Chondrules and calcium–aluminium-rich inclusions (CAIs)
G. J. H. McCall, 2006. "Chondrules and calcium–aluminium-rich inclusions (CAIs)", The History of Meteoritics and Key Meteorite Collections: Fireballs, Falls and Finds, G.J.H. McCall, A.J. Bowden, R.J. Howarth
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Chondrules were first recognized in 1802 by Edward Howard as ‘rounded globules’, but their uniqueness to meteorites was not appreciated until 1863, when Henry Sorby produced excellent microscopic descriptions and Gustav Rose distinguished chondritic meteorites from achondrites. The Rose-Tschermak-Brezina classification was devised by 1885 and was used widely until George Prior, in 1916, replaced it with a simpler classification of chondrites. With this the distinction of ordinary chondrites from carbonaceous chondrites as applied today was formalized. This in turn was widely used until the Space Age of the last half of the 20th century when new classifications of meteorites were proposed based on the work of Keil and Fredriksson, Van Schmus and Wood. Gooding and Keil produced a classification with abundance values for types of chondrules in 1981, and Stöffler, Keil and Scott in 1991 added a classification of shock metamorphism for ordinary chondrites. The fall of the Allende meteorite in Mexico, a CV3 class chondrite, initiated prolific studies on calcium-aluminium-rich inclusions (CAIs) of refractory minerals, which may contain daughter isotopes of extinct parents from the presolar and earliest solar history. Presolar grains such as minute diamonds are recognized in the matrix of chondrules. Radiometric dating has shown that the first chondrules and CAIs formed at about the same time, c. 4566–4567 Ma and chondrules appear to have been forming over 3–5 Ma prior to accretion of meteoritic parent bodies. The origin of chondrules has been covered by a proliferation of hypotheses (some absurd, many well thought out), but in the last 20 years a general consensus seems to have been arrived at that chondrules formed in the outer regions of the solar disk very early on where shock processes raised temperature, in regions where the pressures and concentrations of solid material were higher than the canonical solar value. There was some melting early on but no crystallization from a magmatic melt within the accreted body, as in the case of howardite, eucrite and diogenite (HED) achondrites. Such transient heating models remain incompletely formulated and open to objection on observational astronomy or astrophysical grounds, and a recent alternative model linking chondrule formation with the early active Sun is also being developed. Recent research has suggested that, although ordinary chondrites are the commonest of meteorite classes falling to Earth, this may relate to the fact that we sample in this way only meteorites related to asteroids (Apollos, Amors) with near-Earth passing orbits, and carbonaceous chondrites may be the norm in the main asteroid zone between Mars and Jupiter. Attempts to identify ordinary chondrite parent bodies among asteroids by spectrographic and albedo-based methods have proved unrewarding and there is a need to develop the camera-based detection of fireball traces in the sky alongside collection of the fallen meteorite, only three such cases being up to quite recently recorded for chondrites. There may be as many as 134 asteroidal parent bodies of meteorites, and the H, L, LL and E classes of common chondrites may just represent a few different bodies among these. Sorby’s attribution of chondrules to a ‘fiery rain’ in 1877 and his attribution of them to outer regions of the solar system then occupied by the Sun’s disk appear remarkably percipient from one using only his microscope and quite unaware of the complexities of the solar system that are familiar to 21st century scientists.
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This Special Publication has 24 papers with an international authorship, and is prefaced by an introductory overview which presents highlights in the field. The first section covers the acceptance by science of the reality of the falls of rock and metal from the sky, an account that takes the reader from BCE (before common era) to the nineteenth century. The second section details some of the world's most important collections in museums - their origins and development. The Smithsonian chapter also covers the astonishingly numerous finds in the cold desert of Antarctica by American search parties. There are also contributions covering the finds by Japanese parties in the Yamato mountains and the equally remarkable discoveries in the hot deserts of Australia, North Africa, Oman and the USA. The other seven chapters take the reader through the revolution in scientific research on meteoritics in the later part of the twentieth century, including terrestrial impact cratering and extraordinary showers of glass from the sky; tektites, now known to be Earth-impact-sourced. Finally, the short epilogue looks to the future.
The History of Meteoritics and Key Meteorite Collections should appeal to historians of science, meteoriticists, geologists, astronomers, curators and the general reader with an interest in science.