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Misconceptions and Confusions in U-Pb dating

A Critique of Selva Harris’s Untitled Essay on U-Pb Dating

Alec MacAndrew



Selva Harris published an essay on the web which is reproduced below claiming that U-Pb dating supports the hypothesis of a Young Earth:

http://www.geocities.com/peaceharris/u238/index.html (1)

This is a response to that extraordinary claim.

U-Pb dating uses the relative presence of parent isotopes of uranium (238U and 235U) and their daughter species of lead (206Pb and 207Pb respectively) to determine the age of crystallisation of certain minerals. 238U decays to 206Pb with a half life of 4.47 billion years and 235U decays to 207Pb with a half life of 704 million years. Together they provide two separate decay schemes to determine ages of crystallisation of minerals ranging from about 10 million years, up to and beyond the age of the earth at 4.55 billion years (2) (3)

U-Pb dating is commonly carried out on a limited set of minerals, namely zircon, quartz and apatite. Zircon is a particularly valuable mineral for this purpose for the following reasons:


Harris claims that U-Pb data indicates a young earth. In so doing he makes a number of errors of understanding, interpretation and fact. We will see by the end of this response that there is no justification for claiming that the data supports a Young Earth. On the contrary, other than Gentry, the principle sources that he references will be seen to provide compelling evidence for the date of a major bolide event at Chicxulub at 65 million years, coinciding with the date of a major extinction, which occurred geologically at the K-T (Cretaceous – Tertiary) boundary and which included the extinction of dinosaurs.

Harris’s paper is rather disjointed and I can see no better way to address its fallacies than by categorising my comments into those about the quality of the essay and the errors of fact and reasoning it contains.

The Quality of Harris’s Essay

It is a poorly referenced, poorly written document.


Errors of Fact and Reasoning

Background to the K-T transition and the Chicxulub bolide

In order to put the rest of the argument in context, I include here a very brief review of the background to Harris’s claims. Throughout the world, there is a sudden extinction of a large number of taxa, including the dinosaurs, at the geological boundary known as the K-T (Cretaceous – Tertiary) boundary. There is, world-wide, an ash layer at the boundary which has been independently dated by several different radiometric techniques to approximately 65 million years (3).

However, the cause of this mass extinction was keenly sought but not known until Alvarez et al suggested that it was caused by the impact of a huge asteroid (4) (5) . Subsequent work has identified the Chicxulub crater as being the site of the bolide event which most closely corresponds in time to the K-T boundary (6) . A great quantity of further work has confirmed the Chicxulub crater as the relevant site for the K-T extinction.

Part of that further work is what Harris takes exception to. It consists of studies of ages from U-Pb data in zircons. The ages of zircon crystals both in the breccia surrounding the Chicxulub crater and crystals found at the K-T boundary from a number of remote sites were measured.

Zircon is an extremely hard and stable crystal, which incorporates uranium on crystallisation but little lead. It is therefore ideal for U-Pb dating. Over time, radiogenic lead accumulates in the crystal providing a measure of age. The age for an undisturbed crystal, using the U/Pb ratio from two uranium-to-lead decay schemes, plots on what is known as the concordia curve (2). Lead can be driven out of the crystal under exceptional stress, such as an asteroid impact. If all the lead is removed, the crystal is totally reset and the age plots on the concordia at the age at which the impact occurred. If the lead is partly removed, the age plots on a chord of the concordia (called the discordia) which intersects the concordia at the original age of crystallisation at one end, and at the age of the impact event at the other end.

It has been shown that the physical signs of shocking are proportional to increasing shock and can be revealed by etching (7). Furthermore these signs correlate with the degree of resetting of the U-Pb clock (8) . In this paper, Krogh et al study zircons from the KT layer at a site in Colorado, and conclude that the degree of shock is proportional to the degree of resetting and that the original date of crystallisation is 550My and the date of impact is 65My.

Further work on crystals from the Chicxulub crater itself and from Haiti confirm these results (9) and link the zircons at distant sites with the crater itself, through the date of original crystallisation and the date of the metamorphic event. The data lie on a discordia, which indicates a metamorphic date of ~65Myr; indeed the degree of collinearity of data from different sites and from the crater itself is remarkable. It is a diagram and table from this paper that is the sole original source on this subject that Harris has ever consulted. Further data was subsequently published from zircons from the K-T boundary from even further away in Saskatchewan (10).

