Improving U-Th-Pb electron microprobe dating using monazite age references

This article proposes an improved approach to monazite dating by electron microprobe that includes a “monazite age reference correction” (MARC). During analysis, a set of differing monazite standard reference materials with established isotopic ages are measured at the start of the session. These measurements are used to test the analytical set-up and, if necessary, to calculate MARC factors that can be applied to monazite samples. The MARC is not intended as a way to correct systematic errors due to problems in set-up, but rather as a fine-scale adjustment for factors that cannot be readily assessed during single sessions. Long-term, multi-session calculation of MARC factors allows for precise monitoring of anomalous behavior among monazite age reference materials during individual sessions. The method can also assist in the identification of chemical inhomogeneity in monazite, such as that commonly produced by interaction with metasomatic fluids. A representative set of electron microprobe monazite age reference materials are presented, including two 'reference monazites' that are good examples of monazite with age disturbance induced by metasomatism. Additional modifications to analytic protocols are proposed, including a) corrections for count rate increases during long beam dwell times, and b) improved estimation of background values at line positions by accounting for the effect of mean atomic number.