Geologists and dating (India Basins Half a Billion Years Older Than Thought)

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Joe's spreadsheet uses Dr. Ludwig's add-in Isoplot, which in turn contains some extremely sophisticated statistical formulae that are not readable in the spreadsheet (because it's an add-in). You can find a discussion of the formulae in User's Manual for Isoplot 3.00, with gobs of references for you to digest. To critique the math, you will neeed to be an expert in the fitting of straight-line and curve data in cases where both the abscissa and ordinate contain experimental erros (which in this case are coupled) and the errors are not normally distributed ... i.e. not least-squares.I don't know if Joe is allowed to send you Isoplot even if he wants to. You might have to convince Dr. Ludwig to give it to you. IMHO there's no really good online reference on this subject. That one doesn't make the key point strongly enough. The physics of solidification guarantee that zircons strongly reject lead at solidification and relatively easily accept uranium and thorium, so essentially all of the lead one finds in a zircon is due to radioactive decay of one of those elements. Since the age of the Earth was settled long ago, today the action in geochronology is sub-1% accuracy, and to get that you need to do a common lead correction. But that correction is second-order; it pretty much never affects the age much. IIRC a two percent change would be a very large correction for a zircon.Many correction techniques (I'm not familiar with the particular one Joe used) involve measuring the amount of ²⁰⁴Pb, which is not radiogenic and therefore is common lead, and a model to derive the amounts of other lead isotopes that are common. Ther are techniques that do not depend on measuring ²⁰⁴Pb.So I'm puzzled, and I think Joe's puzzled, about why you want to discuss a second-order correction.You might want to look at GEOCHRONOLOGY V: THE U-TH-PB SYSTEM: ZIRCON DATING which begins:

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Zircon (ZrSiO4) is a mineral with a number of properties that make it extremely useful for geochronologists (Figure 1). First of all, it is very hard (hardness 71/2), which means it is extremely resistant to mechanical weathering. Second, it is extremely resistant to chemical weathering and metamorphism. For geochronological purposes, these properties mean it is likely to remain a closed system. Third, it concentrates U (and Th to a lesser extent) and excludes Pb, resulting in typically very high 238U/204Pb ratios. It is quite possibly nature's best clock. Finally, it is reasonably common as an accessory phase in a variety of igneous and metamorphic rocks.

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The very high 238U/204Pb ratios in zircon (and similar high minerals such as sphere and apatite) provide some special geochronological opportunities and a special diagram, the concordia diagram, has been developed to take advantage of them. The discussion that follows can be applied to any other system with extremely high 238U/204Pb ratios, but in practice, zircons constitute the principal target for Pb geochronologists.

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and goes on to get pretty darned technical. (The entire series of lectures is available at Earth & Atmospheric Sciences 656: Isotope Geochemistry.)Or Zircon: The Key Mineral:

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Uranium and thorium are found in many minerals, of which, however, only a small number are suitable for dating using the U, Th-Pb approach. This is because only a few are adequately retentive towards them and of these the most retentive is zircon. Other useful minerals include pitchblende (or uraninite, U308), monazite, sphene and apatite. In zircon, the concentrations of U and Th average 1330 and 560 ppm respectively. When in pegmatites, zircons contain more of these elements than they do in normal igneous rocks. Uranium and thorium are found in zircon through the isomorphous replacement of Zr4+ (with an ionic radius of 0.87 ) by U4+ (1.O5 ) and Th4+ (1.10 ) as well as through the presence of thorite inclusions. Such substitution is restricted by the differences in the relevant ionic radii Pb2+ is excluded altogether because its ionic radius is 1.32 and it bears a lower charge. The result is that zircon does not contain much lead at the time of its formation and has very high ratios for U/Pb and Th/Pb, making it a valuable geochronometer (Bowen, 1988). For this reason, this mineral is widely utilised in dating by the U, Th-Pb isotope method, for instance by C. J. Allégre et al. (1974).

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Edited by JonF, : No reason given.

I don't know offhand of any measured data that proves this statement; I'm not a crystallographer, and it's one of those things that's so well established that few if any people bother to trace it down. But you could start with the papers referenced in my second link.{ABE: I draw your attention to that which I posted already:

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Such substitution is restricted by the differences in the relevant ionic radii. Pb2+ is excluded altogether because its ionic radius is 1.32 and it bears a lower charge. The result is that zircon does not contain much lead at the time of its formation and has very high ratios for U/Pb and Th/Pb, making it a valuable geochronometer (Bowen, 1988). For this reason, this mineral is widely utilised in dating by the U, Th-Pb isotope method, for instance by C. J. Allégre et al. (1974).

