The use of ASH-15 flowstone as a matrix-matched reference material for laser-ablation U − Pb geochronology of calcite

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.

Towards in situ U–Pb dating of dolomite

“…Uncertainties (2σ ) were propagated on individual unknown ratios such that 207 Pb / 206 Pb (2 %) and 206 Pb / 238 U (4 %) ra-tios of a zircon standard, run throughout the session (mud tank;Black and Gulson, 1978), yielded a single population; this resulted in reasonable mean square weighted deviations (MSWDs) for the calculated ages of calcite RMs. Secondary calcite RMs -ASH-15 (2.9646 ± 0.01 Ma;Nuriel et al, 2021) and Duff Brown (64 ± 0.7 Ma;Hill et al, 2016) -yielded dates within uncertainty of their accepted values (ASH-15: 2.973 ± 0.09, MSWD = 1.3, n = 107; Duff Brown: 63.2 ± 2.3 Ma, MSWD = 1.9, n = 106). Uncertainty correlations are calculated followingSchimtz and Schoene (2007).…”

“…6b). Such a signature is likely to be associated with the mechanism of epigenetic dolomitization governed by diagenetic replacement of pore fluids and re-precipitation of dolomite (Putnis and Putnis, 2007;Olanipekun and Azmy, 2017). In sample EFN-1, a mixture of different zoning patterns can be seen within the fragments of the breccia: dolomite crystals that lack distinctive core-to-rim zones and crystals with bright cores and dark rims (Fig.…”

“…Intracrystalline porosity is usually smaller than a spot size of 85 µm and may include other phases besides dolomite, such as K-feldspar, pyrite, oxides, and bituminous minerals (Fig. 6a; Olanipekun and Azmy, 2017). These phases may include detrital contamination with inherited U-Pb ages, which might lead to mixed ages or ages older than expected.…”

Pace of passive margin tectonism revealed by U-Pb dating of fracture-filling calcite

“…Similar to the previous correction of the 207 Pb/ 206 Pb ratio, this correction includes offset due to both mass bias and detector efficiency differences (i.e., there is no prior gain calibration for the Daly detector array). Accuracy and precision were monitored using a variety of secondary standards including ASH15 33 (2.96 Ma) and Duff Brown Tank 34 (64 Ma). Our full suite of U-Pb data, including reproducibility of primary and secondary standards is available in source data file 3 .…”

U-Pb speleothem geochronology reveals a major 6 Ma uplift phase along the western margin of Dead Sea Transform

“…NIST 614 was used as a primary reference material for 207 Pb/ 206 Pb and 238 U/ 206 Pb inter-element mass bias and instrumental drift corrections. Offsets driven by changes in the plasma conditions, due to matrix effects were corrected using natural calcite sample -WC-1 (Roberts et al, 2017) and ASH-15-D (Nuriel et al, 2020) as secondary reference materials.…”