Characterization of OX1r monazite as a reference material for U–Pb–Th isotopic microanalysis using LA-MC-ICPMS at CPGeo laboratory – IGc- USP

Monazite is a light rare earth element (LREE)-bearing phosphate mineral abundant in peraluminous igneous rocks, aluminous metasedimentary rocks, carbonatites,hydrothermal veins, and as a detrital mineral in clastic sedimentary rocks.It is renowned in Earth Sciences for its utilization as a geochronometer, owing to its high -uranium and -thorium content, low initial non-radiogenic Pb.Despite the widespread use of monazite in U–Pb–Th geochronology, the selection of suitable reference materials remains contentious within the scientific community. Therefore, continuous investigation looking for new reference standards is imperative to ensure accurate dating results with maximal precision.This study aims to evaluate the suitability of Ox1r monazite as a reference material for U–Pb–Th geochronology.

The monazite sample was collected from a pegmatite dikeby J. Orestes Santos and the “red” subsample OX1r was provided to us by Neal McNaughton (Western Australia University). The obtained sample was prepared  in the Sample Preparation Facility of the Geochronological Research Center (CPGeo, IGc-USP, Brazil). The Ox1r crystal was crushed into fragments of around 50–200µm and handpicked  under a binocular microscope prior to be mounted on epoxy resin, and finally polished to expose the internal area of the crystals. To facilitate the identification of the best location to place the spots, the mount was backscattering imaged using a Quanta 250 FEG scanning electron microscope. U–Pb–Th isotopic data were acquired using a NEPTUNE Thermo Scientific  MC-LA-ICP-MS system coupled with an ablation cell (Teledyne) model Excimer 193 (nm), employing a spot diameter of 10μ, a frequency of 6 Hz, and 100% laser energy, resulting in approximately 8.0 mJ of fluency. The reference material USGS 44069 (206Pb/238U ID-TIMS age of 424.9 ± 0.8 Ma (Aleinikoff et al. 2006) was used for correction of mass bias, instrument drift, and elemental fractionation.

The analytical routine included one analysis of the USGS 44069 reference material, one blank reading, and 10 unknown monazite Ox1r analyses. Each analysis had a total acquisition time of 45 seconds, with 10 seconds for background measurement, 30 seconds for integration time, and 5 seconds for the washout period. Isotopic results were processed and corrected using in-house software developed by Siqueira et al. (2014), utilizing an "R" statistical package based on the Python programming language. Geochronological data were plotted using IsoplotR software (Vermeesch, 2018). Only analyses with less than ± 2% discordance were considered for age calculation. Individual uncertainties were reported at the 2σ level, and the confidence level for weighted averages was set at 95%.

A total of 115 analyses were conducted across five analytical sessions on different monazite fragments of Ox1r, resulting in a Concordia age of 507.7 ± 1 Ma (2s) (MSWD = 53; p(x2) = 4.1e-13) and a weighted mean age of 508.4 ± 0.99 Ma (2s) (MSWD = 0.52; p(x2) = 1). These findings closely matched a previously obtained 206Pb/238U ID-TIMS age of 507.3 ± 0.8 Ma (2s), according to William Davis (2005, personal communication). Additionally, the analyses yielded Th-Pb and 207Pb-based common lead corrected 206Pb/238U ages with overall 4% variation (2 SD), indicative of good U-Th-Pb system homogeneity. However, the crystal fragments exhibited inhomogeneity in Th content, with a deviation of 9.35 ± 3.2 wt% (2 SD), and Th/U ratio of 98.5 ± 29 (2 SD), rendering it unsuitable as a compositional reference material.In summary, the Ox1r monazite sample aligns well with calculated and reference ages, suggesting its potential as a calibration reference material for U–Pb–Th geochronology, despite limitations regarding compositional homogeneity.