TS-Mnz: A New Monazite Age Reference Material for U-Th-Pb Microanalysis

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A single megacryst of Neoproterozoic monazite (“TS-Mnz”) was investigated by electron probe microanalysis (EPMA), Raman microspectroscopy, U-Pb dating by isotope dilution-thermal ionization mass spectrometry (IDTIMS), and U-Th-Pb dating and trace element analysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The monazite contains both distinct unaltered domains that are either homogeneous or zoned, and altered domains with secondary inclusions of ThSiO4 along fractures. Altered domains can be easily recognized in backscattered electron images and avoided during microanalysis. The U-Pb ID-TIMS measurements in two laboratories revealed slight reverse discordance of U-Pb data and provided 207Pb/235U mean dates of 908.2 ± 1.5 Ma (95% conf., MSWD = 2.9) and 910.42 ± 0.34 Ma (2σ, MSWD = 1.0). Three individual LA-ICPMS sessions yielded 207Pb/235U mean dates of 899.8 ± 6.9 Ma (95% conf., MSWD = 0.10), 896.3 ± 6.5 Ma (95% conf., MSWD = 0.18) and 901.9 ± 2.0 Ma (95% conf., MSWD = 0.57), and 208Pb/232Th mean dates of 909.2 ± 5.8 Ma (95% conf., MSWD = 0.36), 915.1 ± 11 Ma (95% conf., MSWD = 0.31) and 909.9 ± 1.5 Ma (95% conf., MSWD = 0.56). The lower precision of the LA-ICPMS with respect to the ID-TIMS technique hides discrepancies between ID-TIMS 207Pb/235U dates as well as the reverse discordance, which is common for monazite. The discrepancies between ID-TIMS and LA-ICPMS data are interpreted as related to standardization and individual setup of analytical techniques; however, minor variance of discordance between individual analyzed spots also cannot be excluded. Further electron microprobe U-Th-total Pb dating provided dates of 914.6 ± 2.0 Ma (95% conf., MSWD = 0.99) and 922.9 ± 3.1 Ma (95% conf., MSWD = 4.3) for two fragments of TS-Mnz. The LAICPMS U-Th-Pb round-robin test employing TS-Mnz as a primary reference material and various monazite samples of known age as unknowns provided accurate U-Pb and Th-Pb dates within errors of the reference ages. To conclude, the TS-Mnz despite compositional variations reveals relatively reproducible U-Pb, Th-Pb and U-Thtotal Pb dates and is proposed as a new age reference material for in-situ U-Th-Pb microanalysis with recommended reference mean 207Pb/235U and 208Pb/232Th ages of 910.42 ± 0.34 Ma (2σ) and 910.7 ± 1.3 Ma (95% conf.), respectively.


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