How many different types of radiometric dating are there

These are K-Ar data obtained on glauconite, a potassium-bearing clay mineral that forms in some marine sediment. Woodmorappe fails to mention, however, that these data were obtained as part of a controlled experiment to test, on samples of known age, the applicability of the K-Ar method to glauconite and to illite, another clay mineral. He also neglects to mention that most of the 89 K-Ar ages reported in their study agree very well with the expected ages.

Evernden and others 43 found that these clay minerals are extremely susceptible to argon loss when heated even slightly, such as occurs when sedimentary rocks are deeply buried. As a result, glauconite is used for dating only with extreme caution. The ages from the Coast Range batholith in Alaska Table 2 are referenced by Woodmorappe to a report by Lanphere and others Whereas Lanphere and his colleagues referred to these two K-Ar ages of and million years, the ages are actually from another report and were obtained from samples collected at two localities in Canada, not Alaska.

There is nothing wrong with these ages; they are consistent with the known geologic relations and represent the crystallization ages of the Canadian samples. The Liberian example Table 2 is from a report by Dalrymple and others These authors studied dikes of basalt that intruded Precambrian crystalline basement rocks and Mesozoic sedimentary rocks in western Liberia.

The dikes cutting the Precambrian basement gave K-Ar ages ranging from to million years Woodmorappe erroneously lists this higher age as million years , whereas those cutting the Mesozoic sedimentary rocks gave K-Ar ages of from to million years.

Woodmorappe does not mention that the experiments in this study were designed such that the anomalous results were evident, the cause of the anomalous results was discovered, and the crystallization ages of the Liberian dikes were unambiguously determined.

The Liberian study is, in fact, an excellent example of how geochronologists design experiments so that the results can be checked and verified. The final example listed in Table 2 is a supposed 34 billion-year Rb-Sr isochron age on diabase of the Pahrump Group from Panamint Valley, California, and is referenced to a book by Faure and Powell Again, Woodmorappe badly misrepresents the facts.

The data do not fall on any straight line and do not, therefore, form an isochron. The original data are from a report by Wasserburg and others , who plotted the data as shown but did not draw a billion-year isochron on the diagram. This scatter shows clearly that the sample has been an open system and that its age cannot be determined from these data. Radiometric ages on related formations indicate that the Pahrump diabase is about 1.

Original data from Wasserburg and others As discussed above, one feature of the Rb-Sr isochron diagram is that, to a great extent, it is self-diagnostic. The scatter of the data in Figure 6 shows clearly that the sample has been an open system to 87Sr and perhaps to other isotopes as well and that no meaningful Rb-Sr age can be calculated from these data.

This conclusion was clearly stated by both Wasserburg and others and by Faure and Powell First, the lead data that Kofahl and Segraves 77 cite, which come from a report by Oversby , are common lead measurements done primarily to obtain information on the genesis of the Reunion lavas and secondarily to estimate when the parent magma the lava was derived from was separated from primitive mantle material.

These data cannot be used to calculate the age of the lava flows and no knowledgeable scientist would attempt to do so. We can only speculate on where Kofahl and Segraves obtained their numbers.

The age of 3. The approximate age of , years was the mean of the results from four samples from the Island of Hawaii, which is much younger than Kauai. The Kilauea Submarine Pillow Basalts Many of the rocks seem to have inherited Ar40 from the magma from which the rocks were derived. Volcanic rocks erupted into the ocean definitely inherit Ar40 and helium and thus when these are dated by the KAr40 clock, old ages are obtained for very recent flows.

For example, lavas taken from the ocean bottom off the island [sic] of Hawaii on a submarine extension of the east rift zone of Kilauea volcano gave an age of 22 million years, but the actual flow happened less than years ago.

Two studies independently discovered that the glassy margins of submarine pillow basalts, so named because lava extruded under water forms globular shapes resembling pillows, trap 40Ar dissolved in the melt before it can escape 36 , This effect is most serious in the rims of the pillows and increases in severity with water depth. The excess 40Ar content approaches zero toward pillow interiors, which cool more slowly and allow the 40Ar to escape, and in water depths of less than about meters because of the lessening of hydrostatic pressure.

The purpose of these two studies was to determine, in a controlled experiment with samples of known age, the suitability of submarine pillow basalts for dating, because it was suspected that such samples might be unreliable.

Such studies are not unusual because each different type of mineral and rock has to be tested carefully before it can be used for any radiometric dating technique. In the case of the submarine pillow basalts, the results clearly indicated that these rocks are unsuitable for dating, and so they are not generally used for this purpose except in special circumstances and unless there is some independent way of verifying the results.

Turner, however, made no such comment about excess argon in lunar rocks, and there are no data in his report on which such a conclusion could be based. The statement by Rofahl and Segraves 77 is simply unjustifiable.

The Flow from Hualalai Volcano Volcanic rocks produced by lava flows which occurred in Hawaii in the years were dated by the potassium-argon method. Excess argon produced apparent ages ranging from million to 2. The flow is unusual because it carries very abundant inclusions of rocks foreign to the lava. These inclusions, called xenoliths meaning foreign rocks , consist primarily of olivine, a pale-green iron-magnesium silicate mineral.

They come from deep within the mantle and were carried upward to the surface by the lava. In the field, they look like large raisins in a pudding and even occur in beds piled one on top of the other, glued together by the lava.

