The hawaiite lava flows from the Uinkaret Plateau of the Grand Canyon are
dated by potassium-argon at about 1.2 million years, an age consistent
with the lava flow being younger than the canyon itself. However, a Rb-Sr
isochron plot of samples from the lava flow gives an age of 1.34 billion
years, which is older than even the Cardenas Basalt, some of the
oldest rock in the canyon. The data points are colinear, which is
supposed to indicate a valid isochron. This result shows that the
isochron method is invalid.
One of the requirements for isochron dating is that the samples be
cogenetic, that is, that they come from materials that were
isotopically homogeneous (with respect to each other) when they formed.
Austin's selection of samples violated this assumption. His five
samples came from four different lava flows plus one phenocryst (which
likely solidified in the magma chamber before the flow). Thus,
Austin's conclusion, not the isochron method, is invalid.
Noncogenetic samples such as Austin used are sometimes used
intentionally to determine the age of the common source of the samples.
Austin's results confirm that the lithospheric mantle underlying the
Grand Canyon (the common source for his samples) is older than the
Cardenas Basalt. Geologists have known this all along.
Austin (1988) cited Brooks et al. (1976), showing Austin should have
been aware that noncogenetic samples could produce an isochron for the
age of their molten material's source. His misstatement of the
significance of the isochron is just plain sloppy.
