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525946 
Journal Article 
CHIME dating of monazite, xenotime, zircon and polycrase: Protocol, pitfalls and chemical criterion of possibly discordant age data 
Suzuki, K; Kato, T 
2008 
Yes 
Gondwana Research
ISSN: 1342-937X 
14 
4 
569-586 
English 
This paper outlines the CHIME (chemical Th-U-total Pb isochron method) dating method, which is based on precise electron microprobe analyses of Th, U and Pb in Th- and U-bearing accessory minerals Such as monazite, xenotime, zircon and polycrase. The age-mapping technique that is applicable to young monazite and zircon is also described. CHIME dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation, and then these data are used to construct a "pseudo-isochron" from which all age can be obtained via regression. This method, when coupled with discrimination of possibly concordant age data by chemical criteria such as the (Ca + Si)/(Th + U + Pb + S) ratio for monazite and Ca and S contents for zircon, has the potential advantage of significant precision, and the ability to work with minerals that have a significant initial common Pb component. This technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis. Many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data. Critical factors include the roles of collimator slit, detector gas, background estimation, accelerating voltage, probe current, Xray interferences and count rate in affecting the accuracy, and a way to apply the Th and U interference correction without pure Th- and U-oxides or synthesized pure ThSiO4. The age-mapping procedure for young monazite and zircon includes acquiring PbM alpha (or PbM beta) intensity of individual pixels with multiple spectrometers, correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement, calibrating of intensity with standards and calculating of ages from the Th, U and Pb concentrations. This technique provides age maps that show differences in age domains on the order of 20 Ma with in monazite as young as 100 Ma. The effect of sample damage by irradiation of intense and prolonged probe measurement is also described. (C) 2008 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 
CHIME method; EPMA dating; Age mapping; Monazite; Xenotime; Zircon; Polycrase; ultrahigh-temperature metamorphism; electron-probe microanalysis; empirical correction factors; north china craton; u-pb systems; southern india; microprobe analysis; accessory minerals; east; antarctica; precambrian provenance