Radiometric dating volcanic rock



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I now give a more Raidometric three-source homo scenario that can homo an arbitrary isochron, which could not be detected by a homo homo. These values may be taken as an homo of the very low homo of these elements in the homo and homo of the Earth.


What this does is vocanic the upper parts of the chamber volcaniv uranium and thorium, leaving the radiogenic lead. As this material leaves, that which is first out will be high in lead and low volcanoc parent isotopes. This will date oldest. Magma escaping later will date younger because it is enriched in U and Th. There will be a concordance or agreement in dates obtained by these seemingly very different dating methods. This mechanism was suggested by Jon Covey. Tarbuck and Lutgens carefully explain the process of fractional crystallization in The Earth: An Introduction to Physical Geology. They show clear drawings of crystallized minerals falling through the magma and explain that the crystallized Radiomerric do indeed fall through the magma chamber.

Further, most minerals of uranium and thorium are denser than other minerals, especially when those minerals are in the liquid phase. Crystalline solids tend to be denser than liquids from which they Radiometriv. But the degree to which they are incorporated in other minerals with high melting points might have a greater influence, since the concentrations of uranium and thorium are so low. Now another issue is simply Radiomrtric atomic weight of uranium and thorium, which is high. Any compound containing them is also likely to be heavy and sink to the bottom relative to others, even in a liquid form.

If there is significant convection in the magma, this would be minimized, however. At any rate, there will be some effects of this nature that will produce Radikmetric kinds volcannic changes in concentration of uranium and thorium relative to lead from Radiimetric top to the bottom of a magma chamber. Some of the patterns that are produced may appear to give valid radiometric dates. The latter voclanic be explained away due to various mechanisms. Let us consider processes that could cause uranium and thorium to be incorporated into minerals with a high melting point. I read that zircons absorb uranium, but not much lead.

Thus they are used for U-Pb dating. But many minerals take in a lot of uranium. It is also known that uranium is highly reactive. To me this suggests that it is eager to give up its 2 outer electrons. This would tend datimg produce compounds with a high Radiometric dating volcanic rock moment, with a Radiometric dating volcanic rock charge Radiometrric uranium and golcanic negative charge on the other elements. This would in turn tend to produce a high melting point, since the atoms would attract one another electrostatically. I'm guessing a little bit here. There are a number of uranium compounds with different melting points, and in general it seems that the ones with the highest melting points are more stable.

I would suppose that in magma, due to reactions, most of the uranium would end up in the most stable compounds with the highest melting points. These would also tend to have high dipole moments. Now, this would also help the uranium to be incorporated into other minerals. The electric charge distribution would create an attraction between the uranium compound and a crystallizing mineral, enabling uranium to be incorporated. But this would be less so for lead, which reacts less strongly, and probably is not incorporated so easily into minerals. So in the minerals crystallizing at the top of the magma, uranium would be taken in more than lead.

These minerals would then fall to the bottom of the magma chamber and thus uranium at the top would be depleted. It doesn't matter if these minerals are relatively lighter than others. The point is that they are heavier than the magma. Two kinds of magma and implications for radiometric dating It turns out that magma has two sources, ocean plates and material from the continents crustal rock. This fact has profound implications for radiometric dating. Mantle material is very low in uranium and thorium, having only 0. The source of magma for volcanic activity is subducted oceanic plates.

Subduction means that these plates are pushed under the continents by motions of the earth's crust. While oceanic plates are basaltic mafic originating from the mid-oceanic ridges due to partial melting of mantle rock, the material that is magma is a combination of oceanic plate material and continental sediments. Subducted oceanic plates begin to melt when they reach depths of about kilometers See Tarbuck, The Earth, p. In other words, mantle is not the direct source of magma. Further, Faure explains that uraninite UO sub2 is a component of igneous rocks Faure, p.

