The Chemical State and Occupancy of Radiogenic Pb, and Crystallinity of RW-1 Monazite Revealed by XPS and TEM

Xu Tang,Chen-Hui Fei,Qiu-Li Li,Xiao-Xiao Ling,Bin Zhang,Peng Wang,Jin-Hua Li,Li-Xin Gu

doi:10.3390/min10060504

Abstract

Monazite ((Ce, La, Nd, Th)PO4) is one of the widely used minerals for U–Th–Pb dating in geochronology. To better understand the possible effects of radiogenic Pb on the in situ dating method, a natural monazite U–Th–Pb standard sample (RW-1) was chemically and structurally characterized down to atomic scales by using the combination of Raman spectrum (RM), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The experimental results revealed that radiogenic Pb exists as Pb2+ and substitutes for the Ce site in the monazite crystal lattice. Moreover, TEM imaging demonstrated that monazite is well crystalline revealed by an atomic structure in most areas except for a few tiny defects, which are likely attributed to alpha self-healing from an electronic energy loss of α particles. The characterization of the chemical state and occupancy of radiogenic Pb, and the distribution of Pb and Th in monazite at the nanoscale and atomic scale could provide insight for us to understand the mechanisms of the nanogeochronology.

Highlights

Monazite ((Ce, La, Nd, Th)PO4 ), a light rare earth element (LREE)-phosphate mineral, is one of the widely used minerals for U–Th–Pb dating in geochronology [1,2,3]
Previous study showed that the RW-1 monazite is chemically homogeneous by X-Ray electron probe microanalyzer (EPMA), with the contents of
The chemical state of radiogenic Pb was determined by X-ray photoelectron spectroscopy (XPS) instrument (Thermo Scientific, Inc., Waltham, MA, USA) in two RW-1 monazite chips

Summary

Introduction

Monazite ((Ce, La, Nd, Th)PO4 ), a light rare earth element (LREE)-phosphate mineral, is one of the widely used minerals for U–Th–Pb dating in geochronology [1,2,3]. It is used as nuclear waste disposal material in material science due to the strong bonds between P and O and mineral structure [4,5,6]. Discordant U–(Th)–Pb ages determined from monazite, excluding those reversely discordant caused by excess 206 Pb resulted from high Th content during crystallization [2], have been reported [9,10,11,12]. Recent studies by atom probe microscopy (APM) have demonstrated that radiogenic Pb could be coupled with nanoclusters

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Discussion

Conclusion