Abstract
Abstract. We document that the reliability of carbonate U–Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is improved by matching the aspect ratio of the LA single-hole drilling craters and propagating long-term excess variance and systematic uncertainties. We investigated the impact of different matrices and ablation crater geometries using U–Pb isotope analyses of one primary (WC-1) and two secondary reference materials (RMs). Validation RMs (VRMs) include a previously characterised one (ASH-15D) and a new candidate (JT), characterised by ID-TIMS (intercept age: 13.797±0.031 Ma) with excellent agreement to pooled LA-ICP-MS measurements (13.75±0.11 | 0.36 Ma), a U concentration of approx. 1 µg g−1 and 238U∕206Pb ratios from 5 to 460, defining the isochron well. Differences in ablation crater depth to diameter ratios (aspect ratio) introduce an offset due to downhole fractionation and/or matrix effects. This effect can be observed either when the crater size between U–Pb RM and the sample changes or when the ablation rate for the sample is different than for the RM. Observed deviations are up to 20 % of the final intercept age depending on the degree of crater geometry mismatch. The long-term excess uncertainty was calculated to be in the range of 2 % (ASH-15D) to 2.5 % (JT), and we recommend propagating this uncertainty into the uncertainty of the final results. Additionally, a systematic offset to the ID-TIMS age of 2 %–3 % was observed for ASH-15D but not for JT. This offset might be due to different ablation rates of ASH-15D compared to the primary RM or remaining matrix effects, even when the aspect ratios chosen are similar.
Highlights
Recent improvements in the sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) instruments coupled to a laser ablation system allows us to date very young zircons by the U–Pb method (Guillong et al, 2014) and minerals with very low U concentrations, typically of several parts per billion to tens of parts per million and even lower concentrations of radiogenic Pb, such as carbonates (Li et al, 2014; Methner et al, 2016; Roberts and Walker, 2016; Nuriel et al, 2017)
We first observed that analyses performed with ablation crater diameters larger than those of the primary reference materials (RMs) show a systematic offset towards higher 238U/206Pb ratios, corresponding to younger isochron intercept ages
We introduce the JT vein calcite as a potential validation RM due to its homogeneity and very good spread in relative radiogenic to initial Pb contents
Summary
Recent improvements in the sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) instruments coupled to a laser ablation system allows us to date very young zircons by the U–Pb method (Guillong et al, 2014) and minerals with very low U concentrations, typically of several parts per billion to tens of parts per million and even lower concentrations of radiogenic Pb, such as carbonates (Li et al, 2014; Methner et al, 2016; Roberts and Walker, 2016; Nuriel et al, 2017). Accurate U–Pb dating of carbonates requires a two-step data reduction approach (Roberts et al, 2017) consisting of (1) 207Pb/206Pb mass bias correction based on a homogeneous reference material (typically a standard glass) and (2) a U/Pb inter-element fractionation correction based on the lower intercept in the Tera– Wasserburg concordia diagram using a matrix-matched RM. With this method, carbonates can be dated by laser ablation ICP-MS (LA-ICP-MS) with the advantage of easy availability, high sample throughput and high spatial resolution, allowing us to resolve large differences in U–Pb ratio and cost effectiveness. We document that changes in laser crater aspect ratios between primary RMs and samples may result in significant inaccuracy of U–Pb LA-ICP-MS carbonate dates and propose new analytical strategies to minimise this effect
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