U-Th-PbTOTAL dating of REE-phosphate by electron microprobe: Review and progress

Abstract

Abstract Monazite (Mnz) and xenotime (Xnt) are rare earth element (REE) phosphate minerals that occur in a large variety of rocks. They are known to contain very limited amounts of common Pb, and thus, a date can be calculated by simply measuring the total lead- and actinide-contents. Such a measurement can be done at high spatial resolution with an electron probe microanalyser (EPMA), together with a complete mineral analysis. The U-Th-Pbtotal dating method was developed almost 30 years ago, to provide an in-situ age for several (U, Th)-bearing minerals such as uraninite, Mnz, and Xnt. This technique permits identification of events that are not recorded by other minerals, allows dates to be related to structural or geochemical features, and ultimately reinforces the age interpretation. The principal challenge is the accurate measurement of Pb, which is commonly present at the trace level (< 1,000 ppm). This paper reviews the most recent advances in the U-Th-Pbtotal dating technique of Mnz and Xnt, notably in terms of a) accurate background acquisition using the multipoint background technique, and b) improved peak interference correction for trace and REE elements. This paper also discusses a new method of quantitative element mapping, which helps with the identification of compositional domains in Mnz (or Xnt). Finally, a date cannot be interpreted as an age without the proper context, and several key factors in the interpretation are discussed and illustrated with several case studies.