Publication Date:
2018-06-26
Description:
This study investigates five megacrysts of xenotime (XN01, XN02, XN03, XN04, and XN05) as potential reference materials (RMs) for U-Pb geochronology. These crystals belong to a 300 g xenotime assortment, collected from alluvial deposits in SE Brazil. Electron microprobe and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses show that the selected crystals are internally homogeneous for most rare earth element, (REE, except some light REE) but are relatively heterogeneous for U and Th. The xenotime REE patterns are consistent with an origin from hydrothermal quartz veins in the Datas area that cut greenschist-facies metasediments and that locally contain other accessory phases such as rutile 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 two crystals have slightly different average 206 Pb/ 238 U 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 the ID-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 other established RMs within less than 1% deviation. Other Datas crystals (XN03-05) also display a reproducibility in Pb/U dates better than 1% on LA-ICP-MS, making them good candidates for further testing by ID-TIMS. ©2018. American Geophysical Union. All Rights Reserved.
Electronic ISSN:
1525-2027
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
