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Abstract
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 wholerock isochron and mineral isochron ages. However, our new K-Ar, Rb-Sr, Sm-Nd and Pb-Pb radioisotope data from eleven 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 half-life the greater being the acceleration factor.