A new calibrated strategy for the in situ U–Th–Pb dating of bastnasite by xenotime

Abstract

Bastnaesite becomes an important mineral geochronometer to record REE mineralization time for the REE deposits. As the only reliable primary standard for in-situ U–Th–Pb dating, bastnaesite K-9 is inhomogeneous and...

A new appraisal of ilmenite U–Pb dating method by LA-SF-ICP-MS

“…59 Due to the lack of matrix-matched standards, successful LA-ICP-MS U-Pb dating analyses show that homogeneous and widely distributed non-matrix-matched standards were also effective for a growing number of new minerals to yield highly accurate isotopic ratios and ages with some assistant methods or via optimizing ablation settings to reduce matrix effects. 17,21,24,25,27,31,32,57 Therefore, non-matrix-matched standards play an important role in U-Pb dating methods, especially when they have U/Pb fractionation similar to that of the unknown materials, and t all or most of the ablation settings, e.g., wolframite YGX for scheelite, 31 xenotime XN01 for bastnaesite, 32 and zircon 91500 for rutile. 25 In this analysis, zircon 91500 and three Ti-and Fe-bearing minerals of rutile RMJG, garnet PL-57, and wolframite YGX were selected to match the ilmenite samples.…”

“…As mentioned above, the analyzed conditions, including shorter wavelength ablation, spot size, sampling rate and time, energy density, and the addition of assistant materials, such as H 2 O, H 2 , and N 2 , played an important role in changing U/Pb fractionation. [21][22][23]25,[32][33][34] Both GeoLas and Resolution laser systems in the three analyses were operated at 193 nm wavelength. Thus, the laser system was likely not the principal cause.…”

“…37,38,59,[63][64][65][66] Correspondingly, a mineral that has slightly inhomogeneous U-Pb isotopes will only be proposed as quality control during each analytical session. 30,32 The two-stage calibration method has also become popular in in situ U-Pb dating of U-bearing minerals recently due to the lack of homogeneous standards for some minerals, e.g., apatite, calcite, wolframite, and scheelite. 31,41,55,59 Though containing inhomogeneous U-Pb isotopes and variable common lead, some minerals yielded a uniform and well-constrained lower intercept U-Pb age in the Tera-Wasserburg Concordia diagram and can also be exploited as primary and secondary standard in this calibration method, e.g., WC-1 for calcite and discordant YGX/MTM for wolframite and scheelite.…”

“…26 The data collected from ICP-MS were processed offline using the ICPMSDataCal soware for trace element content and U-Pb age calibration. The detailed calibration equation has been described in previous studies 21,22,31,32 and is also provided as ESI (Table S-1). † Excluding the initial ∼2 s, only the rst ∼25 s of ablation data was used in the calculation to reduce or eliminate down-hole fractionation effects.…”

“…14,15,17 Non-matrix matched calibrations have increasingly been used in LA-ICP-MS U-Pb analysis owing to a lack of matrixmatched reference materials (RMs), e.g., NIST 610 for zircon, monazite, and xenotime; 22 NIST 612 for cassiterite; 23 Plesovice zircon for allanite; 24 zircon 91500 for rutile and garnet; [25][26][27] zircon 91500 and schorlomite for vesuvianite; 28,29 wolframite YGX for ferberite, hübnerite, and scheelite; 30,31 and xenotime XN01 for bastnaesite. 32 However, matrix effects exist for most of the above minerals due to different ablation behaviors, and thus, special conditions have been employed to reduce matrix effects, including adding water vapor and N 2 , 21,22,32 using a large spot size of $44 mm, 25 as well as using a low laser sampling rate, a low energy density, short ablation times (∼30 s), 23,33 and shorter wavelength of ablation laser. 34 Rutile TB-1 has been used as a non-matrix-matched primary standard instead of Mud Tank zircon and BCR-2g to calibrate U/ Pb ratios and obtain the most accurate U-Pb ages within analytical uncertainties for ilmenite samples by LA-ICP-MS due to the lack of matrix-matched RMs.…”