This paper outlines the CHIME (chemical Th-U-total Pb isochron method) dating on the basis of precise electron microprobe analysis of Th, U and Pb in an area of 3-6 μm across within Th- and U-bearing accessory minerals like monazite, zircon, xenotime and polycrase. The approach consists in measuring many spots with sufficient compositional variation in domains of minerals with constant age, and in constructing a "pseudo-isochron" from which an age can be obtained by regression. This method has the potential advantage of substantial precision under the right circumstances. The regression based technique has the ability to work with minerals having substantial initial Pb, and can identify two or more homogeneous domains that separated by an age gap smaller than the analytical error on individual spot analysis of age. Many features that are insignificant in major element analysis can have major manifestation in acquired trace elements. Critical comments include detection limit on microprobe analysis, statistics in X-ray counting, characterization of spectral profiles and correction for elemental interferences. Several other considerations of analytical procedure are more briefly shown, and an age-map procedure is also described .
Monazite is most suitable to the CHIME dating, since it shows a concordant Th-U-Pb relation, contains 5-20 wt.% ThO
2 and 0.1-1.5 wt.% UO
2 that can produce 0.01-0.06 wt.% PbO during 50 Myr, and remains immune to significant Pb-loss even under the sillimanite grade metamorphism. Occasional inconsistence between the CHIME monazite and Rb-Sr whole-rock isochron ages is due to, at least partly, a spurious isochron resulted from heterogeneity of the initial
87Sr/
86Sr ratio within a single magma. The CHIME monazite dating has great chronological potential for the analysis of the detailed sequence of geologic events. The reviewed examples of the CHIME geochronology include (1) the electron microprobe observations of Pb diffusion in metamorphosed detrital monazites from high-grade Ryoke paragneisses, (2) the relationship between the Hikami Granite and Siluro-Devonian clastic rocks in the South Kitakani terrane, and (3) the denudation history of the high T/P Ryoke metamorphic belt.
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