Meteorites, radioactive dating

Voshage H. and Hintenberger H. (1963) The cosmic-ray exposure ages of iron meteorites as derived from the isotopic composition of potassium and the production rates of cosmogenic nuclides in the past. In Radioactive Dating, International Atomic Energy Agency, Vienna, pp. 367-379. 

Lord Rutherford [24] determined the age of a sample of pitchblende, to be 700 million years, by measuring the amount of uranium and radium and helium retained in the rock and by calculating the annual output of alpha particles. The oldest rock found is from Southwest Greenland 3.8 Gyr old [15]. Radioactive dating of meteorites point to their formation and the solidification of the earth about 4.55 0.07 years ago [25]. Since the sun and the solar system formed only slightly before, their age at isolation and condensation from the interstellar medium is taken to be 4.6 Gyr [26]. 

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

We were quite elated, and it appeared that it was a rich field. Now, fifty years later, I must say that it wasn t as rich as we thought. But we have over the years discovered half a dozen natural radioactive elements, and two of these, the samarium-147 with its decay to neodymium-143 and rhenium-187 with its decay to osmium-187, prove to be of use in Nuclear Dating. The importance of rhenium is that it is iron soluble while the other radioactivities are insoluble in metallic iron. In fact, the best half life we have for rhenium-187 was obtained by measuring the osmium-187 to rhenium-187 ratio in iron meteorites which had been dated by other methods. This work was started many years ago by Dr. Herr and others in Germany. The half life is 43,000,000,000 years. 

Nuclear dating has been most helpful in establishing the history of the earth and of the moon and of the meteorites. The fact is, there is no other way of measuring their ages. Prior to the discovery of natural radioactivity in the late 19th century, indirect methods were used to estimate the age of the earth, but there were no real answers until the radioactivity of thorium, uranium, and potassium were discovered and we began to understand atomic structure and to realize that nuclear transformation was essentially independent of the chemical form. 

The discovery of cosmic ray produced 81Kr in meteorites [l]1 introduced a new method of high sensitivity measurements of 81Kr concentrations and cosmic ray exposure dating. The method consists of a direct measurement of both radioactive 81Kr atoms (Ty2 = 2.13 x 105y, [2] and of stable spallation Kr atoms by a... 

In the 81Kr-83Kr dating method, which is discussed in the next paragraph, the production ratio of radioactive and stable Kr isotopes can be evaluated directly from the Kr spallation spectrum in a meteorite according to the relation [1] ... 

The basic idea in radioactive age-dating of rocks (from the Earth, Moon and meteorites) is to find the ratio of daughter to parent in an isolated system. Thus the age inferred is usually the solidification age which is the time since the last occasion when chemical fractionation was halted by solidification. (K-Ar dating gives a gas-retention age which can be slightly shorter.)... 

Age dating The extinct radioactivities can in many cases be used as clocks to determine when demonstrably once-molten meteorite samples solidified. Before this can be done one must know the initial amount of that radioactivity, which is usually taken to be the amount known to have existed in the oldest sample, the one to have... 

The data discussed above are for present-day abundances in the photosphere and meteorites. However, two processes affected the solar abundances over time. The first is element settling from the solar photosphere into the Sun s interior the second is decay of radioactive isotopes that contribute to the overall atomic abundance of an element. The first, discussed in the following, is more important for the sun and large-scale modeling the changes in isotopic compositions and their effects on abundances are comparably minor but important for radiometric dating. The isotopic effects are considered in the solar system abundance table in this section, but are not described at length here. 

In meteorites, short-term cosmic ray flux variations can be recognized by measuring the activity of a radionuclide with a short half-life in meteorites with a known fall date. Longer-term variations are studied by comparing apparent production rates of stable (noble gas) nuclides based on various pairs of a stable and a radioactive nuclide, or by comparing experimental and model data (see Production systematics subsection). 

The major application of TIMS is to the geochronology and tracer studies using terrestrial radiometric systems, for example, U-Th-Pb, Rb-Sr, Sm-Nd, and Lu-Hf. Geochronology exploits the radioactive decay in closed systems to obtain the date of a specific geological event. Tracer studies use the growth of daughter nuclides from radioactive decay to evaluate the interaction between geochemical systems and/or reservoirs. The application of TIMS in cosmochemical analysis is limited however, it is used to measure the isotopic compositions as tracers of nucleosynthesis processes. This includes the measurement of radionuclides observed mainly in meteorites, for example, Mn-Cr, Al-Mg, Fe-Ni, and Ca-K system in addition to the above-mentioned systems. 

For the age of rocks and meteorites, othCT similar methods of dating have been nsed. One method depends on the radioactivity of naturally occurring potassium-40, which decays by positron emission and electron capture (as well as by beta emission). 

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