Datação radiométrica

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Espectrômetro de massas usado para determinar as proporções de isótopos contidos em uma amostra de rocha ígnea.

A datação radiométrica usa as taxas de decaimento de certos átomos radioativos para datar rochas ou artefatos. Geólogos uniformitaristas consideram essa forma de datação como forte evidência de que a Terra tem bilhões de anos. Mas novas pesquisas feitas pelos criacionistas revelaram um grande número de problemas com a datação radiométrica. Em alguns casos, como na datação por carbono-14, a datação radioativa, na verdade, dá forte evidência para uma Terra jovem. Outros métodos, como a datação por potássio-argônio e a datação isocrónica, são baseados em suposições incorretas e são tão inconfiáveis que são inúteis.

Princípios básicos

Isótopos pai e filho comumente usados para estabelecer idades de rochas.

Muitos átomos (ou elementos) existem como numerosas variedades chamadas isótopos, sendo alguns deles radioativos, o que significa que eles decaem ao longo do tempo perdendo partículas. A datação radiométrica é baseada na taxa de decaimento desses isótopos em isótopos não-radioativos, estáveis. Para datar um objeto, os cientistas medem a quantidade de isótopos pai e isótopos filho em uma amostra, e usam a taxa de decaimento atômico para determinar sua possível idade.

Por exemplo, na série 238U-206Pb, 238U é o isótopo pai e os outros são isótopos filho. 206Pb é o isótopo filho final e o que é analisado na datação radiométrica.

Para calcular a idade da rocha, os geólogos seguem este procedimento:

  1. Measure the ratio of isotopes in the rock.
  2. Observe the rate of radioactive decay from the mother to the daughter isotope.
  3. Calculate the time required for the mother isotope to produce all the observed daughter isotope, according to this formula:

Falhou ao verificar gramática (Falha na conversão para PNG; verifique se o latex, dvips, gs e convert foram correctamente instalados): t={\frac {1}{\lambda }}\ln \left(1+{\frac {D}{P}}\right)


where:

  • t is the age of the specimen;
  • D and P are the numbers of daughter and parent isotope today;
  • λ is the decay constant for the parent atom.

The decay constant has dimensions of reciprocal seconds. In the special case in which parent and daughter atoms are present in equal quantities, the age of the specimen is the half-life of the parent isotope:

Falhou ao verificar gramática (Falha na conversão para PNG; verifique se o latex, dvips, gs e convert foram correctamente instalados): t^{{1/2}}={\frac {\ln 2}{\lambda }} [1]

Half-life (t1/2) is the amount of time required for one-half of the nuclei in a radioactive sample to decay into another kind of nucleus. [2]

Suposições

The various isotope dating methods rely upon several assumptions. They are:

  1. Known amounts of daughter isotope (usually zero) at start.
  2. No gain or loss of parent or daughter isotopes by any means other than radioactive decay (closed system).
  3. A constant decay rate.[3]

Challenging the assumption of original composition

The first assumption, that the amount of the daughter isotope in the original rock is known, is the weakest assumption. For example, K-Ar dating assumes that there was no argon in the original rock. But if there was argon in the rock when it originally formed, then the age calculated will be millions of years too high.

To understand this, recall the above formula. The greater the amount of daughter isotope, the greater the apparent age.

The proportion of argon to radioactive potassium in the sample today is observable, and the decay constant of potassium is readily calculable by measuring the amount of argon produced from the decay of 40K after a specified time. But the age of the rock and the proportion of argon to radio-potassium in the sample originally are not observable. As any first-year student of algebra soon learns, a single equation with two unknown variables cannot be solved. In fact, the above formula is far too simple, because it assumes that the amount of daughter isotope was zero at start. The formula below is a proper model that admits the possibility that some daughter isotope was present when the rock formed:

Falhou ao verificar gramática (Falha na conversão para PNG; verifique se o latex, dvips, gs e convert foram correctamente instalados): t={\frac {1}{\lambda }}\ln \left(1+{\frac {D-D_{0}}{P}}\right)


where D0 is the amount of daughter isotope present at start. In order to simplify the formula, scientists generally assume that igneous rock contains no argon when it forms, because the argon, being a noble gas, would escape from the cooling lava.

This assumption has been repeatedly falsified. Fresh volcanic rock is routinely found to have argon in it when it first cools.[4][5] In these cases, lava of a known age of no more than several thousand years (and in one case, no more than ten years) had argon in it when it formed, so that the rock was calculated by K-Ar dating to be millions of years old, even though it was known to be only thousands of years old.

"Calibração" e desconsideração de "Fósseis Fora do Lugar"

Numerous fossils have been found in strata inconsistent with the evolutionary model of Earth's history.[6] These out of place fossils would seem to pose a problem for radiometric dating methods which are still calibrated based on the position of fossils (relative dates) in the geologic column. However, these fossils are not problematic if one simply disregards their existence.