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{emphasis added}So you can go dig up "Bowen, R., 1988. Isotopes in the Earth Sciences. Elsevier Applied Publishers Ltd. Press" to start with.end ABE}FWIW, the RATE group (Baumgardner, Snelling, Austin, and Vardiman: arguably the YECs who are most knowledgable about radiometric dating, and who are definitely interested in discrediting it) accept this fact. From HELIUM DIFFUSION RATES SUPPORT ACCELERATED NUCLEAR DECAY:

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Samples 1 through 3 had helium retentions of 58, 27, and 17 percent. The fact that these percentages are high confirms that a large amount of nuclear decay did indeed occur in the zircons. Other evidence strongly supports much nuclear decay having occurred in the past [14, pp. 335-337]. We emphasize this point because many creationists have assumed that "old" radioisotopic ages are merely an artifact of analysis, not really indicating the occurrence of large amounts of nuclear decay. But according to the measured amount of lead physically present in the zircons, approximately 1.5 billion years worth ” at today’s rates ” of nuclear decay occurred. Supporting that, sample 1 still retains 58% of all the alpha particles (the helium) that would have been emitted during this decay of uranium and thorium to lead.

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{emphasis in original}Note that these knowledgable people, frantically trying to discredit radiometric dating, did not even bother to try to argue that common lead is significant in zircons. Instead they said exactly the opposite; significant amounts of lead in a zircon is prima facie evidence of large amounts of nuclear decay since the zircon formed.But it can easily be proved beyond reasonable doubt that Joe's samples (and almost all the tens of thousands of samples for which this kind of analysis has been carried out) had low common lead. He's already briefly alluded to it, apparently assuming that you know enough to figure it out.If we start with a freshly-formed zircon and plot its position on the concordia diagram, it will plot at (or indistinguishably close to) (0,0) if and only if it has zero (or essentially zero) common lead. As it ages, the point representing it will move to other points on the concordia diagram, tracing a path. Assuming for the moment that that no relevant material is removed or added, those other points will lie on the concordia curve if and only if the point representing the zircon originally plotted at (0,0) and the zircon started with essentially zero common lead.So, when we plot the point representing a zircon todary and observe that it lies on the concordia curve, there are two possibilities:

1. The zircon started with zero (or near-zero) common lead when it formed.
2. The zircon started with noticably non-zero common lead, and some event added or removed relevant material in exactly the proportions required to move its point back onto the concordia curve.

If you investigate possibility 2 mathematically, you will find that the postulated event will have to be significantly sensitive to different isotopes of the same element, ruling out pretty much all chemical and mechanical processes. And any process has to occur in just the right amount to get the point back onto the curve.Tain't gonna happen. Maybe once in a few tens of thousands of tries, but not several times in one study, and not over and over and over again.QED. Zircons start with essentially zero common lead. You can dig up the reasons if you are interested, but the points representing them consistently plotting on the concordia curve proves it. These are very specialized statistical tests, and I wouldn't assume Minitab does them. In this context, "high common Pb" is not very much at all in absolute terms. And did you notice his reason for deducing that they have high common Pb? They didn't plot close enough to the concordia curve.I understand his research and the method well enough to know that the common Pb correction is a tiny fraction of the determined ages.Edited by JonF, : Add marked material

Nope. At least, it's possible and maybe it's done occasionally, but I've never seen it. I'm an amateur and a pro has seen a lot more than I have ...The most common method is to observe that the ratios of various isotopes of lead to each other are pretty much constant worldwide. (In reality, they vary some depending on the source, but are really constant within samples derived from the mantle or derived from the crust or … . But work with me here.) So we measure the amount of ²⁰⁴Pb, which is all common ²⁰⁴Pb since it's not radiogenic. Then can use the observed constant isotope ratios to calculate what amount of the radiogenic isotopes are common. E.g. New constraints on the timing of tectonic events in the Archaean Central Pilbara Craton,Western Australia.:

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A correction was applied for common Pb on the basis of the abundance of 204Pb, which was typically 10 ppm in all standards measured and variable in the samples. This was assumed to be common lead from the mount surface and a correction as described by Compston et al. (1984) was applied, assuming the common Pb component to have the isotopic composition of Broken Hill Pb (204Pb/206Pb=0.0625, 207Pb/206Pb=0.9618 and 208Pb/206Pb=2.2285). Pooled 207Pb/206Pb ages and upper intercept U-Pb concordia ages were calculated using Isoplot (Ludwig, 2001). Cumulative probability diagrams were used to identify different populations within samples. Concordia diagrams show the U-Pb upper intercept ages and the degree of discordance. All samples show discordancy trends that are consistent with radiogenic Pb-loss at zero age. A summary of the interpreted ages is given in Table 5. All errors are 2s errors.

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There are other methods, some not involving ²⁰⁴Pb since it's not measured in some methods. E.g., from the "Why" page of ComPbCorr - Help and documentation - Release 3.1:

