Introductory RemarksAuthor(s): R. HutchisonSource: Proceedings of the Royal Society of London. Series A, Mathematical and PhysicalSciences, Vol. 374, No. 1757, Meteoritic Research (Feb. 4, 1981), pp. 157-158Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/2990352 .

Accessed: 18/06/2014 00:20

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org.

.

The Royal Society is collaborating with JSTOR to digitize, preserve and extend access to Proceedings of theRoyal Society of London. Series A, Mathematical and Physical Sciences.

http://www.jstor.org

This content downloaded from 62.122.76.54 on Wed, 18 Jun 2014 00:20:49 AMAll use subject to JSTOR Terms and Conditions

Proc. R. Soc. Lond. A 374, 157-158 (1981)

Printed in Great Britain

Introductory remarks

BY R. HuTCHIsoN, F.R.S.

Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD, U.K.

For almost two centuries the study of meteorites has been associated with both technical and philosophical developments in various scientific disciplines, examples being the work of H. C. Sorby on optical mineralogy (ca. 1860-1880) and of V. M. Goldschmidt on the distribution of the chemical elements (ca. 1920-1950). The most important of these developments probably took place during the decade from 1965 to 1974. Although it is no coincidence that this period includes the time of the Apollo landings on the Moon, chance also played a part by sending to Earth the Allende and Murchison meteorite showers, without which several important discoveries would have been delayed. The years from 1975 until the present have largely seen the expansion and consolidation of data, and theories, with only a few real innovations. The time was thus ripe for a discussion on meteoritic research as an encouragement to further advances in knowledge.

Meteorites are still our unique source of direct information on the events associated with the, formation of the Solar System. Of particular importance in this respect are the chondrites, those stony meteorites that have not been melted since they formed as aggregates on small planetary bodies around 4500 Ma ago. Chondrites may be composed of up to 70 % of chondrules, which are milli- metre-sized, near-spherical, silicate bodies for which the origin is still controversial. Hypotheses include condensation from a gas (nebula) of solar composition, or fusion of primordial dust by electrical discharge. However, experimental work and observations on the mineralogy and chemical compositions are now tending to favour the formation of chondrules by the melting of pre-existing rocks of diverse composition. Chondrites, therefore, are thought to be composed dominantly of reworked materials, with a minor amount of volatiles such as lead. These volatiles may have been introduced in a solid carrier during accretion, or they may have condensed from a gas soon after accretion of solids had ceased. Since then, chon- drites have remained closed systems, except for those that exhibit evidence of a severe shock event.

Although chondrites are composed largely of reworked or altered materials, chemical closure occurred early, before short-lived radionuclides such as 244Pu had decayed to extinction. This isotope has been used by P. Pellas & D. Storzer as a radioactive 'clock' against which to measure cooling rates in the ordinary chon- drites. (These meteorites have bulk atomic Mg: Si ratios close to 0.95 and oxidation states between those of the highly reduced enstatite chondrites and the highly oxidized carbonaceous chondrites.) The values thus obtained set limits to the

[ 157 ] 6-2

This content downloaded from 62.122.76.54 on Wed, 18 Jun 2014 00:20:49 AMAll use subject to JSTOR Terms and Conditions

158 R. Hutchison (Discussion Meeting)

sizes of the parent planets of each of the three chemical groups into which the ordinary chondrites are subdivided. Such limits are less than those inferred previously from estimates of the rates of cooling by the use of computer modelling of the distribution of nickel between kamacite and taenite; this is sensitive to the (temperature-dependent) rate of diffusion of nickel. The idea that the parent planets of the ordinary chondrites became hot soon after their formation was advanced in the 1 960s to account for the 'metamorphic' textures in many ordinary chondrites. This hypothesis is now under attack from several scientists who argue that ordinary chondritic deposits formed hot and that their textures are a function of the rate at, which they cooled. The new evidence is based on the interpretation of the crystallography of pyroxenes and the structure of metal grains as the result of rapid cooling to below about 700 'C. This is probably com- patible with the thermal history deduced from 244Pu decay.

Most non-chondritic meteorites originated as melts either of iron-nickel metal or of basaltic composition. Their solidification histories have been elucidated by trace element studies, and age and isotopic data indicate that formation of these melts may pre-date the cooling of the chondrites. A major outstanding problem is the identification of the source of heat that caused asteroid-sized bodies to melt. One possibility is the short-lived isotope 26Al, evidence for which has been dis- covered in inclusions in several carbonaceous chondrites. This radionuclide may have been introduced from a nearby supernova. into the matter that evolved into the Solar System. However, the presence in carbonaceous chondrites of anomalous abundances of isotopes with no obvious radioactive progenitor may constitute an even more fundamental discovery. Some anomalies may signify the possible existence of an undiscovered superheavy element, while others may be the signature of stellar condensates that pre-date the Solar System. Yet other unusual isotopic ratios may provide information on the conditions of synthesis of carbon compounds. These, and other problems of the origin and history of the Solar System formed the basis of this Discussion Meeting, which brought together a group of physicists, chemists, geologists and astronomers who share a common interest in meteorites.

This content downloaded from 62.122.76.54 on Wed, 18 Jun 2014 00:20:49 AMAll use subject to JSTOR Terms and Conditions