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Isochron date of young Grand Canyon lava is excessively old (Talk.Origins)

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Response Article
This article (Isochron date of young Grand Canyon lava is excessively old (Talk.Origins)) is a response to a rebuttal of a creationist claim published by Talk.Origins Archive under the title Index to Creationist Claims.


Claim CD014.1:

The hawaiite lava flows from the Uinkaret Plateau of the Grand Canyon are dated by K-Ar at about 1.2 million years old, 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 old, which is older even than 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.

Source: Austin, Steven A., 1992. Excessively old "ages" for Grand Canyon lava flows. Impact 224 (Feb.).


CreationWiki response: (Talk.Origins quotes in blue)

1. 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.

There seems to be no basis for Talk Origins' claim here. Austin makes it clear that he chose samples that should have been isotopically homogeneous.

2. 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.

This assumes that Talk Origins' claim #1 is correct, but there is empirical evidence that contradicts that claim.

Ages of Uinkaret Plateau Rocks in Millions of Years.

K-Ar Rb-Sr Rb-Sr Isochron Pb-Pb Isochron
0.01 1230 - 1310 1300 - 1380 2390 - 2810
1.0 - 1.4 1260 - 1380
2.63 1310 - 1370
3.6 1320 - 1440
3.67 1360 - 1420

Data from: Radioactive Dating Explained - Part 2

The K-Ar and straight Rb-Sr dates strongly disagree with each other. Yet, the straight Rb-Sr dates have a good degree of agreement with each other and the Rb-Sr Isochron, showing that the Isochron is not a result of non-cogenetic samples as Talk Origins claims. This is further supported by the fact that the Pb-Pb Isochron disagrees with the Rb-Sr Isochron date by a billion years.

3. 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.

The fact only works against Talk Origins' claim that the samples were non-cogenetic. Adding this to the additional dating data above show that Talk Origins' claim is wrong and that the samples were cogenetic

His misstatement of the significance of the isochron is just plain sloppy.

It seems more likely that it is Talk Origins that is doing the misstating here.

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