Characterization of xenotime from datas (Brazil) as a potential reference material for in situ u-pb geochronology.

Abstract

This study investigatesfive megacrysts of xenotime (XN01, XN02, XN03, XN04, and XN05) aspotential reference materials (RMs) for U-Pb geochronology. These crystals belong to a 300 g xenotimeassortment, collected from alluvial deposits in SE Brazil. Electron microprobe and Laser Ablation-InductivelyCoupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses show that the selected crystals are internallyhomogeneous for most rare earth element, (REE, except some light REE) but are relatively heterogeneous forU and Th. The xenotime REE patterns are consistent with an origin from hydrothermal quartz veins in theDatas area that cut greenschist-facies metasediments and that locally contain other accessory phases such asrutile and monazite. High-precision U-Pb Isotope Dilution-Thermal Ionization Mass Spectrometry (ID-TIMS)analyses showed slight age heterogeneity for the XN01 crystal not observed in the XN02 sample. The twocrystals have slightly different average206Pb/238U ages of 513.4 ± 0.5 Ma (2 s) and 515.4 ± 0.2 Ma (2 s),respectively. In situ U-Pb isotope data acquired via LA-(Q,SF,MC)-ICP-MS are within the uncertainty of theID-TIMS data, showing homogeneity at the 1% precision of the laser ablation (and probably ion microprobe)technique. U-Pb LA-(MC, SF)-ICP-MS analyses, using XN01 as a primary RM, reproduced the ages of otherestablished RMs within less than 1% deviation. Other Datas crystals (XN03-05) also display a reproducibility inPb/U dates better than 1% on LA-ICP-MS, making them good candidates for further testing by ID-TIMS.