If the date generated by isotope dating analysis agrees with the conventional interpretation of the geological column, paleontologists will accept it as valid. A date that disagrees with that interpretation is dismissed as an anomaly. This is not an example of malfeasance, but rather the result of assuming that the theory of evolution has been proved reliable, and therefore these seeming anomalies are due to contamination or other causes of analytical error. These out of place fossils or rocks are not considered a reason to question the theory. This makes independent testing of these dating methods impossible, since published discrepant dates are rare.[7]

Tipos de Datação Radiométrica

  • Carbon-14 dating: Uses the ratio of 14C to 12C to determine the age of biological remains. Contrary to popular belief, Carbon-14 dating gives solid evidence for a young Earth.[8]
  • Helium diffusion: This dating method, developed by creationists, is based on the rate of Helium diffusion from zircons, which gives many rocks a maximum age of 6,000 +/- 2,000 years.[9]
  • Uranium-Lead dating
  • Potassium-argon dating: K-Ar dating was used for a long time despite being challenged by creationists for its faulty assumptions and data. It is no longer defended as reliable, even by uniformitarian geologists, because it is entirely dependent on the assumption that igneous rocks never have any argon when they initially cool, and that assumption has been repeatedly demonstrated to be false as igneous rock of known age has been "dating" to ages far older than its actual age, because there was Argon in it when it formed.[4][5]
  • Concordia dating: Concordia dating rests on the same assumptions as K-Ar, namely that there was none of the daughter isotope (in this case Lead) in the sample when it originally cooled. Like the assumption in K-Ar, however, this assumption is also unfalsifiable, making this method equally unreliable.[10]
  • Isochron dating: Isochron dating was introduced as an attempted substitute for K-Ar dating, after K-Ar's faulty assumptions were exposed. However, isochron dating bears faulty assumptions of its own. It assumes the homogeneity of the sample when it originally formed, an assumption which is always false in whole rocks, and unfalsifiable in minerals.[11]

Problemas

Artigo Principal: Problemas com a datação radiométrica

Criacionistas have responded to this challenge in varying ways and cited numerous problems with radiometric dating. Creationists admit that there is significant evidence of daughter isotopes well in excess of what could be generated by decay at contemporary observed rates within the timescale they contend to be true.

Some have proposed that the errors could be attributable to excess original daughter isotopes (though isochron dating methods minimize this) and accelerated decay caused by external phenomena. While astronomers have found that magnetars emit radiation that could cause bouts of accelerated decay, and that these bouts may be more common than originally thought, the amount of heat produced by the radiation during the short period presents a problem for creationists.

A more common approach is to allow for accelerated nuclear decay during the early portion of terrestrial history, when those elements which decay naturally were buried far below the crust (or far below the waters of the global flood, in some models), therefore dealing with the heat problem. One possibility for the accelerated decay comes with the possibility of variable speed of light. Other theories simply hypothesize that during certain periods of time God sped up the process; these are called singularities in creation science.

In addition to the above methods of dealing with this challenge, creationists have contended a whole raft of problems with both the older and newer methods of radiometric dating. They cite several examples of discordant dates when multiple methods are tried on the same rock, many anecdotes of dating techniques giving obviously wrong data (including some where rock formed after 1900 was dated as being over 3 million years, such as at Mt. Ngauruhoe[12] and Mt. St. Helens.[4] John Woodmorappe claims that discrepancy in data is prevalent, and accuses scientists of throwing out most of the inaccurate results, giving the illusion of accuracy. He also indicates how mixed families of rock can give anomalous isochron readings, some of which would indicate a negative age for certain rocks. His book, The Mythology of Modern Dating Methods, documents approximately 200 quotes by secular geologists indicating problems with the various dating methods.

Séries de Decaimento Comuns

Artigo Principal: Decaimento radioativo

Referências

  1. "Radiometric Time Scale." In Geologic Time, online edition. USGS Publications Services, December 11, 2000. Accessed October 20, 2008.
  2. Cox, H., Porch, T., Wetzel, J. Chemistry for Christian Schools. Bob Jones University Press; Greenville, South Carolina. (p.533).
  3. Morris JD. "Prologue." In Radioisotopes and the Age of the Earth, Vardiman L, Snelling AA, and Chaffin EF, eds. El Cajon, CA: Institute for Creation Research, 2000, ISBN 0932766196, p. v.
  4. 4,0 4,1 4,2 Austin SA. "Excess Argon within Mineral Concentrates from the New Dacite Lava Dome at Mount St. Helens Volcano." TJ 10(3), 1996. Accessed October 20, 2008.
  5. 5,0 5,1 Snelling AA. "'Excess Argon': The 'Achilles' Heel' of Potassium-Argon and Argon-Argon 'Dating' of Volcanic Rocks." ICR Impact 436, Jan 1999. Accessed October 20, 2008.
  6. Anomalously Occurring Fossils by John Woodmorappe. Creation Research Society Quarterly, Volume 18, March, 1982.
  7. Jensen KG. "The Case of the KBS Tuff." In The Age of the Earth, Randy S. Berg, ed. November, 1997. Accessed October 20, 2008.
  8. Whitelaw RL. "Radiocarbon Dating after Forty Years: Do Creationists See it as Supporting the Biblical Creation and Flood?" CRSQ 29(4), March 1993. Accessed October 20, 2008.
  9. Humphreys DR, Austin SA, Baumgardner JR, and Snelling AA. "Helium Diffusion Age of 6,000 Years Supports Accelerated Nuclear Decay." CRSQ 41(1), June 2004. Accessed October 20, 2008.
  10. Snelling AA. "The age of Australian uranium." Creation 4(2):44-57, June 1981. Hosted at Answers in Genesis. Accessed October 20, 2008.
  11. "Basics of Radioactive Isotope Geochemistry." Geology 656, Cornell University, January 28, 2005. Accessed October 20, 2008.
  12. Snelling AA. "Andesite Flows at Mt. Ngauruhoe, New Zealand, and the Implications for Potassium Argon 'Dating'." Presented at the Fourth International Conference on Creationism, 1998. Accessed October 20, 2008.

Further reading

The radioisotope age dating book and DVD set by the RATE group - (Buy now)

Responses to Anticreationists

Notícias

  • More Fluctuations Found in Isotopic Clocks Researchers are finding small but significant changes in isotope decay rates, and these add credibility to the idea that isotopic processes were once very different from today's processes. ICR Daily Science Updates, August 17, 2012.