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Abstract
The Brahma amphibolites of the Precambrian crystalline basement of Grand Canyon were originally erupted as basalt lavas and subsequently suffered high-grade regional metamorphism. Composed predominantly of hornblende with minor subordinate plagioclase, the collected samples showed no signs of post-metamorphic alteration. K-Ar radioisotope analyses yielded a wide range of model ages, even for adjacent samples from the same outcrop of the same original lava flow. No statistically viable K-Ar isochron age could be obtained because of so much scatter in the data, which is most likely due to 40Ar* mobility within these rocks. By contrast, the Rb-Sr, Sm-Nd, and Pb-Pb radioisotope systems yielded good, statistically consistent, isochron ages of 1240 ± 84 Ma, 1655 ± 40 Ma, and 1883 ± 53 Ma, respectively. These are obviously discordant with one another and with published ages, but there are no clear reasons to reject any of them as unreliable or invalid. One explanation for the discordance is that the decay rates of the parent radioisotopes were different relative to their presently measured rates at some time during the time interval since these rocks formed. We observe that the α-decaying U and Sm yield older ages than the β-decaying Rb, and the heavier atomic weight U yields a PbPb age older than the Sm-Nd age. This pattern in the discordances thus may provide clues into the physics responsible for time variations in the decay process. Obviously, if decay rates have not been constant, radioisotope decay methods do not yield valid absolute ages for rocks.