Five growth domains in a monazite grain (from Mahan et al., 2006)

For more than 20 years, Professor Mike Jercinovic, Williams, graduate students, and postdoctoral fellows have worked to develop the technique of microprobe dating at UMass. The theory is based on the fact that minerals like monazite and xenotime crystalize with significant amounts of U and Th but very little Pb. Thus, virtually all of the Pb in these minerals comes from radioactive decay of U or Th. If we can accurately measure the amount of U, Th, and Pb it is possible to calculate a date (age) of the monazite or xenotime crystal. Because U and Pb are trace elements in monazite and xenotime, the analysis requires a very careful microprobe procedure and a state-of-the-art microprobe instrument.

One of the major goals of igneous or metamorphic petrologic analysis is determining the sequence of minerals that were present during the evolution of the rocks of interest, and ultimately interpreting the sequence of chemical reactions that relate the minerals. Monazite is produced in many metamorphic reactions because it can accommodate many of the trace or Rare Earth elements that are liberated during a reaction. Typical monazite grains tend to have several growth domains or shells (getting younger outward) that represent a series of reactions in the host rocks. The monazite grains are very small and the growth domains within grains are even smaller. Thus, a microprobe with a very small electron beam is required to accurately analyze these tiny domains.

The electron microprobe was originally developed to analyze major elements (> 1 wt. %) in relatively large minerals. As part of this on-going research, Jercinovic and Williams (with Cameca Inc.) developed a new electron microprobe that is optimized to analyze trace elements in very small minerals. The new microprobe is called “Ultrachron”, and it is current running in the Department of Geosciences at UMass. For more information about the Ultrachron microprobe [Click Here].  For more information about monazite dating [Click Here].

References involving UMass researchers [Click Here]