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Wolframite, U/Pb geochronology, LA-ICP-MS, ID-TIMS, French Massif Central
Abstract:
Wolframite has been proposed as a U/Pb geochronometer for direct dating of W mineralisation events, but its isotopic analysis may be hampered by highly variable and low U contents (<200 ppm) and low 206Pb/204Pb ratios (〈300), heterogenous common Pb compositions, post-crystallisation alteration, and the presence of non-cogenetic mineral and fluid inclusions. In situ U/Pb dating of wolframite by laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) can avoid these analytical challenges but requires reference materials to properly correct for matrix and instrumental effects on measured U/Pb ratios. This study presents the U/Pb systematics by LA-ICP-MS of a wolframite sample (MTM-1) from the French Massif Central (FMC) which has been considered for normalisation and validation purposes of U/Pb LA-ICP-MS wolframite data in previous studies. We demonstrate that the MTM-1 wolframite is chemically and isotopically heterogeneous, and more importantly, we show that the previously defined ID-TIMS U/Pb age (334.4 ± 1.7 Ma, 2σ) is invalid due to an inappropriate common Pb correction. We calculate a new U/Pb crystallisation age of 316.7 ± 5.8 Ma (2σ). Based on our new calibration, we also present the U-Th-Pb trace element and U/Pb ages of six W deposits from the FMC, also previously studied by ID-TIMS. We show that U/Pb ages determined by LA-ICP-MS are more robust and geological plausible compared to ID-TIMS ages obtained on the same samples. The concentrations of U (ca 0.1–150 ppm) and Pb (ca 0.01–90 ppm) in FMC wolframite are highly variable and do not ubiquitously correlate to major element composition (such as the Fe/(Fe + Mn) ratio). The chemical variability and isotopic heterogeneity observed in wolframite from the FMC highlights the importance of pre-screening imaging methods (e.g., SEM, EPMA, μXRF) prior to LA-ICP-MS U/Pb analyses. Based on our results, we propose new guidelines for U/Pb LA-ICP-MS wolframite geochronology to increase accuracy and reproducibility in age determinations, as well as improving interlaboratory comparisons.
