Highly sensitive, spatially resolved 230Th–232Th–234U–238U analysis using LA-ICPMS

U-Th dating is frequently used to determine the timing of Earth’s geological, environmental, and
biotic processes from materials formed a few years in the past to over 800 thousand years (kys)
ago [1]. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) could provide
advantages of rapid, in-situ, and spatially lateral scale of 10–100 μm for U-Th dating. However, the
low abundance of ²³⁰Th in most samples together with inter-element fractionation effects have
restricted the use of LA-ICPMS for U-Th dating applications and only a few studies have been
published until today [2].
Here we present a sensitive and quantitative method for the determination of Th and U isotope
ratios via laser ablation-inductively coupled plasma-sector field-mass spectrometry (LA-ICP-SF-MS).
It employs a strategy of solution-based (²²⁹Th-²³³U-²³⁶U) spike addition [3] for the normalization of
mass discrimination and element fractionation effects. A well characterized U-Th spike is added to
the laser generated aerosol by means of a desolvating nebulizer. The ²³³U and ²³⁶U intensity pair is
used to correct for variations in mass discrimination during laser ablation sampling, while the ²²⁹Th
and ²³³U pair monitors and corrects for changes in the ²³⁸U/²³⁰Th and ²³⁴U/²³⁰Th sensitivity ratios.
Thus instrumental artifacts can be effectively reduced and age determination could be improved
as well.
A high-sensitivity ICPMS with jet-interface setup, which provides a significantly detection efficiency
enhanced, approaching a useful yield of 2%, was applied for these experiments. Thereby
sufficiently high signal intensities can be achieved even for the critical isotope 230Th in a real fossil
stalagmite with reported ages of 200 kys [4]. The efficacy of inter-element, mass discrimination,
tailing, and baseline corrections were critically evaluated and optimized. This approach allows for
the revelation of accurate age profiles in various materials and carbonates in particular, and will be
applicable to diverse fields such as paleoclimatology, oceanograpgy, geomagnetics, and
archaeology.
[1] Jian-xin Zhao, Ke-fu Yu, Yue-xing Feng, Quaternary Geochronology, 2009, 4, 423-433.
[2] Steven J. Goldstein, Claudine H. Stirling. Reviews in Mineralogy and Geochemistry, 2003, 52,
23-57.
[3] Hai Cheng et al., Earth and Planetary Science Letters, 2013, 371, 82-91.
[4] Ting-Yong Li et al., Climate of the Past, 2014, 10, 1211-1219.