With the increasingly more widespread use of low-m (m=238U/204Pb) minerals in isotopic dating, the isotopic composition of the initially incorporated Pb should be given a much higher attention, as the correction of initial Pb affects the age. Without knowledge of the isotopic composition of initial Pb, it is not possible to get accurate ages from low- m minerals. In magmatic systems, the initial Pb isotopic composition can be estimated from cogenetic minerals with m = 0. The initial Pb isotopic composition of metamorphic minerals, however, is more elusive as there is no initial homogenization among the metamorphic minerals, which already should be obvious from the sequential nature of minerals reactions preserved in the rock and widely used to constrain the pressure-temperature evolution of the metamorphic rock. Actually besides Pb, it is quite likely that Nd and Hf do not homogenize either.
The key question in not whether, the initial isotopic composition of the daughter element of a geochronometer is homogeneous, but whether its heterogeneity is sufficiently large to affect the age. I have documented such initial heterogeneity and its effect on isotopic dating. Understanding how to distinguish systems where the initial disturbance does not affect the age from those where the age will be off the mark eventually contributes to avoid dating unsuitable samples in the first hand.
Romer, R.L. and Rötzler, J. (2011) The role of element distribution for the isotopic dating of metamorphic minerals. Eur. J. Mineral., 23: 17-23.
Romer, R.L. and Yilin Xiao (2005) Initial Pb-Sr(-Nd) isotopic heterogeneity in a single allanite-epidote crystal: implications of reaction history for the dating of minerals with low parent-to-daughter ratios. Contrib. Mineral. Petrol., 148: 662-674.
Romer, R.L. and Siegesmund (2003) Why allanite may swindle about its true age. Contrib. Mineral. Petrol., 146: 297-307.
Romer, R.L. and Rötzler, J. (2003)Effect of metamorphic reaction history on the U-Pb dating of titanite. In: Vance, D., Müller, A., and Villa I. (eds.) Geochronology: Linking the isotopic record with petrology and textures. Geol. Soc. London, Spec. Publ., 220: 147-158.
Romer, R.L. (2001) Lead incorporation during crystal growth and the misinterpretation of geochronological data from low-238U/204Pb metamorphic mineral. Terra Nova, 13: 258-263.
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