It is called Uranium-lead dating than lead-Uranium dating. So I recommend moving it to the appropriate title. 
I think the assumptions section has not been explained properly. I assume by meaning closed system you mean matter cannot be exchanged with the surroundings. The most favored substance for Uranium lead dating is zircon. It repels lead but incorporates Uranium so it could be assumed all lead in zircon could have come from radioactive decay. And also scientists do not consider it a closed system and assume some lead can escape. So they do it with a statistical model.
But even the best geologic methods are imperfect. Dating a rock involves uranium-lead measurements on many zircons, then assessing the quality of the data. Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge. In these cases, the concordia diagram is a valuable tool.
Uranium lead dating has been applied to other minerals than zircon but many of them do not produce active results. And scientists can differentiate between lead formed by Uranium decay and naturally occurring lead.
|“||The amount of radiogenic lead from all these methods must be distinguished from naturally occurring lead, and this is calculated by using the ratio with 204Pb, which is a stable isotope of the element then, after correcting for original lead, if the mineral has remained in a closed system, the 235U: 207Pb and 238U: 206Pb ages should agree. If this is the case, they are concordant and the age determined is most probably the actual age of the specimen. These ratios can be plotted to produce a curve, the Concordia curve (see CONCORDIA DIAGRAM). If the ages determined using these two methods do not agree, then they do not fall on this curve and are therefore discordant. This commonly occurs if the system has been heated or otherwise disturbed, causing a loss of some of the lead daughter atoms. Because 207Pb and 206Pb are chemically identical, they are usually lost in the same proportions. The plot of the ratios will then produce a straight line below the Concordia curve. G. W. Wetherill has shown that the two points on the Concordia curve intersected by this straight line will represent the time of initial crystallization and the time of the subsequent lead loss."||”|
Scientist can determine if the system is closed but can also identify non radiometric lead. They can calculate the age of a non closed system with proper statistical techniques. EvilFlyingMonkey 12:58, 3 May 2012 (PDT)
- For some dating methods like those involving Carbon, I know they also need to correct for atmospheric levels as well, by trying to figure out what the levels should've been, and then calibrating accordingly. The trouble is we now know atmospheric levels were far different than today's, with oxygen levels around 30-35% compared to the 20% level of today resulting in massive insects:
- As Brent Dalrymple in "The Age of the Earth" points out (p. 87), they don't expect decay from external forces because of how electrons shield isotope nuclei from all but nuclear forces:
|“|| There are two basis reasons why significant changes in rates of decay are not expected. First, the nuclei of atoms are extremely small and well insulated by their cloud of orbiting electrons. These electrons not only separate nuclei sufficiently that they cannot interact, they also provide a 'shield' that prevents ordinary chemical or physical factors from affecting the nucleus. Chemical activity in an atom, for example, occurs almost entirely among the outermost electrons and does not involve the nucleus at all. Likewise the 'compressibility' of a substance may result in slight changes in the configuration of electrons but has no effect on the nucleus.
Second, the energies involved in nuclear changes are 10^6 times greater than those involved in chemical activity and 10^4 to 10^5 times greater than the energies that bind the electrons to the nucleus. Chemical forces, which bind atoms together into molecules, are on the order of 1 electron-volt (eV), while the forces required to remove an electron from an atom are typically in the range of 10 to 100 eV. In contrast, the forces that hold nuclei together are on the order of 10^6 eV, and those that hold quarks, the elementary constituents of protons and neutrons together are on the order of 10^6 eV (Weisskopf, 1983:474). This is the reason why nuclear reactors and powerful particle accelerators are required to penetrate and make changes in atomic nuclei. Except in nuclear reactions, such energies are generally unavailable in natural processes such as those that form, change, and destroy rocks on the Earth and in the Solar System.
- Obviously, the researchers are making some basic presumptions. They are assuming the forces holding atoms together were always this strong, and that atoms themselves did not evolve these powerful forces (because if so decay would happen more easily in the past). They're assuming no nuclear forces within the earth could be aging rocks since they still don't even know what earth's core holds or is like. They're assuming they can calibrate for atmospheric levels correctly, figure out the amounts of initial daughter isotopes, and know that the systems were closed without addition or subtraction. --Jzyehoshua 13:17, 3 May 2012 (PDT)
As for the assumption radioactivity decay could not be affected.
|“||One of the paradigms of nuclear science since the very early days of its study has been the general understanding that the half-life, or decay constant, of a radioactive substance is independent of extranuclear considerations." ||”|
But scientists agree that they may be affected by chemical or physical factors but it is statistically quite insignificant in measurement by radioactive decay.
|“||the existence of changes in radioactive decay rates due to the environment of the decaying nuclei is on solid grounds both experimentally and theoretically. But the magnitude of the changes is nothing to get very excited about. ||”|
Similarly the one recent discovery on the decay rate gets affected with the position with respect to sun too gives a very small change.This may produce an error in measurement of the age of rocks but they will quite low. There will be some error in the result as every experiment should have but it will be an acceptable one. As for my part I was pointing out on text in the article I do not want to go to more topics like that on carbon dating in this page. And I had earlier mentioned how they figure out the concentration of non radiometric lead and they do not consider the system closed always. EvilFlyingMonkey 04:05, 4 May 2012 (PDT)
Agreed about page move
I do agree the page should be moved, too, but would like to hear from Ashcraft first to make sure he doesn't have a reason for the page named as it is. It does appear to be generally called Uranium-lead dating though, not Lead-uranium. I was surprised when I saw the page it was the other way around here as well, and think EFM has a good point. --Jzyehoshua 02:59, 4 May 2012 (PDT)
- About Uranium Lead Dating
- AILSA ALLABY and MICHAEL ALLABY. "uranium-lead dating." A Dictionary of Earth Sciences. 1999. Retrieved May 03, 2012 from Encyclopedia.com: www.encyclopedia.com/doc/1O13-uraniumleaddating.html
- How to Change Nuclear Decay Rates, by Bill johnson
- How to Change Nuclear Decay Rates, by Bill johnson
- Radioactive decay rates vary with the sun's rotation: research