Uraninite is also known as pitchblende. According to plate tectonic theory, continental crust overrides oceanic crust when these plates collide because the continental crust is less dense than the ocean floor. As the ocean floor sinks, it encounters increasing pressures and temperatures within the crust. Ultimately, the pressures and temperatures are so high that the rocks in the subducted oceanic crust melt. Once the rocks melt, a plume of molten material begins to rise in the crust. As the plume rises it melts and incorporates other crustal rocks.

This rising body of magma is an open system with respect to the surrounding crustal rocks. It is possible that these physical processes have an impact on the determined radiometric age of the rock as it cools and crystallizes. Time is not a direct measurement. The actual data are the ratios of parent and daughter isotopes present in the sample. Time is one of the values that can be determined from the slope of the line representing the distribution of the isotopes. Isotope distributions are determined by the chemical and physical factors governing a given magma chamber. Uranium is believed to be able to incorporate itself as a trace material in many other minerals of low density, and so be relatively highly concentrated in the crust.

A lower mantle concentration of uranium is inferred because if the mantle contained the same uranium concentration as the crust, then the uranium's heat of radiactive decay would keep the crust molten. Rhyolites in Yellowstone N. Most genetic models for uranium deposits in sandstones in the U. Most of the uranium deposits in Wyoming are formed from uraniferous groundwaters derived from Precambrian granitic terranes. Uranium in the major uranium deposits in the San Juan basin of New Mexico is believed to have been derived from silicic volcanic ash from Jurassic island arcs at the edge of the continent.

From the above sources, we see that another factor influencing radiometric dates is the proportion of the magma that comes from subducted oceanic plates and the proportion that comes from crustal rock. Initially, we would expect most of it to come from subducted oceanic plates, which are uranium and thorium poor and maybe lead rich. Later, more of the crustal rock would be incorporated by melting into the magma, and thus the magma would be richer in uranium and thorium and poorer in lead. So this factor would also make the age appear to become younger with time. There are two kinds of magma, and the crustal material which is enriched in uranium also tends to be lighter.

For our topic on radiometric dating and fractional crystallization, there is nothing that would prevent uranium and thorium ores from crystallizing within the upper, lighter portion of the magma chamber and descending to the lower boundaries of the sialic portion. The upper portion of the sialic magma would be cooler since its in contact with continental rock, and the high melting point of UO sub 2 uranium dioxide, the common form in granite: The same kind of fractional crystallization would be true of non-granitic melts.

I think we can build a strong case for fictitious ages in magmatic rocks as a result of fractional cystallization and geochemical processes.

The geological processes at homo have a tremendous ability to separate different kinds of elements and minerals. In homo, the implications of phenocrysts were already well understood. Some of the neighboring rocks may have uranium and homo as well although this can be factored in in an isochron-type homo.

As we have seen, we cannot ignore geochemical effects while we consider geophysical effects. Sialic granitic datingg mafic basaltic magma are separated from each other, with uranium and thorium chemically predestined to reside mainly in sialic magma and less in datung rock. Here is yet another volcani that can cause trouble for radiometric dating: As lava rises through the crust, it rocm heat up surrounding rock. Lead has a low melting point, so it will melt early and enter the magma. This will cause an apparent large age. Uranium has a much higher melting point. It will enter later, probably due to melting of materials in which it is embedded. This will tend to lower the ages.

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. Let me make some general comments about isochrons. The idea of isochrons is that one has a parent element, P, a daughter element, D, and another isotope, N, of the daughter that is not generated by decay. One would assume that initially, the concentration of N and D in different locations are proportional, since their chemical properties are very similar. Note that this assumption implies a thorough mixing and melting of the magma, which would also mix in the parent substances as well.

Then we require some process to preferentially concentrate the parent substances in certain places. Radioactive decay would generate a concentration of D proportional to P. By taking enough measurements of the concentrations of P, D, and N, we can solve for c1 and c2, and from c1 we can determine the radiometric age of the sample. Otherwise, the system is degenerate. Thus we need to have an uneven distribution of D relative to N at the start. If these ratios are observed to obey such a linear relationship in a series of rocks, then an age can be computed from them.

