4, and James R. Darling5 Lee F. White et al.
  • 1Department of Natural History, Royal Ontario Museum, Toronto, Ontario, M5S 2C6, Canada
  • 2Department of Earth Sciences, University of Toronto, Toronto, Ontario, M5S 3B1, Canada
  • 3Jack Satterly Lab, University of Toronto, Toronto, Ontario, M5S 3B1, Canada
  • 4Western University, London, Ontario, Canada
  • 5School of the Environment, Geography and Geosciences, University of Portsmouth, PO1 3QL, UK

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.

Lee F. White et al.
Interactive discussion
Status: open (until 10 Jan 2020)
Status: open (until 10 Jan 2020)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
[Subscribe to comment alert] Printer-friendly Version - Printer-friendly version Supplement - Supplement
Lee F. White et al.
Lee F. White et al.
Viewed  
Total article views: 100 (including HTML, PDF, and XML)
HTML PDF XML Total Supplement BibTeX EndNote
85 13 2 100 5 0 0
  • HTML: 85
  • PDF: 13
  • XML: 2
  • Total: 100
  • Supplement: 5
  • BibTeX: 0
  • EndNote: 0
Views and downloads (calculated since 29 Nov 2019)
Cumulative views and downloads (calculated since 29 Nov 2019)
Viewed (geographical distribution)  
Total article views: 57 (including HTML, PDF, and XML) Thereof 55 with geography defined and 2 with unknown origin.
Country # Views %
  • 1
1
 
 
 
 
Cited  
Saved  
No saved metrics found.
Discussed  
No discussed metrics found.
Latest update: 05 Dec 2019
Publications Copernicus
Download
Short summary
The generation of highly precise and accurate ages requires crushing of the original sample so that individual mineral grains may be separated out for dating. Here, we use a focused ion beam to extract grains directly from a subset of a sample, effectively performing micro-surgery to isolate individual crystals from the rock itself. This approach opens the door to high precision dating for a variety of precious planetary materials that have previously been under-explored by scientists.
The generation of highly precise and accurate ages requires crushing of the original sample so...
Citation