They occur in all different types of rocks throughout the earth’s crust, and have several interesting properties that make them particularly well suited to the purpose. Consequently they crop up frequently in more informed discussions about the age of the earth.
The most important feature of zircons is that they accept certain elements into their crystalline structure as impurities, but not others. They can contain up to about 1% uranium when they first form, but they strongly reject lead. This is because uranium atoms have the right size and valency to fit into their crystalline structure, whereas lead atoms do not. A freshly formed zircon crystal will contain no more than a few parts per trillion of lead (Gehrels, 2010); anything more than that must be radiogenic in origin. One part per trillion corresponds to about four months’ worth of radioactive decay.
Another important property is that they are very hard (7.5 on the Mohs scale of hardness — somewhere between quartz and topaz). That, and their small size (typically a fraction of a millimetre) makes them strongly resistant to weathering. Zircons also have a very high melting point — over 1,800°C — and a closure temperature well in excess of 1,000°C. The closure temperature is the temperature above which the impurities in the zircons start to become mobile and can thus diffuse out, thereby resetting the radiometric “clocks.”
Uranium decays to lead via a number of intermediate elements, with a half-life of 700 million years for 235U → 207Pb, or 4.5 billion years for 238U → 206Pb. Naturally occurring uranium consists of about 99% 238U and only about 0.72% 235U.
What does this mean for the age of the earth? For starters, if there has been any leakage, lead will have been preferentially lost, while if there has been any contamination, uranium will have been preferentially gained. This means that the U-Pb model age of the zircons will always be a lower limit. If it is incorrect, the zircons will be older than the naive dating method indicates. This does not help the young-earth cause.
In fact, it gets better than that. Because there are two isotopes of uranium, which decay at different rates, it is possible to determine not only whether there has been any leakage, but also when, and to what extent. This is done using a concordia diagram.
More importantly, this also means that after six thousand years, zircons should not contain more than about eighteen parts per billion of lead. Thus, any zircons which contain significantly more than that will blow the young earth timecale right out of the water. As for accelerated nuclear decay — which, as we’ve already seen, is science fiction — that would have released enough heat to melt the zircons and reset the “clocks.”
Since lead atoms do not fit naturally into a zircon’s crystal structure, zircons that have seen a large amount of decay tend to become brittle, eventually completely losing their crystalline structure and becoming amorphous. This process is called metamictisation. On heating, the impurities will anneal out, restoring the crystal structures of the zircons to their original, pristine condition. Again, we should not expect to find metamict zircon crystals in a young earth.
In practice, uranium-lead dating of zircons is one of the most reliable and accurate radiometric dating methods that scientists have at their disposal. It can give ages that are accurate to between 0.1% and 1%.
Now of course there is another product of uranium-lead nuclear decay: helium from alpha particles. What happens to the helium? That is the subject of next week’s post.
Radiometric dating deniers like to portray results far in excess of six thousand years as just “rationalisations” or “just-so stories.”
Take, for example, this article on creation.com, which claims that “long-age geologists” just disregard (or “reinterpret”) any data that doesn’t fit their preconceived notions. The author, Tas Walker, correctly points out that radiometric dating doesn’t always give the results we’d expect, and that when this happens, scientists come up with an alternative, explanation for the results. He gives some examples of such explanations: heating events since the rocks cooled; xenocrysts; contamination; inherited ages; and so on.
But then he says this:
No matter what the radiometric date turned out to be, our geologist would always be able to ‘interpret’ it. He would simply change his assumptions about the history of the rock to explain the result in a plausible way. G. Wasserburg, who received the 1986 Crafoord Prize in Geosciences, said, ‘There are no bad chronometers, only bad interpretations of them!’ In fact, there is a whole range of standard explanations that geologists use to ‘interpret’ radiometric dating results.
