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

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.

The study of the valence and distribution of radiogenic Pb in monazite by using XPS and TEM

<p>Monazite((Ce,Y,La,Th)PO<sub>4</sub>) is an important phosphate mineral and is one of the widely used minerals for U-Th-Pb dating in geochronology. In this study, we have examined the crystallinity, the valence and coordination of radiogenic Pb in a natural RW-1 monazite standard (ThO<sub>2</sub> up to13.5 wt% and Pb up to ~5000 ppm) with a <sup>207</sup>Pb/<sup>235</sup>U age of 904.15 ± 0.26 Ma from a Norwegian pegmatite by using laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy(XPS) and transmission electron microscopy (TEM). The Raman spectrum analysis revealed that this monazite is well crystalline and is not damaged by α-particles. The results of XPS and TEM suggest that the radiogenic Pb produced by the α-decay of U and Th is divalent and radiogenic Pb atom substitutes the Ce-site within the monazite crystal lattice. The qualitative analyses conducted on the HAADF-STEM data reveal heterogeneous distribution of radiogenic Pb within the monazite crystal lattice. This is the first work on the determination of the oxidation state, the atomic location and distribution of radiogenic Pb in a natural monazite (CePO<sub>4</sub>). The deeply study of radiogenic Pb in monazite at the nanoscale and atomic scale provides a good insight for us to understand the mechanisms of nano-isotopic mobility and the nano-geochronology  that has been poorly understood so far.</p>