However, the radioisotope dating of these Grand Canyon rocks clearly demonstrates that the disagreement, or isochron discordance, is pronounced.
Even when the calculated error margins are taken into account the different radioisotope dating methods yield completely different "ages" that cannot be reconciled—1240±84 Ma (Rb-Sr), 1655±40 Ma (Sm-Nd), and 1883±53 Ma (Pb-Pb) (see diagram).
The best isochron plots, where all the variation from the line of best fit to the data incorporates all the analytical errors, yielded an Rb-Sr isochron age of 1240±84 Ma, an Sm-Nd isochron age of 1655±40 Ma, and a Pb-Pb isochron age of 1883±53 Ma.
Most people believe that when the different radioisotope dating methods are used on the same rock unit they all yield the same age.
Results obtained usually signify the "date" of the metamorphism, but they may also yield the "age" of the original volcanic (or sedimentary) rock.
The "age" or "date" is calculated from the amount of the daughter isotope produced by radioactive decay of the parent isotope.
In Grand Canyon, the "date" of metamorphism of the basalt lavas to form these Brahma amphibolites has been determined as 1690-1710 Ma (million years ago), based on U-Pb dating of minerals in the overlying Vishnu Schist and underlying Rama Schist that formed during the metamorphism.
It is also claimed that the original basalt lavas were erupted between 17 Ma, based on U-Pb dating of "original" zircon grains in metamorphosed felsic (granitic) volcanic layers within the Brahma and Rama Schists.
Deep inside the Inner Gorge of Grand Canyon, northern Arizona, are the crystalline basement rocks that probably date back even to the Creation Week itself.
This includes two samples only 0.84 meters apart that yielded K-Ar model ages of 1205.3±.2±73 Ma.
The computer program Isoplot was used to plot isochrons and calculate isochron ages from the other radioisotope analyses.
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