>> This is a comment on a post of Glen Morton (14 Sept) in which he
>> critiques the Rb/Sr dating of Grand Canyon rocks by Steve Austin.
>> He suggest's that Steve's methodology is flawed, because whole
>> rock samples were used rather than specific minerals within a
>> rock. I do not want to discuss Steve Austin's interpretations,
>> but I am mystified by Glen's remarks about methodology. This
>> post explains why.
>> GM: " ... Steve has taken several rocks and used them to
>> construct an isochron. This is not the way the method works.
>> Using multiple rocks creates problems. You are supposed to create
>> your isochron by using several minerals in the same rock."
>> ..... Steve created his date by using several rocks. Steve
>> violates the assumptions of the method."
>> From all this I conclude that Steve Austin is following standard
>> practice in his Rb/Sr work. There may be objections to the
>> interpretations placed on the data, but the data itself should
>> be allowed to stand. Furthermore, if the objection to Steve
>> Austin's methodology were to be correct, this would cast doubt
>> on all "whole rock" isochrons - and the majority of Rb/Sr dates
>> should be discarded.
I think Glenn got it slightly wrong. Using multiple rocks is fine, *if*
you have good reason to believe that they were formed at the same time.
A geologist should normally be able to determine if that is the case.
Austin is apparently using rocks that are "demonstrably not cogenetic"
(the words of Brent Dalrymple, a radio-dating expert). That violates
the assumptions of isochron dating, and means that any resulting date is
meaningless.
Using non-cogenetic rocks should cause data points not to fit on a line.
But Austin apparently was able to get a line *by throwing away points that
did not fit on it*. That is also a violation of the way isochron dating
should be done.
-- Jim Foley Symbios Logic, Fort CollinsJim.Foley@symbios.com (303) 223-5100 x9765* 1st 1.11 #4955 * "I am Homer of Borg! Prepare to be...OOooooo! Donuts!!!"