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+--Forum: Young-Earth Antievolution
+---Topic: attacking radometric dating started by Taz
Posted by: Taz on July 07 2006,17:21
No references provided, so does anyone know where these data were found?
How Old Are the Cardenas Basalt Lavas?
[from “The Fallacies of Radioactive Dating of Rocks: Basalt Lava Flows in Grand Canyon” by Andrew A. Snelling, Ph.D., in the premier issue of ANSWERS magazine]
These radioactive dating methods have been used to calculate an absolute age of 1,103±66 million years for the Cardenas Basalt lavas. (The number after the ± symbol refers to the error margins in the “age” determination so that 1,103±66 million years means that the age is between 1,037 and 1,169 million years.) So it would seem! However, a closer examination of the results from all such studies reveals the fallacies of the radioactive dating methods.
The claimed age of 1,103±66 million years was obtained using the rubidium-strontium isochron method with 10 samples and has been regarded as the best radioactive dating result for any Grand Canyon rock unit. Nevertheless, potassium-argon model “ages” for each of 15 individual Cardenas Basalt samples range from 577±12 to 1,013±37 million years, while the potassium-argon isochron “age” obtained using 14 samples is only 516±30 million years. This is less than half the rubidium-strontium isochron “age” of 1,111±81 million years obtained using 19 samples. It is also less than the claimed Cambrian age of the Tapeats Sandstone that sits on top of, and well above the Cardenas Basalt lavas (Figure 4). Worse still, the samarium-neodymium isochron “age” obtained using 8 samples is 1,588±170 million years—more than three times the potassium-argon isochron “age” of 516±30 million years!
So what is the correct “age” of the Cardenas Basalt lavas?
• (a) 516±30 million years (the potassium-argon isochron age)
• (b) 1,111±81 million years (the rubidium-strontium isochron age)
• © 1,588±170 million years (the samarium-neodymium isochron age)
• (d) None of the above
How can we know for sure which is the correct age when there is no independent cross-check?
< http://www.answersingenesis.org/articles/am/v1/n1/radioactive-dating >
Posted by: creeky belly on July 09 2006,19:55
< Isochron analysis of Rb-Sr, Ar-Kr >
About halfway down the page:
Larson, et al. (1994, p. 266-267) found a correlation between the K-Ar dates and weight percent K2O in the samples. Samples with anomalously high K2O values are associated with younger dates so they proposed a perfectly reasonable (and testable! explanation for the bad K-Ar dates from the Cardenas Basalt:
The explanation that seems most consistent with the data is that the progressive decrease in the dates is the result of increased loss of Ar associated with preferential burial alteration of those flows containing the higher contents of K2O. The more felsic the flow, the greater its viscosity, and the greater the content of mesostasis material containing large quantities of K2O and, therefore, the greater the likelihood of Ar loss during burial metamorphism.
Therefore it's clear that Woodmorappe misquoted McKee and Noble and was very selective in the presentation of data to support his claims. A full examination of the data shows the reliability of the Rb-Sr method for dating the Cardenas Basalt and a testable explanation for the argon loss and unsuitability of the Cardenas for K-Ar dating methods.
This publication has more direct answers to the geomorphology questions, and why the date analysis is difficult, but much closer to the middle answer of 1100 Ma:
"Tectonic inferences from the ca. 1255–1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis" J. Michael Timmons, et al
Geological Society of America Bulletin, vol.117, no.11-12, pp.1573-1595, Dec 2005