Re: Isochrons
Bill Hamilton (hamilton@predator.cs.gmr.com)
Mon, 22 Apr 1996 15:28:56 -0400Steven Schimmrich quoted David Tyler:
>> The Grand Canyon lavas mentioned above are the ones studied by Steve Austin -
>> some discussion took place on this last year. Austin has obtained values of
>> 1.3 billion years for the Tertiary basalts of Grand Canyon: something for
>> which he is using to question the appropriateness of using Rb/Sr dating. The
>> important point here is that his findings are is close agreement with Leeman
>> (1974). To nit-pick at possible selective use of data seems to be an unwise
>> response to Austin. The argument of Brooks et al (1976) is that Austin has
>> found a pseusoisochron - that conveys information about the earth's mantle
>> and mechanisms of petrogenesis. At the moment, unless I can get my question
>> (previous paragraph) answered, I think the magma mixing explanation is to be
>> considered the most likely - which implies that the isochrons have
>> geochemical, but not geochronological, information.
>
>Glenn Morton wrote:
>
>> This whole thing with the bad Rb/Sr dates ignores the fact that we know
>> they are wrong because other radiometric dates give a more reasonable age.
>> The dating of the lava dam (which gives a billion years age with Rb/Sr)
>> gives 1.16 Myr with K-Ar. This date also agrees with the date derived
>> from stratigraphy. This always seems to be ignored. The bad Rb/Sr dates
>> at Grand Canyon are always spoken of as if this is the only dating method
>> applicable to them.
>>
>> One of the explanations given by Brooks et al is that some of the
>> lithosphere has become trapped below the continent accounting for the
>> problematic Rb/Sr dates. If this is correct then it is quite possible
>> that the magma melt which gave rise to the Cardenas Basalt also gave rise
>> to the late Cenozoic volcanism. This would explain the discrepancy
>> between the K/Ar date and the Rb/Sr d
>>
>> I would also like to point out that Steve is quite selective in the way he
>> handles contradictory evidence. He cites (p. 123 of his Grand
>> Canyon:Monument to Catastrophe) a 117 million year old K/Ar date on the
>> Vulcan Throne Basalt in the text but only informs the reader at the end of
>> the footnote buried at the end of the chapter that olivine is not a
>> suitable mineral for K/Ar ying that piece of info.
>>
>> He then cites the 1.2 Myr date I mention above but never discusses their
>> significance. Argon being a gas is able to mostly leave the mineral when
>> the lava erupts. It's date fits with local geologic evidence and Steve
>> simply mentions the age and then moves on. He also cites a 10,000 year
>> K/Ar date for one of the upper lavas and a 230,000 year date for another.
>>
>> Because of all this Brooks et al's explanation seems quite reasonable.
>
...
>I do have a question about Austin's work. Where has it been published?
>I'd like to see what he's written about this and the details of his study.
Several years ago Chris Stassen apparently wrote to Steve Austin asking for
his data from the Grand Canyon work. Steve sent him some, which Chris
forwarded to Brent Dalrymple. I have hard copy of Dalrymple's response.
Dalrymple questioned the validity of Austin's data and made several remarks
about practices he had apparently warned Austin about in the past --
practices that might lead to erroneous results. Austin's reaction when he
saw Dalrymple's report was quite interesting: He got upset with Chris
Stassen, grumbling that now he would "have to respond" to Dalrymple.
Bill Hamilton | Chassis & Vehicle Systems
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