Other
Scientific paper
Oct 1977
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977gecoa..41.1457l&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 41, Issue 10, pp.1457-1471
Other
9
Scientific paper
Stepwise Pb-removal experiments, using both vacuum volatilization and HF-leaching techniques, on acid-washed K-feldspar concentrates from Precambrian igneous rocks show that all contain some unsupported radiogenic Pb. Two types of radiogenic Pb were recognized. One has a "normal" isotopic composition, with relative abundances of 206 Pb, 207 Pb and 208 Pb consistent with the age and U-Th contents of the rocks. The other type of unsupported radiogenic lead in the feldspars is apparently pure 206 Pb, derived from long-term migration and accumulation of radioactive daughter(s) of 238 U. This "pure 206 pb" lead occupies different sites from the "normal" radiogenic lead, and tends to show a release maximum during vacuum volatilization at about 1150°C. The usefulness of stepwise vacuum volatilization may be limited by the tendency of a small amount of radiogenic lead to concentrate in the least volatile fraction. Stepwise partial HF attacks appear to give at least as good separation of radiogenic from original feldspar lead, and are recommended as a routine procedure for isotopic analyses of Precambrian feldspars. The fact that most of the five K-feldspars examined contained unsupported radiogenic lead implies that caution must be used in applying total-sample lead analyses of Precambrian feldspars to problems of lead-isotope evolution in crustal rocks.
Ludwig Kenneth R.
Silver Leon T.
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