Age constraint on Burmese amber based on U–Pb dating of zircons

Elemental fractionation effects during analysis are the most significant impediment to obtaining precise and accurate U-Pb ages by laser ablation ICPMS. Several methods have been proposed to minimize the degree of downhole fractionation, typically by rastering or limiting acquisition to relatively short intervals of time, but these compromise minimum target size or the temporal resolution of data. Alternatively, other methods have been developed which attempt to correct for the effects of downhole elemental fractionation. A common feature of all these techniques, however, is that they impose an expected model of elemental fractionation behavior; thus, any variance in actual fractionation response between laboratories, mineral types, or matrix types cannot be easily accommodated. Here we investigate an alternate approach that aims to reverse the problem by first observing the elemental fractionation response and then applying an appropriate (and often unique) model to the data. This approach has the versatility to treat data from any laboratory, regardless of the expression of downhole fractionation under any given set of analytical conditions. We demonstrate that the use of more complex models of elemental fractionation such as exponential curves and smoothed cubic splines can efficiently correct complex fractionation trends, allowing detection of spatial heterogeneities, while simultaneously maintaining data quality. We present a data reduction module for use with the lolite software package that implements this methodology and which may provide the means for simpler interlaboratory comparisons and, perhaps most importantly, enable the rapid reduction of large quantities of data with maximum feedback to the user at each stage. Copyright 2010 by the American Geophysical Union.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009GC002618

Improved laser ablation U‐Pb zircon geochronology through robust downhole fractionation correction

[1] Zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences. Modern microbeam techniques such as SIMS and LA-ICPMS have been successfully applied to in situ U-Pb zircon age determinations, at spatial resolutions of 20–30 μm or less. Matrix-matched calibration by external standardization of well-characterized natural zircon references is a principal requirement for precise microbeam U-Pb zircon age determination due to fractionation effects between Pb and U, which usually result in an external age error exceeding 1%. Alternatively, zircons with a closed U-Pb system can be directly dated by measurement of 207Pb/206Pb isotopic ratio without external standardization, which has been a common practice for zircons older than 1.0 Ga, but not for relatively young (<1.0 Ga and particularly Phanerozoic) ones because of limitations of analytical precision. We describe in this paper a method of 207Pb/206Pb measurement on Phanerozoic zircons using a new generation of large radius magnetic sector multicollector Cameca IMS-1280 SIMS. In combination with multicollector mode, a Nuclear Magnetic Resonance (NMR) magnet controller and oxygen flooding techniques, we achieve precisions of 207Pb/206Pb ratio of <0.1% and 0.1 ∼ 0.2%, propagating to Pb/Pb age errors <0.4% and 1–3% (excluding U decay constant uncertainties), for zircons of latest Neoproterozoic and late Paleozoic to Mesozoic age, respectively. Therefore, the multicollector SIMS is capable of direct determination of zircon Pb/Pb ages as young as Mesozoic age with uncertainties of geological significance. This technique is useful for direct dating of zircons in thin sections. Moreover, it has significance for dating of some other U-rich minerals (i.e., baddeleyite and zirconolite) that are not suitable for SIMS U-Pb dating by external standardization.

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Precise determination of Phanerozoic zircon Pb/Pb age by multicollector SIMS without external standardization

The effect of three different cone combinations on the performance of laser ablation MC-ICP-MS (Neptune plus) for the in situ Hf isotope analysis of zircon were investigated. The signal sensitivities of Hf, Yb and Lu were improved by a factor of 1.4 and 2.5, respectively, with using the X skimmer cone + standard sampler cone and the X skimmer cone + Jet sample cone compared to the standard arrangement (H skimmer cone + standard sample cone). However, when using the high-sensitivity Jet sample cone, the instrumental mass fractionation for hafnium displayed a large non-linear component that could not be corrected using the normal mass fractionation laws. The magnitude of this non-linear mass fractionation was strongly related to the central gas flow rate. The in situ Hf isotope analysis of zircon standards 91500 and Mud Tank using the Jet cone displayed large deviations (410–470 ppm) at the optimum central gas flow rate for Hf, which seriously deteriorated the performance of the Jet cone. The addition of 4 ml min−1 nitrogen to the central gas flow in laser ablation MC-ICP-MS was found to not only increase the sensitivity of Hf by a factor of 2.1, but also suppress this non-linear mass fractionation. The determined Yb/Hf and Lu/Hf ratios at their corresponding optimum makeup gas flow rates for Hf intensity were found to be reduced by factors of 2 and 1.3 in the presence of nitrogen, respectively, which would benefit the accurate in situ determination of Hf isotopes in high-content Yb and Lu samples. Compared to the standard arrangement, the corresponding precision (2σ) of 176Hf/177Hf for single spot analysis of zircon standard 91500 was improved from 224 ppm to 50 ppm by using the newly designed X-skimmer cone and Jet sample cone in combination with the nitrogen addition technique. The determined 176Hf/177Hf ratios are in excellent agreement with published values in five reference zircon standards (91500, GJ-1, Mud Tank, Penglai and Plesovice). Our first Hf isotopic results from zircon standard M257 (0.281544 ± 0.000018; 2SD, n = 151) showed that it was fairly homogeneous in Hf isotopes. These results clearly demonstrate that the present analytical method has the potential to become an important tool for the pursuit of high-quality in situ Hf isotope data for zircons.

Improved in situ Hf isotope ratio analysis of zircon using newly designed X skimmer cone and jet sample cone in combination with the addition of nitrogen by laser ablation multiple collector ICP-MS

Nous proposons un nouveau materiel de reference potentiel - des megacristaux de zircon naturel provenant d'un basalte alcalin du debut du pliocene (region de Pengiai, nord de l'ile de Hainan, Chine meridionale) destine a l'analyse microfaisceaux des isotopes de l'O et de l'Hf, et la datation U-Pb. Les megacristaux de zircon de Penglai se sont reveles, apres un grand nombre de mesures par LA-MC-ICP-MS et SIMS, assez homogenes en terme d'isotopes de l'O et de l'Hf. Les determinations precises des isotopes de l'O par IRMS et des isotopes de l'Hf par MC-ICP-MS solution sont en bon accord avec la moyenne statistique des analyses microfaisceaux. La valeur moyenne δ 18 O de 5.31 ± 0.10‰ (2s) par IRMS et la valeur moyenne 176 Hf/ 177 Hf de 0.282906 ± 0.0000010 (2s) par solution MC-ICP-MS sont les meilleures valeurs de reference pour les zircons de Penglai. Les mesures SIMS ainsi que celles par dilution isotopique-TIMS donnent, compte tenu des incertitudes analytiques, des âges 206 Pb/ 238 U conformes, et l'âge prefere 206 Pb/ 238 U est estime a 4.4 ± 0.10 Ma (intervalle de confiance de 95%). Leur jeune âge et leurs teneurs variables et elevees en Pb commun font que les zircons de Penglai ne sont pas utilisables comme materiel de reference primaire pour l'etalonnage d'echantillons inconnus dont les âges U-Pb doivent etre determines par analyses microfaisceaux; Toutefois, ils peuvent etre utilises comme materiel de reference secondaire pour le controle qualite de la determination de l'âge U-Pb des zircons jeunes (en particulier pour ceux < 10 Ma).

/pdf/penglai-zircon-megacrysts-a-potential-new-working-reference-4jw0hy1d4g.pdf

Jirri Slama

Papers

Plesovice zircon : A new natural reference material for U-Pb and Hf isotopic microanalysis