The effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis: A model
- Virginia Polytechnic Institute and State Univ., Blacksburg (USA)
The initial Pb isotopic composition of anatectic granites is modelled as a mixture of Pb derived from major minerals that have characteristically low {sup 238}U/{sup 204}Pb and {sup 232}Th/{sup 204}Pb ratios, with Pb contributed from dissolved accessory minerals with characteristically high {sup 238}U/{sup 204}Pb and {sup 232}Th/{sup 204}Pb ratios. The initial Pb isotopic composition of anatectic granites is a function of (a) the age, type, modal distribution, and heterogeneity in the initial U and Th content of the accessory minerals, (b) variation in melt composition and temperature during anatexis, (c) the fraction of the source melted, and (d) the extent to which the melt was homogenized prior to crystallization. The differential behavior of accessory minerals during anatexis can explain the common yet enigmatic decoupling of the Pb isotopes from other chemical and isotopic systems in granite rocks. Granitic plutons derived by anatexis of the same source material are not required to have the same initial Pb isotopic composition. Low temperature, peraluminous granites are more likely to exhibit homogeneous initial Pb isotopic compositions dominated by the Pb contained within the major mineral assemblage of the source material. In contrast, the initial Pb isotopic composition of higher temperature, metaluminous or peralkaline anatectic granites is more likely to be heterogeneous, unless the melts are homogenized during emplacement, and will record a larger fraction of the radiogenic Pb component contained in the accessory mineral assemblage of the source material.
- OSTI ID:
- 6053463
- Journal Information:
- Geochimica et Cosmochimica Acta; (USA), Vol. 55:1; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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