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

• Main Authors: L. F. White, K. T. Tait, S. L. Kamo, D. E. Moser, J. R. Darling
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-07-01
• Series: Geochronology
• Online Access: https://gchron.copernicus.org/articles/2/177/2020/gchron-2-177-2020.pdf

<p>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 <span class="inline-formula">Xe⁺</span> plasma and <span class="inline-formula">Ga⁺</span> 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&thinsp;<span class="inline-formula">µm</span> in length and from 5 to <span class="inline-formula">≤0.1</span>&thinsp;<span class="inline-formula">µg</span> in mass. In total, six domains of Phalaborwa baddeleyite extracted using a <span class="inline-formula">Xe⁺</span> plasma FIB (pFIB) yield a weighted mean <span class="inline-formula">²⁰⁷Pb∕²⁰⁶Pb</span> age of <span class="inline-formula">2060.1±2.5</span>&thinsp;Ma (0.12&thinsp;%; all uncertainties <span class="inline-formula">2<i>σ</i></span>), within uncertainty of reference values. The smallest extracted domain (ca. <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">10</mn><mo>×</mo><mn mathvariant="normal">15</mn><mo>×</mo><mn mathvariant="normal">10</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="61pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="84f4b31fb299567abff6b1714c00e33a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-2-177-2020-ie00001.svg" width="61pt" height="10pt" src="gchron-2-177-2020-ie00001.png"/></svg:svg></span></span>&thinsp;<span class="inline-formula">µm</span>) yields an internal <span class="inline-formula">²⁰⁷Pb∕²⁰⁶Pb</span> age uncertainty of <span class="inline-formula">±0.37</span>&thinsp;%. Comparable control on cutting is achieved using a <span class="inline-formula">Ga⁺</span>-source FIB instrument, though the slower speed of cutting limits potential application to larger grains. While the U–Pb data are between 0.5&thinsp;% and 13.6&thinsp;% 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 <span class="inline-formula">2060.20±0.91</span>&thinsp;Ma (0.044&thinsp;%). 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 <span class="inline-formula">²⁰⁷Pb∕²⁰⁶Pb</span> (and potentially U–Pb in concordant materials) age analysis, allowing dating of a wide variety of targets and processes newly accessible to geochronology.</p>