Mathematics – Logic
Scientific paper
Jun 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.2467k&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 61, Issue 12, pp.2467-2474
Mathematics
Logic
6
Scientific paper
Comparison of results from the isotope dilution and the single-grain zircon evaporation methods on highly discordant samples shows that the evaporation method can yield accurate crystallization ages on some zircon populations that display complex discordant patterns. Three granite gneiss samples from the Western Gneiss Region, Norway, which crystallized at approximately 1660 Ma and experienced profound lead loss in zircon during Caledonian metamorphism at 400 Ma, were analyzed by both the isotope dilution (Tucker et al., 1990) and evaporation methods. 207 Ph/ 206 Pb evaporation ages on one sample agree with the isotope dilution crystallization age. Two other samples produced a range of anomalously young 207 pb/ 206 Pb evaporation ages. Analyses at the highest evaporation temperatures from one of these two samples were consistent with crystallization ages determined by the isotope dilution method, but it would be difficult to interpret them correctly without independent age information. The anomalously young results are generally easy to recognize by the pattern of increasing 207 Ph/ 206 Ph ages with increasing evaporation temperature. A more serious problem, however, is that some grains produced 207 Ph/ 206 Pb age plateaus over a range of increasing evaporation temperatures that cannot be correlated with any documented geologic event in the region. The evaporation analyses yielding the youngest 207 Pb/ 206 Pb ages also display the most common lead, suggesting that common lead may diffuse into the zircon grains during loss of radiogenic lead. A pegmatite from Massachusetts, USA, contains zircons with visible xenocrystic cores. The 207 Ph/ 206 Pb evaporation ages at lower temperatures agree well with the 299 Ma crystallization age determined by the isotope dilution method on the zircon rims (R. D. Tucker, unpubl. data). At higher evaporation temperatures, a range of older 207 Pb/ 206 Pb ages are produced by mixing Pb evaporated from both the zircon rims and xenocrystic cores. Large changes in the 206 Pb/ 208 Pb ratio accompany the increase in 207 Pb/ 206 Pb ages and indicate mixing of two distinct zircon domains. Thus, in addition to providing useful 207 pb/ 206 Pb age information, the evaporation method may help distinguish between the discordance produced by diffusional lead loss and the mixing of two domains of differing age.
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