Radioisotopes in the Diabase Sill (Upper Precambrian) at Bass Rapids, Grand Canyon, Arizona: An Application and Test of the Isochron Dating Method

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by Andrew A. Snelling, Ph.D., Steven A. Austin, Ph.D., and William A. Hoesch, M.S.

Presented at the Fifth International Conference on Creationism, Pittsburgh, Pennsylvania, August 4–9, 2003. Published in: Proceedings of the Fifth International Conference on Creationism, R. L. Ivey (Ed.), pp. 269–284, 2003.

© 2003 Creation Science Fellowship, Inc., Pittsburgh, PA, USA. Published with permission. All rights reserved.


The five-point Rb-Sr whole-rock isochron age of 1.07 Ga for the diabase sill at Bass Rapids, Grand Canyon, has been regarded for 20 years as an excellent example of the application of conventional radioisotopic dating. Initial thorough isotopic mixing within the sill is ideal for yielding concordant whole-rock isochron and mineral isochron ages. However, our new K-Ar, Rb-Sr, Sm-Nd, and Pb-Pb radioisotope data from 11 whole-rock samples (eight diabase, three granophyre) and six mineral phases separated from one of the whole-rock diabase samples yield discordant whole-rock and mineral isochron “ages.” These isochron “ages” range from 841.5±164 Ma (whole-rock K-Ar) to 1375±170 Ma (mineral Sm-Nd). Although significant discordance exists between the K-Ar, Rb-Sr, Sm-Nd, and Pb-Pb radioisotope methods, each method appears to yield concordant “ages” internally between whole rocks and minerals. Internal concordance is best illustrated by the Rb-Sr whole rock and mineral isochron “ages” of 1055±46 Ma and 1059±48 Ma, respectively. It is therefore argued that only changing radioisotope decay rates in the past could account for these discordant isochron “ages” for the same geologic event. Furthermore, these data are consistent with alpha decay having been accelerated more than beta decay, and with the longer the present halflife the greater being the acceleration factor.


Diabase, Sill, Grand Canyon, Potassium-Argon, Rubidium-Strontium, Samarium-Neodymium, Lead-Lead, Radioisotopic Dating, Model Ages, Whole-Rock Isochron Ages, Mineral Isochron Ages, Discordance, Decay Constants, Accelerated Decay

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