Abstract
Abstract. Latest advances in laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) allow for accurate in situ U−Pb dating of carbonate material, with final age uncertainties usually >3 % 2σ. Cross-laboratory reference materials (RMs) used for sample-bracketing are currently limited to WC1 calcite with an age of 254.4±6.5 (2σ). The minimum uncertainty on any age determination with the LA-ICPMS method is therefore ≥2.5 %, and validation by secondary RMs is usually performed on in-house standards. This contribution presents a new reference material, ASH-15, a flowstone that is dated here by isotope dilution (ID) thermal ionization mass spectrometry (TIMS) analysis using 37 sub-samples, 1–7 mg each. Age results presented here are slightly younger compared to previous ID isotope ratio mass spectrometry (IRMS) U−Pb dates of ASH-15 but within uncertainties and in agreement with in situ analyses using WC1 as the primary RM. We provide new correction parameters to be used as primary or secondary standardization. The suggested 238U∕206Pb apparent age, not corrected for disequilibrium and without common-lead anchoring, is 2.965±0.011 Ma (uncertainties are 95 % confidence intervals). The new results could improve the propagated uncertainties on the final age with a minimal value of 0.4 %, which is approaching the uncertainty of typical ID analysis on higher-U materials such as zircon. We show that although LA-ICPMS spot analyses of ASH-15 exhibit significant scatter in their isotopic ratios, the down-hole fractionation of ASH-15 is similar to that of other reference materials. This high-U (≈1 ppm) and low-Pb (<0.01 ppm) calcite is most appropriate as a reference material for other speleothem-type carbonates but requires more-sensitive ICP-MS instruments such as the new generation of single-collector and multi-collector ICP-MS. Reference materials with high-Pb and low-U or both low-U and low-Pb compositions are still needed to fully cover the compositional range of carbonate material but may introduce analytical challenges.
Highlights
Age results presented here are slightly younger compared to previous isotope dilution (ID) isotope ratio mass spectrometry (IRMS) U−Pb dates of ASH-15 but within uncertainties and in agreement with in situ analyses using WC1 as the primary reference materials (RMs)
We show that LA-ICPMS spot analyses of ASH-15 exhibit significant scatter in their isotopic ratios, the down-hole fractionation of ASH-15 is similar to that of other reference materials
All analyses were performed on ASH-15-D-K yellow Pliocene layer, abbreviated here as “ASH-15” unless specification of ASH-15-D, E, etc. is indicated
Summary
Recent advances in laser ablation techniques applied to multi-phase carbonates allow for accurate dating of a variety of sample types, including calcite cements (Li et al, 2014; Godeau et al, 2018; Anjiang et al, 2019; Holdsworth et al, 2019), hydrothermal veins (Coogan et al, 2016; MacDonald et al, 2019; Piccione et al, 2019), fault-related veins, breccia cement, slickenfibers (Ring and Gerdes, 2016; Roberts and Walker, 2016; Goodfellow et al, 2017; Nuriel et al, 2017, 2019; Hansman et al, 2018; Parrish et al, 2018), and speleothems (Woodhead and Petrus, 2019). Finding the right matrix-matched reference material (RM) is a major hurdle for LA analyses of carbonates because of the variety of mineralogy (calcite, dolomite, and aragonite), textures, composition (e.g., high-magnesium calcite, high common lead), and ages (e.g., low radiogenic lead in young samples). Textural differences such as microcrystalline, fine- and coarse-grained material, between the unknown and RMs can contribute to high uncertainties due to differences in ablation efficiency, down-hole fractionation, and differences in crater morphology (e.g., Guillong et al, 2020; Elisha et al, 2020). Observed deviations are potentially up to 20 % of the final intercept age depending on the degree of crater geometry mismatch and are related either to downhole fractionation and/or matrix effects (Guillong et al, 2020)
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