This caricature is very misleading. As we have seen, geologists do not just look for a plausible sounding interpretation based on changing assumptions; they have to rigorously demonstrate which interpretations are consistent with the data and which ones are not. In fact, they can often discover a whole lot of other useful information about the rock strata than just their age — information such as their thermal history, for example. They most certainly are not just a case of making up excuses, or of choosing whichever interpretation you like in order to get the results that you want, and it is quite frankly dishonest to portray them as if they were.
What about unpublished results?
In any case, this caricature does not explain why up to 95% of results do not require any such “rationalisation” or “changing assumptions.” To accommodate this high degree of concordance, radiometric deniers instead sometimes claim that discordant results tend to be thrown out, or filed away in a drawer somewhere. But could this be happening often enough to undermine the credibility of radiometric dating?
If it is, the amount of data that they must be withholding is colossal. This article gives some calculations that show that merely to get meaningful isochron plots from random data (remember, isochron dating requires all the points to lie on a straight line) you would have to be throwing away several hundred results for every one that you accept. It gives a couple of examples of single isochrons from the scientific literature that would have cost as much as $1.7 million if they really had been the result of cherry-picking random numbers.
But let’s suppose that all isochron plots are all due to mixing, and do as a result give meaningless “ages.” If this is the case, you would still have to be throwing away perhaps as many as a hundred results for every one that you accept in order to achieve 95% concordance.
Radiometric dating is expensive. Geochron Laboratories quotes $750 for each point on a Rb-Sr isochron graph, and $800 per point for Sm-Nd. An isochron graph requires at least three points, and in some cases can consist of a dozen or more. Since researchers frequently use more than one dating method, the cost of dating a single sample can easily exceed $10,000. It’s fair to say that it costs as much as a small family car.
This means that if scientists really are cherry-picking data in this way, they must be spending a million dollars or more per published result on what amounts to wholescale scientific fraud. As we’ve already seen, the number of radiometric results in the literature runs into the hundreds of thousands, with tens of thousands of new results being added every year. The amount of money being squandered must run deep into the tens if not hundreds of billions of dollars worldwide.
Where, then, are the accountants and auditors complaining about this colossal waste of money? Where are the scientists working in other fields, competing with it for funding, creating a stink because they have lost out on research grants because of it? Where are the documents blowing the gaff on it on Wikileaks? And where are the US Senators — some of whom are YECs themselves — calling for the obvious fix to the problem of requiring all radiometric studies to be pre-registered?
Ultimately, the claim that “long age geologists” don’t accept results unless they match their preconceived notions is nothing more than a conspiracy theory — and a bad one at that. It may sound plausible when you first hear it, but it only takes some simple arithmetic to see it descend into absurdity. It simply doesn’t make sense.
Radiometric deniers love to point out examples of cases where different dating methods give different results on the same rock strata. The most comprehensive attempt to catalogue these that I’m aware of is an article by John Woodmorappe in the September 1979 issue of the Creation Research Society Quarterly entitled Radiometric Geochronology Reappraised, which lists about 300 examples that he had culled from the scientific literature.
Now anomalous results do happen from time to time, and lists such as these do look pretty impressive, but they greatly exaggerate their frequency, extent and significance. Here are a few points to bear in mind.
1. Discrepancies are the exception, not the rule.
As we saw last week, the number of radiometric results in the scientific literature runs into the hundreds of thousands, and quite possibly even into the millions. This being the case, three hundred bad dates represents only a tiny fraction of the overall data.
How many results overall are discordant? Without doing an exhaustive literature search, which would take forever, it is hard to tell. However, one thing is clear: when radiometric dating is correctly applied, they are very much in the minority.
This Talk Origins article cites geochronology expert Brent Dalrymple, an expert in radiometric dating with lots of real-world experience, as estimating that only between 5% and 10% of radiometric results are discordant. The article doesn’t explain how he arrived at that figure though.
