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Friday, April 14, 2006

Bad Answers in Genesis?

The following feedback was posted at AIG (Answers in Genesis) today

Feedback


The publication defends an article by Andrew Snelling that appeared in their 'journal' Creation.

Article

I had not seen this article before, so I went and had a look. The main premise is that Snelling collected samples of amphibolite from the Grand Canyon and found (gasp) many different ages using different isotopic systems. The 'data' are shown here:



Intrigued, as always, I went to dig a little deeper into the methods used. Generally speaking, one of the best dating methods for an amphibolite metamorphic rock would be to separate out amphiboles for Ar-Ar dating. Using the argon-argon variant of K-Ar allows many cross checks for excess argon or argon loss that are invisible to standard K-Ar dating. At the very least, a better approach would be to date the rocks using individual minerals. The worst possible method would be to measure the age on an inhomogeneous whole rock sample. In the footnotes, the methods are described as follows:

'Whole rock? samples were analyzed in all cases?K-Ar at Activation Laboratories, Ancaster, Ontario, Canada; Rb-Sr, Sm-Nd and Pb-Pb at the PRISE Laboratory, Research School of Earth Sciences, Australian National University, Canberra, Australia.

He then goes on to say that these results (all millions of years) give evidence for accelerated decay. This is a complete non-sequiter. They give evidence of sloppy methodology on the part of Snelling. Nothing more, nothing less. In fact, it gets even worse. In the introduction, Snelling makes the following statement:

Based on radiometric dating, long-age geologists say that the basalt lavas erupted 1,745 million years ago and were metamorphosed some 1,700 million years ago.

The references cited for this are the following:

1741-1750 million years (Ma) based on U-Pb (uranium-lead) ?dating? of ?original? zircon grains in metamorphosed felsic (granitic) volcanic rock layers within the Brahma and Rama Schists. See Ilg, B.R., Karlstrom, K.E., Hawkins, D.P. and Williams, M.L., Tectonic evolution of Paleoproterozoic rocks of Grand Canyon: Insights into middle-crustal processes, Geological Society of America Bulletin 108:1149?1166, 1996, and Hawkins, D.P., Bowring, S.A., Ilg, B.R., Karlstrom, K.E. and Williams, M.L., U-Pb geochronologic constraints on the Paleoproterozoic crustal evolution of the Upper Granite Gorge, Grand Canyon, Arizona, Geological Society of America Bulletin 108:1167?1181, 1996. 1690?1710 Ma based on U-Pb ?dating? of minerals (monazite, xenotime and titanite) that formed in the overlying Vishnu Schist and underlying Rama Schist during the metamorphism. See Hawkins et al., ref. 4, and Hawkins, D.P. and Bowring, S.A., U-Pb monazite, xenotime, and titanite geochronological constraints on the prograde to post-peak metamorphic thermal history of Paleoproterozoic migmatites from Grand Canyon, Arizona, Contributions to Mineralogy and Petrology 134:150?169, 1999.

Note that these authors dated individual minerals that have different closure temperatures to estimate conditions and timing of metamorphism. In fact, the ages are perfectly in line with their expected age progression (i.e. the zircons are older than the metamorphic minerals). Here we have a case of geochronology done right supplanted by geochronology done wrong (if you believe Snelling).

In fact, the abstract by Hawkins and Bowring (1999) gives a wonderful illustration of the power of geochronology (done right) in interpreting metamorphic history:

The petrology and U-Pb geochronology of pelitic migmatite and calc-silicate gneiss reveal a detailed prograde to post-peak metamorphic thermal history for a single outcrop of Paleoproterozoic supracrustal rocks in the eastern part of the Grand Canyon. Metamorphic monazite from paleosomal pelitic schist grew on the prograde path beginning at about 1708 Ma and continued to grow until about 1697 Ma. The U-Pb dates for magmatic xenotime and monazite from peraluminous granite and pegmatite leucosomes indicate that partial melting, which involved the breakdown of muscovite to sillimanite, commenced at about 1702 Ma, prior to the metamorphic peak. Partial melting continued until about 1690 Ma, the youngest U-Pb date from magmatic monazite in the leucosomes. Field and petrographic evidence, as well as inheritance patterns in monazites from the leucosomes, suggest that some of the leucosomes appear to represent in situ partial melts that did not escape the source region. Between 1702 and 1690 Ma, the migmatite package heated to peak metamorphic conditions of about 720 °C and 6 kbar, cooled to about 675 °C at a cooling rate >30 °C/million years, and decompressed to about 4 kbar. The U-Pb geochronological data for metamorphic titanite from a calc-silicate gneiss exhibit a clear relationship between grain size and the 207Pb/206Pb date indicating that the titanite crystals record cooling ages. These data, combined with the titanite Pb diffusion data of Cherniak (1993), yield a cooling rate of 5.4m0.9+1.7 °C/million years, integrated over the interval 1690 to 1676 Ma and suggest that by 1675 Ma, the cooling rate slowed to less than 2 °C/million years. The rapid decompression during the peak of metamorphism and the change in cooling rate immediately following peak metamorphism are interpreted to reflect large-scale tectonic processes associated with the accretion of juvenile crust to the margin of Laurentia. Juvenile arc crust appears to have been assembled, accreted and stabilized into Laurentian lithosphere in less than 30 million years.

In short, this is a classic example of geochronology done poorly. Snelling either deliberately chose the worst rocks and the worst methods in order to guarantee strange results or was grossly incompetent. Well, ok maybe both. I also notice that the data are not given in the creation article. If someone has a more detailed description of the results, it'd be fun to look at the individual data points more carefully. Lastly, Snelling implicitly suggests that if geochronology is not correct every single time, then it can never be trusted. I wonder if he ever gets on a airplane?

Cheers
Joe Meert

5 Comments:

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At 6:36 PM, Blogger Mike Haubrich said...

I just didnt want your comment to only be spam. I came over from Pharyngula and appreciate your discussion of Snelling's methodology.

 
At 7:35 PM, Blogger tfk said...

Gee Mike,

You might be jumping to conclusions about that first response.
Perhaps Joe has stumbled on the single most convoluted way in web history for a rockhound to get a date. Maybe he did all this work JUST to get this very message from his CLOSE PERSONAL friend, "anonymous".

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PS. Good luck on the dating scene...

 
At 11:51 PM, Anonymous Dahan said...

Great article and good luck with the hotties!

 
At 8:13 AM, Anonymous Abie said...

Thanks for dissecting this paper : general bogus claims are easily debunked even by laymen, but crucial and precise questions like the choice of a atation method can only be explained by specialists...

 

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