Isochron Discordances and the Role of Inheritance and Mixing of Radioisotopes in the Mantle and Crust
by Andrew A. Snelling, Ph.D.
RATE II: Radioisotopes and the Age of The Earth: Results of a Young-Earth Creationist Research Initiative, (Volume II), L. Vardiman et al., eds. (San Diego, CA: Institute for Creation Research and the Creation Research Society, 2005)
New radioisotope data were obtained for ten rock units spanning the geologic record from the recent to the early Precambrian, five of these rock units being in the Grand Canyon area. All but one of these rock units were derived from basaltic magmas generated in the mantle. The objective was to test the reliability of the model and isochron “age” dating methods using the K-Ar, Rb-Sr, Sm-Nd, and Pb-Pb radioisotope systems. The isochron “ages” for these rock units consistently indicated that the α-decaying radioisotopes (238U, 235U, and 147Sm) yield older “ages” than the β-decaying radioisotopes (40K, 87Rb). Marked discordances were found among the isochron “ages” yielded by these radioisotope systems, particularly for the seven Precambrian rock units studied. Also, the longer the half-life of the α- or β-decaying radioisotope, and/or the heavier the atomic weight of the parent radioisotope, the greater was the isochron “age” it yielded relative to the other α- or β-decaying radioisotopes respectively. It was concluded that because each of these radioisotope systems was dating the same geologic event for each rock unit, the only way this systematic isochron discordance could be reconciled would be if the decay of the parent radioisotopes had been accelerated at different rates at some time or times in the past, the α-decayers having been accelerated more than the β-decayers. However, a further complication to this pattern is that the radioisotope endowments of the mantle sources of basaltic magmas can sometimes be inherited by the magmas without resetting of the radioisotope “clocks” during ascent, intrusion, and extrusion in the earth’s crust. This is particularly evident in recent or young rocks. Some evidence of opensystem behavior was also found, and crustal contamination of some of the rock units was evident from their isotope geochemistry. Nevertheless, the overall systematic trend of radioisotope “ages” in the rock units according to their relative positions in the geologic record confirms that accelerated radioisotope decay was the dominant factor operating through earth history, with inheritance and mixing of radioisotopes from the mantle and crust as contributing factors in producing anomalous “ages.” Within the Biblical framework of earth history, it is thus postulated that accelerated radioisotope decay accompanied the catastrophic geologic processes operating early in the Creation week and during the subsequent global Flood. Furthermore, because each of the three assumptions of conventional radioisotope dating—known initial conditions, closed-system behavior, and constancy of decay rates—have been clearly shown to be subject to failure, the radioisotope methods cannot, and should not, be relied upon to produce absolute “ages” for the earth’s rock strata.
radioisotope dating, isochron ages, global Flood, decay rates, RATE II
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