The bigger c1 is, the older the rock is. That is, the more daughter product relative to parent product, the greater the age. Thus we have the same general situation as with simiple parent-to-daughter computations, more daughter product implies an older age. This is a very clever idea.

However, there are some problems with it. First, in order to have a meaningful isochron, it is necessary to have an unusual chain of events. Initially, one volcnic to have a uniform ratio of lead isotopes in the magma. Usually the concentration of uranium and thorium varies in different places in rock. This will, over the assumed Rafiometric of years, produce uneven concentrations of lead vlocanic. To even this out, one has to have a thorough mixing of the magma. Even this is problematical, unless Radjometric magma is very hot, and no external material enters.

Now, after the magma is thoroughly mixed, the uranium and thorium will also be thoroughly mixed. What has to happen next to get an isochron is that the uranium or thorium has to concentrate relative to the lead isotopes, more in some places than others. So this implies some kind of datijg fractionation. Then the system has to remain closed for a long time. This chemical fractionation will most likely arise by some minerals incorporating more or less uranium or thorium relative to lead. Anyway, to me it seems unlikely that this chain of events would occur. Another problem with isochrons is that they can occur by mixing and other processes that result in isochrons yielding meaningless ages.

Sometimes, according to Faure, what seems to be an isochron is actually a mixing line, a leftover from differentiation in the magma. Fractionation followed by mixing can create isochrons giving too old ages, without any fractionation of daughter isotopes taking place. To get an isochron with a false age, all you need is 1 too much daughter element, due to some kind of fractionation and 2 mixing of this with something else that fractionated differently. Since fractionation and mixing are so common, we should expect to find isochrons often. How they correlate with the expected ages of their geologic period is an interesting question.

There are at least some outstanding anomalies. Faure states that chemical fractionation produces "fictitious isochrons whose slopes have no time significance. As an example, he uses Pliocene to Recent lava flows and from lava flows in historical times to illustrate the problem. He says, these flows should have slopes approaching zero less than 1 million yearsbut they instead appear to be much older million years. Steve Austin has found lava rocks on the Uinkeret Plateau at Grand Canyon with fictitious isochrons dating at 1. Then a mixing of A and B will have the same fixed concentration of N everywhere, but the amount of D will be proportional to the amount of P.

This produces an isochron yielding the same age as sample A. This is a reasonable scenario, since N is a non-radiogenic isotope not produced by decay such as leadand it can be assumed to have similar concentrations in many magmas.

Dating rock Radiometric volcanic

Magma from the ocean floor has little U and little U and probably little lead byproducts lead and lead Magma from melted continental material probably has more of both U and U and lead and lead Thus we can get an isochron by mixing, that has the volcaniv of the younger-looking continental Radiometrci. The age will not roci depend on how much crust is incorporated, as long as it is non-zero. Austin to apply Geocron's potassium-argon dating to his sample of dacite known to be only six years old. But let's ask the obvious question. If there wasn't yet enough argon in the rock to be detectable, and the equipment that was used was not sensitive enough to detect any argon, how was enough argon found that such old results were returned?

There are Radkometric possible Radiometric dating volcanic rock that the old dates were returned. The first has to do with the reason Geocron's equipment was considered useful only for high concentrations of argon. There would always be a certain amount of argon volcainc the mass spectrometer left over from previous experiments. If the sample being tested is old enough to have significant argon, this leftover contamination would be statistically insignificant; so this was Radiometrci for Geocron's normal purposes. But for a sample with little or no argon, it would produce a falsely Raduometric result.

This was undoubtedly a factor in Dr. The second possibility is that so-called "excess argon" could have become trapped in the Mount St. This is where we find the bulk of the confusing complexity in Austin's paper and in those of his critics. The papers all go into great detail describing the various ways that argon-containing compounds can be incorporated into magma. These include the occlusion of xenoliths and xenocrysts, which are basically contaminants from existing old rocks that get mixed in with the magma; and phenocrysts, which are crystals of all sorts of different minerals that form inside the rock in different ways depending on how quickly the magma cools.

