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Evaluating the reliability of U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) carbonate geochronology: matrix issues and a potential calcite validation reference material
Ist Teil von
Geochronology, 2020-06, Vol.2 (1), p.155-167
Ort / Verlag
Copernicus Publications
Erscheinungsjahr
2020
Quelle
EZB Electronic Journals Library
Beschreibungen/Notizen
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.