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:
It incorporates uranium but little lead during crystallisation and therefore a high proportion of radiogenic lead (ie lead which has arisen by the radioactive decay of uranium) in zircon has resulted from the in situ decay of uranium
It is extremely resistant to chemical weathering or leaching. Once crystallised, it is very difficult to add or remove lead or uranium – it is a closed system
It is extremely resistant to physical weathering – it is hard and crystals can survive for a long time.
Under extreme conditions, such as those that pertain in a major bolide event, a quantity of the lead is removed from the crystal matrix, thus partially or totally resetting the crystal. A set of such shocked crystals provides data that allows the date of original crystallisation and the date of resetting to be determined.
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.
In a document of less than four pages, more than two are devoted to an extremely elementary introduction to the principles of U-Pb dating, which contain several errors (see below)
It has one ‘reference’ – a .gif copy of a scanned paper by Gentry which is almost impossible to read, which is about polonium haloes in coal and which is completely irrelevant to the subject at hand.
Harris makes a number of unsupported but rather
insulting claims in the essay. He goes so far as to accuse scientists of fraud: ‘To preserve conventional wisdom, we must also throw away rocks which contain vary little radiogenic lead’. It would be bad enough to make such an unsupported accusation if it turned out to be true, but as we shall see, not only is it unsupported, but Harris’s indecent haste to condemn scientists as unprincipled fraudsters has rebounded on him.
It has hyperlinks to bit map files of a concordia/discordia plot and a table of primary and derived uranium and lead isotope data from a key paper relating to the dating of zircons from the breccia and ejecta of Chicxulub, the bolide event associated with the K-T extinction. Harris fails to reference the paper properly.
What is worse, Harris bases a key point in his argument on criticising the contents of this same paper and the methods of its authors even though he has never read it. In fact, it is certain that Harris has only ever seen the scanned plot and its caption, and the table of primary and derived data and no other part of the content of this or the other papers on the subject. It is certain, because I personally posted the scanned elements of the paper at the link he provides, in response to his plea to ‘post the data’.
The fact that he has not read any original research on the subject and that he is relying on nothing more than the concordia/discordia plot and its caption, has led him into a serious and embarrassing (for him) misinterpretation of the research, as we shall see. Harris's hubris is unbounded. He claims to have found fundamental flaws that render an entire radiometric dating technique invalid (a technique which professional scientists have spent their entire careers in developing and using). But even more amazingly, he has done so by reviewing a single elementary explanation of the technique and without reading so much as one whole primary research report relevant to the subject. Needless to say, this is breathtakingly poor scholarship, and unsurprisingly, it has led him into embarrassingly erroneous claims, as we shall see.
Harris bases a major plank of his argument on a process which I shall call inhomogenous mixing. He makes no attempt to critically consider whether the data actually supports this hypothesis. (It doesn’t).
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.
He says: ‘To check whether a sample of igneous rock falls on the concordia curve, a geologist first needs to determine (206Pb/204Pb)o. If molten lava flows through a lead mining site which does not contain any uranium impurity, the ratio of (206Pb/204Pb)o would be approximately 17. If lava flows through a uranium mining site containing no primordial lead, this ratio would be infinite since uranium does not decay to 204Pb. And when this magma solidifies, whatever radiogenic lead produced while the magma flowed would be preserved in the rock’ There are three errors here. The first is that U-Pb is not used for dating igneous rock per se but is used only with crystals, such as zircon, which are known to be closed and which are known to incorporate uranium but little lead on crystallisation. Second, in order to determine whether a sample of zircon falls on the concordia, one needs to plot 206Pb/238U versus 207Pb /235U (not 206Pb/204Pb). The quantity of 204Pb is a measure of how much background lead (the sum of common lead – the small quantity of lead present at crystallisation - and laboratory lead introduced by the measuring process) is present and therefore must be corrected for in determining the quantity of radiogenic lead present. The third error is the reference to the original 206Pb/204Pb ratio being 17 in the presence of common lead (these two isotopes of lead are found naturally occurring in open systems in the
ratio 19 today – the ratio was lower in the past, and indeed the 206Pb/204Pb ratio
is a measure of the age of formation of lead ores) and being infinity in the absence of common lead since as any high school student will tell you zero divided by zero is not infinity but indeterminate.
He says: ‘Often there would be many samples which fall on a straight line below and above the concordia curve. Instead of admitting that the value used for (206Pb/204Pb)o could be wrong, one could make the hypothesis that there has been lead loss, uranium loss, lead addition and uranium addition to the rock after it solidified’ (206Pb/204Pb)o cannot be wrong in such a way as to produce a discordia straight line. The ratio (206Pb/204Pb)o will be of the order of 19 or less in the presence of lead and will be indeterminate in the absence of common lead (not infinite as Harris says). Of course in the latter case, since we have no common lead to correct for, the error on the determination of radiogenic lead will be minimised.