Page after page of chemical compositions, mineral breakdowns, charts and graphs, and all sorts of discussion of practically every last molecule found in the Mount St. Summarizing both arguments, Dr. Austin claims that xenoliths and xenocrysts were completely removed from the samples before testing, and that the wrong results are due to phenocrysts, which form to varying degrees in all magma, and thus effectively cast doubt on all potassium-argon testing done throughout the world. It's important to note that his arguments are cogent and are based on sound geology, and are often mischaracterized by skeptics. He did not simply use the wrong kind of radiometric dating as an ignorant blunder.

He was deliberately trying to illustrate that even a brand-new rock would show an ancient age, even when potassium-argon dating was properly used. Austin's critics charge that he ignored the probable likelihood that the limitations of Geochron's equipment accounts for the results, just as Geochron warned. Carbon is a radioactive isotope of carbon, with a half-life of 5, years, [25] [26] which is very short compared with the above isotopes and decays into nitrogen. Carbon, though, is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere and thus remains at a near-constant level on Earth.

The carbon ends up as a trace component in atmospheric carbon dioxide CO2. A carbon-based life form acquires carbon during its lifetime. Plants acquire it through photosynthesisand animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to take in new carbon, and the existing isotope decays with a characteristic half-life years. The proportion of carbon left when the remains of the organism are examined provides an indication of the time elapsed since its death. This makes carbon an ideal dating method to date the age of bones or the remains of an organism. The carbon dating limit lies around 58, to 62, years. However, local eruptions of volcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon and give inaccurate dates.

The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon by a few percent; conversely, the amount of carbon was increased by above-ground nuclear bomb tests that were conducted into the early s.

Also, an increase in the solar wind or the Earth's magnetic field above the current value would depress the amount of carbon created in the atmosphere. Fission track dating method[ edit ] Main article: This involves inspection of a polished slice of a material to determine the density of "track" markings left in it by the spontaneous fission of uranium impurities. The uranium content of the sample has to be known, but that can Radiometric dating volcanic rock determined by placing a plastic film over the polished slice of the material, and bombarding it with slow neutrons. This causes induced fission of U, as opposed to the spontaneous fission of U. The fission tracks produced by this process are recorded in the plastic film.

The uranium content of the material can then be calculated from the number of tracks and the neutron flux. This scheme has application over a wide range of geologic dates. For dates up to a few million years micastektites glass fragments from volcanic eruptionsand meteorites are best used. Older materials can be dated using zirconapatitetitaniteepidote and garnet which have a variable amount of uranium content. The technique has potential applications for detailing the thermal history of a deposit. The residence time of 36Cl in the atmosphere is about 1 week.

Thus, as an event marker of s water in soil and ground water, 36Cl is also useful for dating waters less than 50 years before the present. Luminescence dating methods[ edit ] Main article: Luminescence dating Luminescence dating methods are not radiometric dating methods in that they do not rely on abundances of isotopes to calculate age. Instead, they are a consequence of background radiation on certain minerals. Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C graph. Click on the "Show Movie" button below to view this animation.

How are Samples Processed? Clicking on the "Show Movie" button below will bring up an animation that illustrates how a K-Ar sample is processed and the calculations involved in arriving at a date. This is actually a mini-simulator, in that it processes a different sample each time and generates different dates. K-Ar Processing Limitations on K-Ar Dating The Potassium-Argon dating method is an invaluable tool for those archaeologists and paleoanthropologists studying the earliest evidence for human evolution. As with any dating technique, there are some significant limitations.

The technique works well for almost any igneous or volcanic rock, provided that the rock gives no evidence of having gone through a heating-recrystallization process after its initial formation.


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