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Common lead is lead of non-radiogenic origin incorporated into a mineral during its initial formation, in subsequent recrystallization processes or by contamination during analysis. As the presence of even small amounts of unsupported lead in a zircon or other datable mineral will increase its apparent U-Th-Pb ages, the presence of uncetected or uncorrected common lead is very detrimental to U-Pb dating. The use of plasma-ionization mass spectrometry with in-situ laser-ablation microsampling (LAM-ICPMS) is a new and promising analytical approach to U-Pb dating of U-enriched minerals (e.g. zircon). The method used to compensate for the presence of common lead in thermal or secondary ionization mass spectrometryuses the minor, non-radiogenic isotope 204Pb as a monitor of common lead, and the signals of the radiogenic isotopes 206Pb, 207Pb and 208Pb are corrected in proportion to their relative abundances in common lead. Unfortunately, this approach cannot generally be applied to LAM-ICPMS analyses, at least not when a quadrupole mass spectrometer is used. This problem arises primarily because the low peak/background ratio of the 204Pb peak is compounded by the ubiquitious presence of Hg in the argon nebulizer gas; 204Hg interferes on 204Pb, while the 202Hg peak is so small that reliable measurement is difficult, if not impossible, and hence an overlap correction of sufficient precision is seldom feasible. Popular methods for common lead correction of such U-Pb analyses make assumptions of ideal concordance of 206Pb/238U and 207Pb/235U or 208Pb/232Th (e.g. Ludwig 2001), which may not always be justified. ComPbCorr uses a different approach to common lead correction, described by Andersen (2002), which neither requires knowledge of the amount of 204Pb present, nor assumes that corrected compositions plot on the concordia.

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I don't know what method or variation of a method Joe used, and I don't much care … because I know that the correction is a very small percentage of the reported age.

I don't see anywhere on that page where that link says that initial lead isotope ratios are done in the manner you described for U-Th-Pb analyses. You described a method using multiple samples from a common source with diffferent ratios to plot a line and look at its intercept. That's isochron dating. AFAIK that's not done U-Th-Pb dating.Common lead correction i is obtaining the initial ratios.(There is a Pb-Pb isochron, but the point representing the initial ratio does not lie on any axis and must be obtianed from some imdependent method.)

Yup, you got the Rb/Sr section right. Being pedantic, it's "co-genetic" or common origin rather than necessarily the same sample, but you've got the gist.

I note that you are avoiding the evidence that the common Pb correction is always small (well unber 10%) in zircons.

I don't see what's so magical about Joe's data; raw data is available in many papers, many of which are online, some of which are free and many of which can be bought for a small sum.While browing on this topic I stumbled across a thread which probably clarifies what peaceharris is looking for … he thinks that the dates somehow reflect mixing "when it solidifies … uranium would attract other uranium atoms and lead would attract its own kind." It's pretty confused, but anyone interested can look at u-pb concordia.And peaceharris continues to ignore the high-school chemistry of atomic size and valence effects.

Radiometric dating measures the age of rocks and minerals, not age of the atoms that make them up. So all of your message is meaningless gobbledygook, except for one sentence: Yup, you got it. It's not really "belief", it's a conclusion from masses of experimental and theoretical evidence.

You certainly don't understand how radiometric dating is carried out. They probably can predict, at lease roughly; I don't know.However, even if they cannot your "it also follows" is false. Common Pb corrections are not based on any calculations of the amount of Pb incorporated under any conditions at any time. They are based on the isotopic composition of the Pb that was incorporated, no matter what its amount, and a model for how that evolved over time. Common Pb corrections are applied to minerals (e.g. Apatite, Allanite, Anatase, Epidote, Garnet, Rutile, Titanite/Sphene, Thorite, Tritomite, and Pitchblende/Coffinite/Uraninite) in which much higher proportions of common lead are found than are found in zircons. It isn't done all that often because the results have large uncertanties because a large common lead correction introduces possible errors of its own.You ignorance of the fundamentals of dating reveals that you do not understand how radiometric dating works and are incapable of analyzing the data. But you could fix that … yourself. You are asking Joe to teach you a graduate student course for free in an unsuitable medium. IMHO he'd be totally justified in telling you to go pound sand.You want to do the calculations, learn to do the calculations. Register as a student in geosciences at your local university. Or teach yourself from the many materials available on the Web and in libraries.Edited by JonF, : No reason given.

Hey, Joe: is there a technical advantage to using LA-MC-ICP-MS rather than SHRIMP or some variation thereof, or did you use that mostly because it was in the lab around the corner?

Thanks, interesting.I've been writing a web page explaining this stuff to the layman … been doing it for years, and have no idea when it'll be done … and I'd like to inbclude a few paragaphs on instrumentation. Any suggestions for further reading?

Boy, I'd love to see your reference for that claim! Of course you'll never supply one.You obviously don't understand the theory behind discordia dating. Nontetheless, everybody prefers concordant dates; that's why techniques have been developed for obtaining them, and the vast majority of dates obtained recently are concordant. Like Joe's concordant dates.You guys are all alike. Focus on one small individual topic, ignore the big picture and especaily ignore the elephant in the room … consilience Yes, for several reasons; the major one of which is that it's a second-order correction. Says Mr. meteorites-hit-these-zircons!

I already posted the money quote from the RATE article to which that refers. He ignored it.Lord only knows how he's convinced himself that the common lead correction is significant and not just a slight second-order effect.And like prety much all YECs, he ignores the consilience between radiometric and non-radiometric methods and between wildly different radiometric maethhods. He hopes that finding a problem in one little niche will bring the entire structure crashing down. The fact that the RATE boys realize that the only hope for YEC is total restructuring af all physics, and that they also realize that this requires massive non-Biblical miraculous interventions, is anathema to peaceharris' kind.