ELECTRON MICROPROBE PETROCHRONOLOGY: NEW DATA AND NEW CHALLENGES FROM THE ADIRONDACK MOUNTAINS, NEW YORK

Petrochronology involves the incorporation of chronometer phases into the petrologic and tectonic evolution of their host rocks, such that direct age constraints can be placed on petrologic and structural processes. The electron microprobe has a central and critical role to play in establishing the linkage between chronometer phases and their host assemblage. In addition to characterizing the composition of the major silicate phases, the probe can be used to locate and map compositional zonation in accessory phases including chronometers such as monazite, xenotime, zircon, titanite, etc. One particularly effective technique for integrating petrology and geochronology is to combine high-resolution images of chronometer phases with large-scale x-ray maps of polished sections. Chronometers can be evaluated in the context of their local compositional environment. This has proven to be useful for placing the chronometer domains within the reaction history of the host rock. Key chronometer domains, once selected, can be dated by the most appropriate tool, or by multiple techniques. For relatively old or small domains in monazite or xenotime, EPMA can provide precise age constraints. However, the analysis strategy must be designed as a trace element analysis from the beginning. Unlike major element analysis, background analysis is both critical and challenging, and new methodology is required, currently either wavelength scanning or multipoint analysis. Errors on the order of 2% are possible with standard instruments, and errors close to 1% can be achieved with optimized instruments. Recent work in the Adirondack Mountains provides an example of the power and challenges of EPMA Petrochronology. Integrated compositional and geochronological analyses constrain the time of prograde metamorphism, two stages of melting, hydrothermal alteration, and retrograde metamorphism. However, the record from individual rocks is heterogeneous, especially concerning which events are recorded in specific rocks. It is increasingly important to identify monazite-producing reactions and to incorporate these into thermodynamic models.