He says: ‘The author, in his letter to Nature interpreted the discordia line to be caused by the impact of meteorites on zircon crystal. Samples marked CH8, CH9, CH10 and CH11 when dated by the decay of 238U gave results in the range of 63 million to 73 million years. These samples are interpreted as follows: When the asteroids hit these crystals about 65 million years ago, all 206Pb was "squeezed out", thus resetting the crystal. Thus the current 206Pb/238U ratio represents the date for the rock's last metamorphoses’. This is incorrect. These samples are not completely reset but fall on the discordia line somewhat above its intercept with the concordia. The fact that they are discordant means that the age according to the 206Pb and the 207Pb decay schemes are different because of partial lead removal. So although the 206Pb data gives ages for these crystals in the range that Harris quotes, they plot on a discordia which cuts the concordia at 57.3+/-4 Myr which is the actual age of resetting. Harris’s confusion about this point is fairly typical of his poor understanding of the science.
He says: ‘Geologists had concluded that the KT layer was 65 million years old long before radiometry was discovered’ This is simply untrue. Radiometric dating is the method by which absolute dating of the K-T layer is achieved. In this, as in so much else, Harris is wrong.
He says: ‘To preserve conventional wisdom, we must also throw away rocks which contain vary little radiogenic lead. The author of the above link wrote, "Data for sample C5 and C6 are imprecise due to a very low radiogenic content and are omitted for clarity." ‘. This is the crux of the matter. Harris accuses Krogh et al (9), of ‘throwing away’ critical data. He claims that crystals C5 and C6, omitted for clarity in the concordia plot of ref 9
are the critical pieces of data and that Krogh et al have corruptly thrown them away to deceive us, the editors of Nature and the referees on the paper. This is a wild and erroneous accusation. If Harris had bothered actually to
read the references he would know that the data for C5 and C6 were first published in ref 8 where they were not ‘omitted for clarity’. And what does the data of C5 and C6 show (according to Harris they will show a young earth)? In fact C5 and C6 are not heavily shocked and they plot near the concordia at the upper end of the discordia chord around 490 – 520 Myr. This discordia cuts the concordia at 550Myr (age of crystallisation) and 65.5 Myr (age of resetting) . In fact C5 and C6 are very small crystals weighing around 1µg, which means that they have low absolute radiogenic lead content (and low uranium content). It does not mean that they contain no radiogenic lead or that they in any way support a recent date of crystallisation.
(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)
He says: ‘The straight line obtained in the discordia plot can be understood by considering the following analogy. A mixture of oil and water is solidified. Assume that carbon consists of 12C and 13C and oxygen consists of 16O and 18O. We randomly select small equal volume portions of this solidified mass for analysis. The value of 12C/16O in these samples is taken as the abscissa and the value of 13C/18O is taken as the ordinate. This plot would fall on a straight line. At locations where there was a tiny oil droplet, we would obtain a point close to the origin.’ There are several serious things wrong with this explanation and analogy. In essence, Harris claims that the discordia line is the consequence of inhomogeneous mixing of two compounds each containing one measured element in the form of two isotopes. In other words, he claims that the discordia is the consequence of inhomogeneous mixing and random sampling of uranium and lead where the ratio of uranium isotopes and the ratio of lead isotopes is fixed. So what is wrong with this?
All of the isotopes in Harris’s analogy are stable. The condition he described will remain that way forever, completely unlike the situation with radioactive isotopes where the proportion of elements changes over time
The existence of 206Pb and 207Pb is the consequence of a decay scheme from 238U and 235U with half lives of 4.47 billion years and 704 million years respectively. There would be vanishingly little 206Pb and 207Pb in a world only 6500 years old.
Most significantly of all, if the discordia was due to inhomogeneous mixing as Harris has stated the straight line would pass through the origin (as he acknowledges). But a discordia is a chord of the concordia and does not pass through the origin. It cuts the 206Pb/238U axis above the origin. So the discordia, which results from variable resetting of crystals by a major metamorphic event, results in data which is totally different from inhomogeneous mixing.
He says: ‘The straight line obtained in discordia dating has nothing to do with radiometry and lead loss. This fact is proven by the fact that non-radiometrically related species C and O also can produce a similar plot.’ As we have seen above, this is entirely spurious. The discordia has nothing whatsoever to do with inhomogeneous mixing. The fact is that ‘that non-radiometrically related species C and O’ do not produce a similar plot.
He says: ‘To determine the formation date of a particular geologic strata, a geologist should not omit rocks containing very little radiogenic lead, as these are the most important samples. These samples represent an upper bound for the formation of that strata.
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
Conclusion
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:
His claims about the ratio of radiogenic to non-radiogenic lead present as common lead is arithmetically flawed
His claim that the absolute age of the K-T boundary at 65Myr was determined before radiometric dating is wrong
His interpretation of discordant data near complete resetting is incompetently wrong
His claim that scientists have incompetently or fraudulently ignored data that refutes the date of K-T is absolutely unfounded and wrong and the data all support the 65Myr date
His explanation for the discordia based on inhomogeneous mixing is entirely refuted
His references of Gentry’s work are irrelevant
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.
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
