DEVELOPMENT OF AN OPTIMIZED ELECTRON MICROPROBE FOR GEOCHRONOLOGIC AND TRACE ELEMENT ANALYSIS

Microprobe monazite geochronology represents a major tool for petrologic and tectonic analysis of metamorphic rocks. At the University of Massachusetts, we have developed a multi-step process for monazite geochronology involving: stage-scan mapping to identify all monazite in a polished thin section; high resolution compositional and age mapping to characterize age zonation within individual crystals; and high-resolution trace element analysis for dating mapped domains. These techniques have already had a profound effect on tectonic interpretations (and on interpretations of previous geochronologic data) in several orogenic belts. Although modern electron microprobes can already yield useful age constraints, significant improvements are possible that have the potential to dramatically improve the technique. We have begun a collaborative project (with Cameca Inc) to develop and construct: new electron microprobe optimized for trace element analysis and specifically, monazite geochronology. Hardware modifications include a redesigned electron gun and column for operation at high-current and small spot size, spectrometer redesign to accommodate very large crystals, and redesigned backscatter and anticontamination systems. Software modifications involve multi-spectrometer counting, background modeling, flexible overlap correction, and integrated compositional and age mapping. In addition, new synthetic and natural standards will be developed and fully characterized. All modifications are aimed at increasing the numerical and spatial resolution of the electron microprobe, particularly for trace element analysis. Our goal is to test the limits of microprobe monazite geochronology and to firmly establish this technique within the spectrum of geochronologic techniques. Many of the improvements will be exportable to existing microprobe laboratories or will be available on new microprobes. In addition, once instituted, the new completely optimized microprobe at UMass will be the core of a Monazite Geochronological Research Center, available for investigation of specialized geochronological problems and for correlation of results from other laboratories.