“A Cheap Date in Economically Trying Times”. Geochronology of Rare-earth Phosphate using the Electron Microprobe Analyzer; Analytical Protocol Design and Geological Applications.
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The concept of Deep Time is of great importance in the history of science, and determination of ages for geologic events is fundamental to our understanding of the rates and periodicity of Earth-scale processes. For geologists, perhaps the most important set of natural “clocks” by which deep time is measured are minerals which incorporate significant amounts of the elements thorium and uranium within their crystalline lattice; the decay of these elements to isotopes of lead is a strict function of time, and measured ratios of the isotopes of Th, U, and Pb are convertible to units of time; time elapsed since some event in the history of the analyzed mineral.
This presentation will focus on the properties and use of trace elements in a rare earth phosphate mineral, monazite as a recorder of geologic heating, cooling, and pressure-related events that can be linked to crustal-scale tectonic processes. The novel application of the Electron Microprobe Analyzer (EPMA) for trace element analysis and mapping of monazites for both elucidation of the growth history of the mineral and determination of the ages of distinguishable growth zones, in contrast to the more traditional (and costly) SIMS method of radiogenic age determinations will be discussed. Development of a protocol for accurate and precise EPMA analysis of trace elements in monazite will be described and results will be compared with isotopic ages determined by SIMS. Examples of applications of this technique to determining temperature-time histories of metamorphic terrains from central New England, New Mexico, Pennsylvania, and the Indian-Nepalese Himalaya will be presented.
Dr. Pyle’s presentation is based on his research while a Research
Associate in Rensselaer Polytechnic Institute’s Department of Earth and
Environmental Sciences. He is currently
employed at BMPC in Schenectady, NY.