U series dating problems. Uranium

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Uranium

u series dating problems

Initially, one has to have a uniform ratio of lead isotopes in the magma. Uranium has a much higher melting point. They found similar excess radium at Mount St. Lava erupting earlier would come from the top of the magma chamber, and lava erupting later would come from lower down. Lead could easily reside in impurities and imperfections in the crystal structure. Initially, we would expect most of it to come from subducted oceanic plates, which are uranium and thorium poor and maybe lead rich. The mineral makeup of an igneous rock is ultimately determined by the chemical composition of the magma from which it crystallized.

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Uranium

u series dating problems

The rest of the mixing comes from source 3. Back to their method of dating the paintings: Anyone who has been to a cave knows stalactites, stalagmites, columns, and draperies form gradually in caves. This is how geologists explain away the old isochron at the top of the Grand Canyon. Equilibrium Imagine a bucket with a hole in it. At any stage in the crystallization process the melt might be separated from the solid portion of the magma. So this is a valid mixing, and we are done. Thus we can assume that zircons would incorporate some lead in their impurities, potentially invalidating uranium-lead dates obtained from zircons.


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Uranium

u series dating problems

So we see that it is actually not much harder to get an isochron yielding a given age than it is to get a single rock yielding a given age. Quick links to Footnotes: 1 Pike, et al. But, is this test always done? U-Pb dating attempts to get around the lack of information about initial daughter concentrations by the choice of minerals that are dated. Subduction means that these plates are pushed under the continents by motions of the earth's crust. The 234Th decays to 234Pa in a matter of months, which almost instantly decays to 234U; so it is pointless to try to measure the thorium or protactinium. The concentration of Pb is usually so much higher than U, that a 2- to 3-fold increase of U doesn't change the percent composition much e. Discrepancies between multiple 14C determinations on a single painted motif have been common, as are discrepancies between the dates of different chemical e.

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U

u series dating problems

To me this indicates a high melting point for its minerals, as those with a low melting point might be expected to concentrate in the magma remaining after others crystallized out. What this does is deplete the upper parts of the chamber of uranium and thorium, leaving the radiogenic lead. To even this out, one has to have a thorough mixing of the magma. For example, zircons are thought to accept little lead but much uranium. Can one really assume that no thorium was present in the water that evaporated to form the flowstone? Therefore, when the water evaporates in the cave, leaving the minerals behind on the cave wall, there is some uranium but no thorium when the flowstone first formed. To be cautious therefore, we base our interpretation of the dates for samples O-21 and O-48 on dates corrected with our assumed rather than measured detrital value.

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More Bad News for Radiometric Dating

u series dating problems

Some process is causing the differences in the ratios of these magmatic rocks. There is a simple relationship between tau and the half-life. Each method is named after the isotopes measured to obtain the date, mostly a daughter and its parent. Understanding the equilibrium for a given decay series, helps scientists estimate the amount of radiation that will be present at various stages of the decay. All we need to know is, surprisingly, found in a table on the New York State Department of Health website. So this factor would also make the age appear to become younger with time.

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U

u series dating problems

I read that zircons absorb uranium, but not much lead. We now have so many things that can make radiometric dating go wrong, and isochrons don't remedy the situation at all, that I think the weight of evidence of radiometric dating is nullified. Principles of Isotope Geology: John Wiley and Sons, Inc. Eventually the rate that the water is leaking out of the bucket equals the rate at which the hose is filling it, so the water level remains the same. As an example, he uses Pliocene to Recent lava flows and from lava flows in historical times to illustrate the problem. It is also known that uranium is highly reactive.

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(PDF) U

u series dating problems

If this is so, the magma would initially be poor in thorium and uranium and rich in lead, and as it cooled it would become rich in thorium and uranium and poor in lead. In many instances the melt which has migrated from the initial magma chamber will undergo further segregation. The general idea is that many different minerals are formed, which differ from one another in composition, even though they come from the same magma. The actual data are the ratios of parent and daughter isotopes present in the sample. This process will generate an igneous rock of yet another composition. How much thorium would it take to produce a false old age for a modern formation? This could produce an arbitrary isochron, so this mixing could not be detected.

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More Bad News for Radiometric Dating

u series dating problems

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. More U is found in carbonate rocks, while Th has a very strong preference for granites in comparison. Would this cause trouble for our explanation? 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. Suppose also that N2 and N3 differ significantly. Evolutionary Anthropology: Issues, News, and Reviews. In other words, mantle is not the direct source of magma.

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U

u series dating problems

It is known that the crystal structure of zircons does not accept much lead. He was also not happy with the published dates on the flows in the Nevada Atomic Bomb Test site where one of the volcanic flows showed a reversal of isotope ratios and gave a value of 20,000 years in the future! Such a large variety of igneous rocks exists that it is logical to assume an equally large variety of magmas must also exist. Time to Equilibrium In order to figure out how long it takes to reach equilibrium, we need to know decay rates. One obvious problem with this is that one has to scrape part of the painting off the wall in order to analyze the carbon content, destroying part of the painting. Just a small measurement error in Y results in huge errors in the calculated age. And now radiometric dating has had its foundation removed from under it.

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More Bad News for Radiometric Dating

u series dating problems

Mechanisms that can create isochrons giving meaningless ages: Geologists attempt to estimate the initial concentration of daughter product by a clever device called an isochron. Illustration of how the earliest formed minerals can be separated from a magma by settling. Anyway, to me it seems unlikely that this chain of events would occur. Although these minerals crystallize in the order shown, this sequence is not a true reaction series. From my reading, isochrons are generally not done, as they are expensive. A number of processes could cause the parent substance to be depleted at the top of the magma chamber, or the daughter product to be enriched, both of which would cause the lava erupting earlier to appear very old according to radiometric dating, and lava erupting later to appear younger. For that reason, igneous rocks of granitic composition are strongly enriched in U and Th compared to rocks of basaltic or ultramafic composition.

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