A more detailed picture comes from this account by geology professor Joe Meert (scroll down to the section “A brief discussion regarding the integrity of radiometric dating”), in which he records a discussion with some YECs on the subject. The figures that they cited were, unsurprisingly, higher, claiming that up to 15-20% or so of dates were discordant. However, he cited fifteen years of his own research, in which he found that fewer than 5% of his results were anomalous, and that each and every one of them had a reasonable explanation.
Even if the figure is as high as 20%, this leaves a serious question for radiometric denialists. Why do so many of the hundreds of thousands of results in the literature show little or no discordance? This would never happen if radiometric dating really were consistently unreliable, and certainly not if it were so off-base that it could not distinguish between thousands and billions.
In any case, there are certain important data sets that show little or no discordance — specifically, meteorites and lunar rocks returned from the Apollo missions. Meteorites in particular are very, very consistent in showing ages between 4.4 billion and 4.6 billion years by up to six different methods. Because they have very simple geological histories and have not been subjected to weathering, erosion, and metamorphic and tectonic processes found on Earth, there is little or no scope for “evolutionists” to fish around for alternative interpretations in order to get the results they want, as creation.com claims to be standard practice. Furthermore, these data sets are relatively small (a few hundred or so samples), so allegations of cherry-picked results are completely unrealistic.
2. Showing that one method fails under specific conditions does not prove that all methods fail everywhere.
There are over forty different isotopes used in radiometric dating. Each of them has a different half-life, applies to a different range of ages, involves different experimental procedures, and works in different situations on some minerals but not others.
Let’s say, for example, that you tried to use carbon-14 dating on a traffic cone, and got an age of 5,000 years. This is quite a plausible result, because traffic cones are made of plastic, which has been heavily processed from oil deposits and will therefore have been heavily contaminated by significant amounts of modern carbon. However, what does this tell us? Simply that carbon-14 dating doesn’t work on traffic cones. It doesn’t tell us whether or not it works on bones, wood, or ancient seeds. And it certainly doesn’t tell us whether or not uranium-lead dating works on zircon crystals in granites.
Before radiometric dating techniques can be used in the field on new types of materials, they are first tested against samples of those materials whose ages are already known. Many of Woodmorappe’s discordances you read about are the results of these preliminary experimental tests. It is dishonest to cite these preliminary tests as evidence that field results using well-established methods are also unreliable.
3. Showing that a method fails when pushed to its limits does not prove that it fails everywhere.
Take, for example, this report of an attempt to date some lava from a lava dome that formed on Mount St Helens in 1986. The potassium-argon “ages” reported ranged from 0.35 million years to 2.8 million years, which the YEC researcher, Steve Austin, described as “preposterous.” In reality, it was his methodology that was preposterous.
Samples less than 5 M.Y. old, or containing less than 0.1%K will incur a 50% surcharge, reflecting the special care and additional analyses required. We cannot analyze samples expected to be younger than 2 M.Y.
Geochron Laboratories did not have the advanced state-of-the-art equipment needed to process samples that young, and consequently, contamination and “memory effects” of lingering argon from previous analyses would have been a very real issue.
The half-life of 40K is 1.25 billion years — roughly a thousand times greater than the ages reported by Austin. This study was the equivalent of using a weighbridge to measure out flour, sugar and eggs when baking a cake for a family of four, then claiming that weighbridges are unreliable when it comes out all mushy.
I’m sorry, but this is called “gaming the system,” pure and simple.
Another example here is the RATE project’s claims of discovery of primordial radiocarbon in ancient coals and diamonds. The quantities discovered were very low, close to the limits of the capabilities of many radiocarbon labs, and showed clear patterns that were consistent with contamination, with heavily processed samples showing significantly more 14C than unprocessed ones. Although the RATE team claimed to have taken great care to take contamination into account, radiocarbon experts such as Kirk Bertsche noted that the procedures they followed were incorrect.