Harris claims that scientists have manipulated this extensive data by ‘throwing away’ two data points and that the evidence supports a young earth. We shall see.

What are the errors in Harris’s essay

Harris makes an number of fundamental errors of fact and reasoning in his essay. I list these below more or less in order of appearance in his document.

(For sample C5, the 238U age is 507+/- 12 Myr and the 235U age is 519Myr.

For sample C6, the 238U age is 496 +/- 12 Myr and the 235U age is 493 Myr)

Gentry realized this while dating coal and wrote, "To obtain 238U/206Pb ratios that more accurately reflect the amount of lead from in situ U decay, a search was made for sites with even higher ratios, for such areas possibly contained negligible amounts of extraneous Pb." Gentry found samples with the 238U/206Pb ratio as high as 4*104 and 6*104. Ratios as high as this indicate that the formation of coal beds took place less than 100k years ago.’ As we have seen, the scientists who gathered and interpreted the data for zircons in breccia and ejecta of the Chicxulub bolide do not omit crystals with little radiogenic lead. Gentry’s claims are entirely irrelevant here. Gentry was attempting to demonstrate the existence of polonium haloes and attempting to reason a young earth from these (unsuccessfully I might add). It would be surprising if even he claims the U/Pb ratios in coal were evidence for a young earth. There is no way that U/Pb ratios in coal can possibly mean anything, as coal is not a closed system for uranium and lead. That means that these elements can leach in and out of coal making it entirely inappropriate for U/Pb dating


Harris’s claims about the unreliability of U/Pb dating and his claims that it supports a young earth have been shown to be entirely erroneous. He is altogether too ready to think that he is able to find fundamental problems in the work of professional scientists, and goes so far as to accuse us of incompetence or deliberate deceit on the basis of his superficial and incomplete data and profound ignorance of the science. We have seen that:

There is hardly a phrase past the elementary introduction to U-Pb dating that does not contain a serious error. Harris’s attempt to refute U-Pb dating is badly off target.  The effort required to properly confront the high density of errors in Harris's essay far exceeds the effort required to create it in the first place. In any serious intellectual community, Harris’s rather bad failure on this topic would so destroy his reputation that no-one would take him seriously again. However, creationists’ target audience is not the scientific community, but the unsophisticated and unscientifically trained public that are more easily fooled. Creationists bounce back again and again after being seriously embarassed, since their audience is so uncritical. I hope that this example can be used in future to remind others of Harris’s lack of careful scholarship and his premature willingness to accuse scientists of fraud. His carelessness and poor understanding of the subject has rebounded on him.

1 http://www.geocities.com/peaceharris/u238/index.html

2 This site gives a basic introduction to radiometric dating including the principles of isochrons, the concordia curve and the discordia chord. It does so more accurately and in a more unbiased way than Harris does in his essay. He uses an illustration from this site in his paper without acknowledging or referencing it:


 3 Dalrymple, G. Brent (1991) The Age of the Earth. Stanford University Press

 4 Alvarez et al, 1980. Extraterrestrial cause for the Cretaceous-Tertiary boundary extinction. Science 208, 1095-1108

 5 Alvarez, L.W. 1983. Experimental evidence that an asteroid impact led to the extinction of many species 65 million years ago. Proceedings of the National Academy of Science 80: 627-642

 6 Alvarez et al 1995. Emplacement of Cretaceous-Tertiary boundary shocked quartz from Chicxulub Crater. Science 269: 930-935

 7 Bohor et al, 1993, Impact-shocked zircons: discovery of shock induced textures reflecting increasing degrees of shock metamorphism, Earth Plan Lett 119, 419 – 424

 8 Krogh et al, 1993, Fingerprinting the K/T impact site and determining the time of of impact by U-Pb dating of single shocked zircons from distal ejecta, Earth Plan Lett 119, 425 – 429

 9 Krogh et al, 1993, U-Pb ages of single shocked zircons linking distal K/T ejecta to the Cicxulub crater, Nature 366, 731 – 734.

 10 Kamo and Krogh, Chicxulub crater source for shocked zircon crytals from the Cretaceous – Tertiary boundary layer, Saskatchewan: Evidence from new U-Pb data, Geology 23, 281 - 284



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