Dating gl basses
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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.
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.
Keywords: 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 This paper was originally published in the Proceedings of the Fifth International Conference on Creationism, pp.
269–284 (2003) and is reproduced here with the permission of the Creation Science Fellowship of Pittsburgh.
The 1.1-billion-year rubidium-strontium isochron date for the Cardenas Basalt is widely regarded as perhaps the best ‘age’ obtained for Grand Canyon strata.12 Similarly, the diabase sill at Bass Rapids (fig.1) has yielded a 1.07-billion-year rubidium-strontium isochron date,3 providing apparent confirmation of the relationship between the diabase sills and the Cardenas Basalt flows.The Bass Rapids diabase sill provides every indication that it was well mixed isotopically when it was intruded, even though during cooling the sill segregated mineralogically and chemically by crystal settling to produce a granophyre on top of the diabase.Such a condition of initial thorough isotopic mixing of the original magma body followed by rapid chemical segregation is suited to the assumptions of whole-rock and mineral isochron dating.How then are the radioisotope daughters distributed through the granophyre and diabase, and through the mineral phases of the latter?The various radioisotope pairs would be expected to give concordant whole-rock isochron and mineral isochron ‘ages.’ However, published potassium-argon model ‘ages’ for the diabase sills (and the Cardenas Basalt) are significantly younger than their associated rubidium-strontium isochron ‘ages’.45 Mafic igneous rocks occur as sills, dikes, and flows in the thick succession of strata making up the middle Proterozoic Unkar Group of the Grand Canyon, Arizona (fig. The Unkar Group sedimentary sequence is comprised of four formations—in ascending order, the Bass Limestone, Hakatai Shale, Shinumo Quartzite, and the Dox Sandstone—which are overlain by the 300 meter plus thick flow sequence of lava flows of the Cardenas Basalt.67 The younger Precambrian sediments of the Nankoweap Formation and the Chuar Group overlie this Unkar Group succession, which unconformably rests on the early Proterozoic metamorphic and igneous crystalline basement.813 Thus the relationship between them is obscure because the direct feeders to the flows have never been recognized among the available diabase outcrops.