HR: 0800h
AN: V51A-0333    [Abstracts]
TI: Analytical Aspects of EPMA for Trace Element Analysis in Complex Accessory Minerals
AU: * Jercinovic, M J
EM: mjj@geo.umass.edu
AF: University of Massachusetts, Department of Geosciences 611 N. Pleasant St., Amherst, MA 01003, United States
AU: Williams, M L
EM: mlw@geo.umass.edu
AF: University of Massachusetts, Department of Geosciences 611 N. Pleasant St., Amherst, MA 01003, United States
AU: Lane, E
EM: edlane@mail.utexas.edu
AF: University of Texas at Austin, Department of Geological Sciences, Austin, TX 78712, United States
AB: High-resolution microanalysis of complex REE-bearing accessory phases is becoming increasingly necessary for insight into the chronology of phase growth and tectonic histories, and in understanding the mechanisms and manifestations of growth and dissolution reactions. The in-situ analysis of very small grains, inclusions, and sub-domains is revolutionizing our understanding of the evolution of complexly deformed, multiply metamorphosed, rocks. Great progress has been made in refining analytical protocols, and improvements in instrumentation have yielded unprecedented analytical precision and spatial resolution. As signal/noise improves, complexity is revealed, illustrating the level of care that must go into obtaining meaningful results, and in adopting an appropriate approach to minimize error. Background measurement is most critical for low concentration elements. Errors on net intensity values resulting from improper background measurement alone can exceed 50% relative. Regression and modeling of the background spectrum is essential, and must be carried out independently for each spectrometer, regardless of instrument. In complex materials such as REE- bearing phosphates, high concentrations of REEs and actinides create difficult analytical challenges as numerous emission lines and absorption edges cause great spectral complexity. In addition, trace concentrations of "unexpected" emission lines such as those from sulfur, or fluoresced from nearby phases (Ti, K), cause interferences on both measured peaks and background regions which can result in very large errors on target elements (U, Pb, etc.), on the order of 10s to 100s of ppm. Characteristic X-ray emission involving electron transitions from the valence shell are subject to measureable peak shifts, in some cases significantly affecting the accuracy of results if not accounted for. Geochronology by EPMA involves careful measurement of all constituent elements, with the calculated date dependant on the resulting Pb, U, and Th concentrations. Cancellation of systematic errors, such as those arising from improper background estimation procedures, can yield "reasonable" dates despite large inaccuracies in the obtained concentrations. If not recognized, these systematic errors can result in age errors of 10s of m.y. Rigorous trace element strategies (distinct form traditional major-element protocol) must be followed in order to obtain meaningful ages from a variety of if domain compositions. Individual compositional domains in complexly zoned accessory phases should be sampled until statistical adequacy is met, and overall results for a domain should be evaluated on the basis of compositional homogeneity and the resulting propagation of counting errors. Weighted means of corresponding domains can then begin to reveal a more meaningful dataset where complete compositions can be related to calculated ages. Although reliable trace element secondary standards are not currently available, an appropriate range of age standards of differing composition can at least constrain accuracy, and reveal inappropriate procedures.
DE: 3694 Instruments and techniques
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
DE: 5475 Tectonics (8149)
DE: 9820 Techniques applicable in three or more fields
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting