Great Dyke

Snelling (2009) Misrepresents Radiometric Dates of the Great Dyke of Zimbabwe, Africa

Kevin R. Henke, Ph.D., December 28, 2019

Young-Earth creationists (YECs) are infamous for misquoting references and leaving out critical information in the references that they use that actually refute their arguments (e.g., Oard 2009a; 2009b; https://sites.google.com/site/respondingtocreationism/home/documents ; Woodmorappe 1999; https://www.noanswersingenesis.org.au/henke_on_woody.htm ). As just one example of several in his chapter in Oard and Reed (2009), YEC Snelling (2009, p. 189) fails to mention some critical information in Mukasa et al. (1998) on the radiometric dating of the Great Dyke of Zimbabwe, Africa.

Snelling (2009, p. 189) mentions three consistent and independent Rb-Sr dates of 2,477 +/- 90, 2,455 +/-16, and 2,467 +/- 85 million years old for the Great Dyke. Other dates for the dike are somewhat older and include: a Sm-Nd whole rock result of 2,586 +/- 16 million years, a Pb-Pb mineral and whole rock date of 2,596 +/- 14 million years old, and U-Pb date on the mineral rutile of 2,587 +/- 8 million years. Snelling (2009, p. 189) cites Mukasa et al. (1998) and claims that the younger Rb-Sr dates were dismissed as resulting from alteration from a hydrothermal event even though Dr. Snelling claims that there were no observational or other geochemical evidence to support such an event. Snelling (2009, p. 189) states:

“Thus, even though three independent studies all yielded an identical Rb-Sr age for this intrusion, it was rejected in favor of the approximately 120 m.y. older, identical radiometric age obtained by the Sm-Nd, Pb-Pb, and U-Pb methods. The reason given for rejecting the younger Rb-Sr age was that hydrothermal alteration must have reset the Rb-Sr system, in spite of the fact that there is no observational evidence, or any other geochemical indicators, of such hydrothermal alteration!” [my emphasis]

A difference of 120 million years seems immense until it is realized that it’s less than 5% for these ancient rocks.

Even though they were careful to select the least altered samples, Mukasa et al. (1998, p. 357) notes that the rocks of the Great Dyke show “strong alteration in general.” Specifically, the feldspars are the least altered minerals in the dike, but still show some alteration. Pyroxenites show “pervasive and intense alteration” even at depths up to 400 meters (Mukasa et al. 1998, p. 357). So, why does Snelling (2009, p. 189) fail to mention this critical observational evidence of alteration? Clearly, the mineralogy of the Great Dyke was altered by a later event, which explains the post-emplacement resetting of the Rb-Sr “clock.” While mafic melts tend to be emplaced at temperatures of 1000 to 1200C (Hall 1996, p. 29), the Rb-Sr "clock" does not start for most minerals until temperatures drop below about 400C; i.e., the Rb-Sr closure temperature (Nebel and Mezger 2008, p. 227). After considering the possibility of slow magmatic cooling, low-grade metamorphism, and ion-exchange involving hydrothermal fluids, Mukasa et al. (1998, pp. 363, 366) argue that later hydrothermal processes best explain why the Rb-Sr dates are somewhat younger than the Sm-Nd, Pb-Pb and U-Pb dates.

In a more recent study of this dike that is not mentioned in Snelling (2009), Nebel and Mezger (2008) obtained a more precise Rb-Sr date of 2,543.0 +/- 4.4 million years for the Great Dyke after carefully handling and cleaning their samples. Nebel and Mezger (2008, p. 231) conclude:

“Therefore, it is demonstrated for the first time that if carefully handled, the Rb–Sr isotope system can be applied to high-precision dating of mafic intrusions as old as the Archean, even where a rock experienced weak isotope disturbance on a whole rock scale, e.g., associated with fluid infiltration.”

Nebel and Mezger (2008, p. 231) argue that the hydrothermal event that produced their Rb-Sr date of 2543.0 +/- 4.4 million years occurred about 30 million years after the emplacement of the dike, which Nebel and Mezger (2008, p. 231) date at 2,575 million years. When more recent evidence and even the references that he uses are fully and carefully examined, Dr. Snelling’s accusations against radiometric dating fall apart.

References

Hall, A, 1996. Igneous Petrology, 2nd ed., Addison Wesley Longman Ltd, Essex, UK, 551pp.

Mukasa, S.B., A.H. Wilson, and R.W. Carlson. 1998. A Multi-element Geochronologic Study of the Great Dyke, Zimbabwe: Significance of the Robust and Reset Ages. Earth and Planetary Science Letters, v. 164, pp. 353-369.

Nebel, O. and K. Mezger. 2008. Timing of Thermal Stabilization of the Zimbabwe Craton Deduced from High-Precision Rb-Sr Chronology, Great Dyke. Precambrian Research, v. 164, pp. 227-232.

Oard, M.J. 2009a. Landslides Win in a Landslide over Ancient 'Ice Ages', chapter 7 in M.J. Oard and J.K. Reed (editors). 2009. Rock Solid Answers: The Biblical Truth Behind 14 Geological Questions, Master Books: Green Forest, AR, pp. 111-123.

Oard, M.J. 2009b. Do Varves Contradict Biblical History?, chapter 8 in M.J. Oard and J.K. Reed (editors). 2009. Rock Solid Answers: The Biblical Truth Behind 14 Geological Questions, Master Books: Green Forest, AR, pp. 125-148.

Snelling, A. A. 2009. “Radiometric Dating: Challenging an Icon of Evolutionary Geology, “ chapter 11 in M.J. Oard and J.K. Reed (editors). 2009. Rock Solid Answers: The Biblical Truth Behind 14 Geological Questions, Master Books: Green Forest, AR, pp. 185-206.

Woodmorappe, J., 1999, The Mythology of Modern Dating Methods, Institute for Creation Research, El Cajon, CA, 118 pp.