| Popis: |
Dating methodology by the electron probe microanalyser (EPMA) of (Th, U)-bearing minerals, highlighting monazite,acquired greater than ever importance in literature, particularly due to its superior spatial resolution, as well as versatility,which allow correlating petrological processes at times registered only in micro-scales in minerals and rocks with absoluteages. Although the accuracy is inferior to the one achieved with conventional isotopic methods in up to an order of magnitude,EPMA is the instrument that allows the best spatial resolution, reaching a few μm3 in some conditions. Quantifi cation ofminor and trace elements with suitable precision and accuracy involves the own instrumental and analytical set-ups anddata treatment strategies, signifi cantly more rigorous when compared with those applied in conventional analyses. Th-U-PbTdating is an example of these cases. Each EPMA is a unique machine as for its instrumental characteristics and respectiveautomation system. In such a way, analytical procedures ought to be adjusted for laboratory specifi cities. The analyticalstrategies and data treatment adopted in the Electronic Microprobe Laboratory from Instituto de Geociências of Universidadede São Paulo, Brazil, with a JEOL JXA8600S EPMA, and a ThermoNoran-Voyager 4.3 automation system, are presentedand compared with the ones used in other laboratories. The infl uence of instrumental factors and spectral overlaps on Th,U, and Pb quantifi cation is discussed. Applied procedures to interference correction, error propagation, data treatment, andfi nal chemical age presentation as well as to sampling and analyses are emphasized. Some typical applications are discussed,drawing attention to the most relevant aspects of electron microprobe dating. |