4. Discrepancies of a factor of two or three do not prove that all methods must be out by a factor of a million or more.
About three quarters of the discrepancies on Woodmaroppe’s list come within a factor of two of the expected result, about 90% within a factor of three, and more than 97% within a single order of magnitude. Furthermore, about two thirds of the time, the dates reported are too small. This does not help the Literal Six Day Young Earth timescale, which requires the dates reported to be consistently too large by a factor of between a thousand and a million.
A factor of a million is a colossal error. It is like demonstrating that the whole of London would fit into a rucksack, or that you could buy a four bedroom house in Chelsea for five pounds, or that the life and ministry of Christ recorded in the Gospels happened in the space of fifteen minutes just yesterday afternoon.
A minority of discrepancies of a factor of two or three, the majority of them underestimates, falls far, far short of demonstrating that all results are consistently overestimates by a factor of up to a million. Especially when you consider that…
5. The differences are unsurprising, well understood, and useful.
Different results from different radiometric methods are not unexpected, given that many rocks have had a complex history. In fact, the differences usually provide a lot of useful information that extends well beyond the ages of the samples.
One condition which results in different dating methods giving different results is when a rock formation such as granite takes a long time to cool. This is because radiometric dating measures the time since the rock cooled below the “closure temperature.” Closure temperatures can be determined experimentally and are different for different radioisotopes. Consequently the differences in radiometric ages can tell us how long the rock took to cool.
When plotted on a graph, it can be seen that these give a rather nice cooling curve:
Far from falsifying radiometric dating, different results from different methods provide additional information about a rock formation’s thermal history as well as its age. This extra information is of particular importance in oil exploration, where geologists need to know the thermal history of the oil deposits as well as their ages in order to determine whether they’re going to yield anything useful.
Woodmorappe claimed, in response to a critique to his list, that cooling explanations are just a “rationalisation,” and that the imbalance between overestimates and underestimates was evidence of cherry-picking by the scientific community, with “nonsensical” results more likely to be withheld from publication. We shall see next week that his allegations of cherry-picking have no merit whatsoever, but for now it should be clear that these explanations are no mere “rationalisation,” but are in fact individually testable.
6. Some claims of discordant dates are blatantly dishonest.
One of the articles that Woodmorappe cited was that of a Rb-Sr isochron that allegedly gave an age of 34 billion years — seven times the age of the earth and two and a half times the age of the universe.
This sounds like a massive poke in the eye for isochron dating until you see the “isochron” in question:
This is not an isochron. Anyone who knows anything about isochron dating will be aware that you need all the points to lie on a straight line in order to get a valid date. When they don’t, the only correct interpretation of the graph is “undateable due to contamination and leakage,” and the 1.09 billion and 34 billion year lines were purely there to illustrate the fact.
For what it’s worth, this is a variant on what scientists call “quote mining” — taking a particular quote completely out of context and misrepresenting it as saying something that it is not. A charitable view here would be merely that Woodmorappe had misunderstood the graph, wasn’t getting his facts straight, or was perhaps being a little bit careless, but scientists who get quote mined do not take such a charitable view. They consider it to be a form of lying.
Discordant dates do not prove what YECs claim that they prove.
There is a vast difference between showing that something doesn’t always work and showing that it never works, and similarly, there is a vast difference between showing that something can occasionally be out by a factor of two, and showing that it is consistently too large by a factor of up to a million.
Disagreements between different dating methods may give us leave to take individual results with a pinch of salt. This may have some bearing on subjects such as the Shroud of Turin, or the occasional archaeological discovery relating to Bible times. But they present no challenge whatsoever to the overall corpus of data as a whole. In particular, they fall far, far short of falsifying the geological column with its display of a general progression of species through the ages, let alone demonstrating that all radiometric results could plausibly be out by up to six orders of magnitude. Radiometric dating still presents an insurmountable barrier to the young-earth timescale and the discordances highlighted by the young-earth literature barely put a dent in it.