The history of meteorite age determinations
(very persuasive evidence)
J. R. De Laeter
Department of Applied Physics, Curtin University, GPO Box U1987, Perth, WA 6845, Australia [email protected]
The determination of the age of the Earth has been of scientific interest over hundreds of years, but it was not until radioactivity was discovered at the close of the 19th century that the possibility of a physical estimate became possible. The discovery of isotopes, a means of measuring isotope abundances by mass spectrometry, and the establishment of the U, Th-Pb geochronological system gave impetus to the search for the age of the Earth, but many unsuccessful attempts were made before Clair Patterson measured the isotopic composition of lead in iron meteorites in 1956, to produce an age of 4550 Ma, which is still generally accepted today as an excellent estimate of the age of formation, not only of the Earth, but of the solar system itself.
A mere 4 years were then to elapse before the dawn of a new era, to decipher the timing of events in the early history of the solar system, was heralded by John Reynolds exciting discovery that excess 129Xe, a daughter product of the now extinct radionuclide 129I, was present in a stony meteotite. This enabled a formation interval, between the nucleosynthesis of elements in stars and the formation of meteorite parent bodies, to be determined. The last 40 years of the 20th century have witnessed the investigation of a wide array of short-lived radioactive systems by virtue of the fact that their respective daughter products have been identified in meteoritical material by painstaking mass spectrometric-based research, thus allowing a chronology of early solar system events to be established. This formation interval is less than a few million years.
Thus, meteorites were the key to determining both the age of formation of the Earth and of the solar system, together with the early chronology of the solar system. However, meteorites had more secrets to reveal. The third age of meteorites is a measure of the time they have spent in space. The bombardment of meteoroids by cosmic rays produces spallation products, some of which are radioactive. Despite the slow production of these radionuclides and their associated daughter products, the long periods of unprotected time spent in space allowed the accumulation of these nuclides, so that when the fragments arrived on Earth, the radioactive systems could be analysed to provide the exposure ages of the meteorites in space.
Most stony meteorites have exposure ages up to 80 Ma, stony-irons 10-180 Ma and irons up to 2300 Ma, indicating the importance of mechanical strength in their survival in space. There is also evidence of clustering of exposure ages in some meteorite classes, which provide information on the frequency of collisional events and orbital trajectories. A clustering of exposure ages at approximately equal to 7 Ma for asteroidal-sourced meteorites indicate that collisions were prevalent at that time. When mete target="_blank">orites arrive on the Earths surface, the source of radionuclide production ceases, but they continue to decay with their characteristic half-lives until retrieved for radiochemical analysis. The activity of such radioactive systems in finds, compared with corresponding meteorite falls, give their terrestrial age on the Earths surface.