Thus we can assume that zircons would incorporate some lead in their impurities, potentially invalidating uranium-lead dates obtained from zircons. And yet we expect that uranium-lead ratios are determined by radiometeric decay alone or at least sometimes! An Introduction to Physical Geology. So one could argue that any variations in Pb ratios would have to result from radioactive decay. But how does Bowen's reaction series account for the great diversity of igneous rocks? This can happen, for example, if the earlier formed minerals are heavier than the liquid portion and settle to the bottom of the magma chamber as shown in Figure 3. Contamination and fractionation issues are frankly acknowledged by the geologic community. These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio. Furthermore, if there are special circumstances that invalidate the method, then this raises questions about the method in general. Recall that olivine is composed of a single tetrahedra and that the other minerals in this sequence are composed of single chains, double chains, and sheet structures, respectively. U-Pb dating attempts to get around the lack of information about initial daughter concentrations by the choice of minerals that are dated. Can we find evidence that shows that an explanation of radiometric dates in terms of a young geologic column is more plausible than an explanation in terms of an old geologic column? So this factor would also make the age appear to become younger with time. If the reverse happens before mixing, the age of the isochron will be decreased. Geologists are aware of the problem of initial concentration of daughter elements, and attempt to take it into account. If minerals that crystallize at the top of a magma chamber and fall, tend to incorporate a lot of uranium, this will tend to deplete uranium at the top of the magma chamber, and make the magma there look older.
I assume he would have mentioned if any others had been done. So there may not be anything to explain. Finally, ages can also be determined from the U—Pb system by analysis of Pb isotope ratios alone. Now, after the magma is thoroughly mixed, the uranium and thorium will also be thoroughly mixed. Assuming a closed system means that nothing on the outside of the rock affected the sample. It depends how fast it happened. Mechanisms that can alter daughter-to-parent ratios What happens when magma solidifies and melts and its implications for radiometric dating The following quote from The Earth: Subduction means that these plates are pushed under the continents by motions of the earth's crust. Most genetic models for uranium deposits in sandstones in the U. But it could be measuring the apparent age of the ocean floor or crustal material rather than the time of the lava flow. There will be a concordance or agreement in dates obtained by these seemingly very different dating methods. For that reason, igneous rocks of granitic composition are strongly enriched in U and Th compared to rocks of basaltic or ultramafic composition. Another point is that of time. Note that this assumption implies a thorough mixing and melting of the magma, which would also mix in the parent substances as well. Subducted oceanic plates begin to melt when they reach depths of about kilometers See Tarbuck, The Earth, p. Not all radioactive isotopes follow this kind of behavior; others have irregular rate changes that still have no explanation. I also recall reading that geologists assume the initial Pb isotope ratios vary from place to place anyway. Now, this would also help the uranium to be incorporated into other minerals. I would also mention that there are some parent-to-daughter ratios and some isochrons that yield ages in the thousands of years for the geologic column, as one would expect if it is in fact very young. Another interesting fact is that isochrons can be inherited from magma into minerals. Mantle material is very low in uranium and thorium, having only 0. Then the surrounding crystal surface would continue to grow and close up the gap, incorporating a tiny amount of magma. Thus we produce the desired isochron. It's not only a matter of incorporation in minerals either, as one sometimes does whole rock isochrons and I suppose parent-daughter ratios of whole rock, which would reflect the composition of the magma and not the incorporation into minerals. I think we can build a strong case for fictitious ages in magmatic rocks as a result of fractional cystallization and geochemical processes. To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating.
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