At 12:01 PM 8/22/01 -0700, you wrote:
Cases in point.
1: I know you are aware of the Uinkaret Plateu lava flows which flowed down
into Grand Canyon. These have been dated by a Pb-Pb isochron age in the
neighborhood of 2.6 billion years and by a Rb-Sr isochron date somewhere
around 1.34 billion years. Everyone rejects these computed ages because it
is obvious that the lava cannot be that old. They must be younger than
Grand Canyon which is variously dated between 1 and 5 million years. An
assortment of reasons concerning the "original abundances of isotopes" have
been proposed to try to explain the discrepancy, but the most important
point is that the dates are rejected because it is "known" that the lava
cannot be that old. The rocks are assumed to be young therefore computed
dates which indicate vast ages are rejected.
2: Also in Grand Canyon are the Cardenas Basalts which are dated by a Rb-Sr
isochron near 1.07 billion years. Because the Cardenas Basalts are among the
oldest exposed rocks in Grand Canyon everyone assumes that the Cardenas
Basalt's are old enough for the computed dates to be accepted as valid.
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