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.