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How did Polonium Haloes form in old rocks?
Gentry is a physicist, not a geologist. This will cause them to retain argon and appear too old. About the Rocks Geologists classify rocks into three main categories - sedimentary, igneous, and metamorphic - based on the way in which they form. Zircons exclude lead, for example, so U-Pb dating can be applied to zircon to determine the time since lava cooled. As the magma or lava cools, this path will consist entirely of hot magma or lava, and so the argon will have a free path, and will continue to enter the magma as it cools. Mica is thought to exclude Sr, so it should yield good Rb-Sr dates. Indeed, there are a number of conditions on the reliability of radiometric dating. So argon is being produced throughout the earth's crust, and in the magma, all the time. So when one combines all of the above figures, the statement that there are only 10 percent anomalies or 5 percent or whatever, does not have any meaning any more. However, it does not seem likely that sedimentary rocks would be this hot very often, except near lava or magma flows. This shows that rocks can adsorb a large amount of argon relative to the argon needed to give them old K-Ar ages, and also suggests that old K-Ar ages can be produced by external argon from the mantle. This follows because, as each parent atom loses its identity with time, it reappears as a daughter atom. It may also be that lava is produced by melting the bottom of continents and successively different layers are melted with time, or there could be a tendency for lighter isotopes to come to the top of magma chambers, making the lava there appear older. Potassium is present in most geological materials, making potassium-argon dating highly useful if it really works. When burial pressures and temperatures get too great, the rocks melt completely, becoming new igneous rocks. Age estimates on a given geological stratum by different radiometric methods are often quite different sometimes by hundreds of millions of years. But if we really understand what is going on, then we should be able to detect discrepant dates as they are being measured, and not just due to their divergence from other dates.
Let me illustrate the circulation patterns of argon in the earth's crust. So magma should have at least 20 times as much argon as a rock million years old by K-Ar dating. These haloes were considered to be the result of damage to the crystal structure of the host minerals caused by high energy alpha particles. Fortunately, neither effect has been significant in the case of older samples submitted for carbon dating. Clearly, more work is required to resolve all of these questions. He states that geologists are aware of this problem, and make allowances for it. Lead dating The presence of radon gas as a member of the uranium-decay scheme provides a unique method for creating disequilibrium. Beta particles are created when a neutron breaks down into a proton and an electron - the beta particle thus is an electron, only in this case it comes from the nucleus. Such checks include dating a series of ancient units with closely spaced but known relative ages and replicate analysis of different parts of the same rock body with samples collected at widely spaced localities. Anomalies of radiometric dating If a date does not agree with the expected age of its geologic period, and no plausible explanation can be found, then the date is called anomalous. The Branching Ratio Dr. This could happen because of properties of the magma chambers, or because of argon being given off by some rocks and absorbed by others. The effect of alpha particles in crystalline materials, whose physical properties vary depending on orientation, is less straight forward. And quite a few other dates are often much, much farther off. The percentage of published dates that are considered as anomalous has little bearing on the question. And as I said above, I'm also interested to know how much of the fossil-bearing geologic column can be dated by isochrons, and how the dates so obtained compare to others. Volcanic rocks, because they are able to cool and crystalize rapidly, have a very fine-grained texture; the individual mineral grains are too small to see easily with the naked eye. Historically, the decay constants used for the various radiometric dating systems have been adjusted to obtain agreement between the results obtained. When these minerals occur as inclusions in certain other minerals, most notably the mica family, they are often seen to develop discoloration, or "pleochroic" haloes. This distinction formerly fell to bismuth, with an atomic number of 83, until its only primordial isotope , bismuth, was found in to decay very slowly. A third source of radioactive isotopes is provided by the uranium - and thorium -decay chains. Gentry's ion-beam work, however, was not able to produce multiple bands or the sharply defined concentric ring structure of certain haloes. Many sedimentary uranium ores are not. If the minerals in the lava did not melt with the lava, one can obtain an age that is too old. The importance of internal checks as well as interlaboratory comparisons becomes all the more apparent when one realizes that geochronology laboratories are limited in number. Both facts will tend to produce artificially high K-Ar ages in these flows which will not be seen in modern lava flows in the same manner.
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