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Lead-isotope inhomogeneity in Precambrian igneous K-feldspars

Ludwig, Kenneth R. and Silver, Leon T. (1977) Lead-isotope inhomogeneity in Precambrian igneous K-feldspars. Geochimica et Cosmochimica Acta, 41 (10). pp. 1457-1471. ISSN 0016-7037. doi:10.1016/0016-7037(77)90251-4.

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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.

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Additional Information:© 1977 Pergamon Press. Received 8 February 1977; accepted in revised form 21 June 1977. The study was part of a Ph.D. thesis by Ludwig, and was supported by N.S.F. grants GA 15989 and EAR 74-00155A01.
Funding AgencyGrant Number
NSFGA 15989
NSFEAR 74-00155A01
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Caltech Division of Geological Sciences2800
Issue or Number:10
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Official Citation:Kenneth R. Ludwig, Leon T. Silver, Lead-isotope inhomogeneity in Precambrian igneous K-feldspars, Geochimica et Cosmochimica Acta, Volume 41, Issue 10, October 1977, Pages 1457-1471, ISSN 0016-7037,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42395
Deposited By: Ruth Sustaita
Deposited On:12 Nov 2013 22:12
Last Modified:21 Jul 2022 15:16

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