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igneous and metamorphic rocks with zircon, baddeleyite, perovskite, monazite, titanite, rutile, xenotime, pitchblende, thorite, and thorianite; whole rock carbonates; single-mineral grains from sediments Thomas Edvard Krogh - Director, Geochronology Laboratory, Royal Ontario Museum, Toronto. Coeditor of Proceedings of the Sixth International Conference on Radiocarbon and Tritium Dating. Olson - Emeritus Professor of Geology, Whitworth College, Spokane, Washington.The fixed ratio of lead-204 to the primordial amounts of the other lead isotopes may be used as the baseline to estimate the extra amounts of radiogenic lead present in rocks as a result of decay from uranium and thorium. The longest-lived radioisotopes are The relative abundances of the four stable isotopes are approximately 1.5%, 24%, 22%, and 52.5%, combining to give a standard atomic weight (abundance-weighted average of the stable isotopes) of 207.2(1).