The study by Funkhouser and Naughton 51 was on the xenoliths, not on the lava. The xenoliths, which vary in composition and range in size from single mineral grains to rocks as big as basketballs, do, indeed, carry excess argon in large amounts. Quite simply, xenoliths are one of the types of rocks that cannot be dated by the K-Ar technique. Funkhouser and Naughton were able to determine that the excess gas resides primarily in fluid bubbles in the minerals of the xenoliths, where it cannot escape upon reaching the surface.

Studies such as the one by Funkhouser and Naughton are routinely done to ascertain which materials are suitable for dating and which are not, and to determine the cause of sometimes strange results. They are part of a continuing effort to learn. Two extensive K-Ar studies on historical lava flows from around the world 31 , 79 showed that excess argon is not a serious problem for dating lava flows.

In nearly every case, the measured K-Ar age was zero, as expected if excess argon is uncommon. An exception is the lava from the Hualalai flow, which is so badly contaminated by the xenoliths that it is impossible to obtain a completely inclusion-free sample.

Initial 87Sr There is really no valid way of determining what the initial amounts of Sr87 in rocks were. As discussed above in the section on Rb-Sr dating the simplest form of Rb-Sr dating i. Such samples are rare, and so nearly all modern Rb-Sr dating is done by the isochron method.

The beauty of the Rb-Sr isochron method is that knowledge of the initial Sr isotopic composition is not necessary it is one of the results obtained. A second advantage of the isochron method is that it contains internal checks on reliability. Look again at the isochron for the meteorite Juvinas Figure 3. The data are straightforward albeit technically complex measurements that fall on a straight line, indicating that the meteorite has obeyed the closed-system requirement.

The decay constants used in the calculations were the same as those in use throughout the world in The age of 4. Initial 40Ar There is far too much Ar40 in the earth for more than a small fraction of it to have been formed by radioactive decay of K This is true even if the earth were really 4. In the atmosphere of the earth, Ar40 constitutes This is around times the amount that would be generated by radioactive decay over the hypothetical 4.

Certainly this is not produced by an influx from outer space. Thus it would seem that a large amount of Ar40 was present in the beginning.

Since geochronologists assume that errors due to presence of initial Ar40 are small, their results are highly questionable.

First, there is not more 40Ar in the atmosphere than can be accounted for by radioactive decay of 40K over 4. An amount of 40Ar equivalent to all the 40Ar now in the atmosphere could be generated in 4. Current estimates of the composition of the Earth indicate that the crust contains about 1.

The 40Ar content of the atmosphere is well known and is 6. Thus, the Earth and the atmosphere now contain about equal amounts of 40Ar, and the total could be generated if the Earth contained only ppm potassium and released half of its 40Ar to the atmosphere. Second, there have been sufficient tests to show that during their formation in the crust, igneous and metamorphic rocks nearly always release their entrapped 40Ar, thus resetting the K-Ar clock.

In addition, scientists typically design their experiments so that anomalous results, such as might be caused by the rare case of initial 40Ar, are readily apparent. The study of the Liberian diabase dikes, discussed above, is a good example of this practice. First, if it is assumed that there is a uniform distribution of Sr87 in the rock, then it is assumed that there is also a uniform distribution of Rb It only requires that the Sr isotopic composition, i.

Even though the various minerals will incorporate different amounts of Sr as they cool and form, the Sr isotopic composition will be the same because natural processes do not significantly fractionate isotopes with so little mass difference as 87Sr and 86Sr.

Second, Slusher has confused isotopes and elements. Rb and Sr are quite different elements and are incorporated into the various minerals in varying proportions according to the composition and structure of the minerals.

There is no way to correct for this natural isotopic variation since there is no way to determine it. This renders the RbSr87 series useless as a clock. He has used an invalid analogy and come to an erroneous conclusion. Isochrons and Mixing Lines Arndts and Overn 8 and Kramer and others 78 claim that Rb-Sr isochrons are the result of mixing, rather than of decay of 87Rb over long periods: It is clear that mixing of pre-existent materials will yield a linear array of isotopic ratios.

We need not assume that the isotopes, assumed to be daughter isotopes, were in fact produced in the rock by radioactive decay. Thus the assumption of immense ages has not been proven. The straight lines, which seem to make radiometric dating meaningful, are easily assumed to be the result of simple mixing.

Kramer and others 78 and Arndts and Overn 8 have come to an incorrect conclusion because they have ignored several important facts about the geochemistry of Rb-Sr systems and the systematics of isochrons. First, the chemical properties of rubidium and strontium are quite different, and thus their behavior in minerals is dissimilar. Both are trace elements and rarely form minerals of their own.

It is chemically similar to potassium and tends to substitute for that element in minerals in which potassium is a major constituent, such as potassium feldspar and the micas muscovite and biotite.

It commonly substitutes for calcium in calcium minerals, such as the plagioclase feldspars. The chemical properties of rubidium and strontium are so dissimilar that minerals which readily accept rubidium into their crystal structure tend to exclude strontium and vice versa. Thus, rubidium and strontium in minerals tend to be inversely correlated; minerals high in rubidium are generally low in strontium and vice versa.

This relation, however, is a natural consequence of the chemical behavior of rubidium and strontium in minerals and of the decay of 87Rb to 87Sr over time, and has nothing to do with mixing.


Radiometric Dating. How radiometric dating works in general. Why methods in general are inaccurate. Why K-Ar dating is inaccurate. The branching ratio problem. hcqn.tk is the place to go to get the answers you need and to ask the questions you want.

Total 1 comments.
#1 21.10.2018 в 18:13 I_Luv_Converse:
I think a deep note