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VIII Evidence from Meteorites

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The nature and significance of meteoritic matter. Monica M. Grady

The Natural History Museum, Cromwell Road, London SW7 5BD. UK.

Meteorites are fragments broken from asteroids, with a compositional variation that spans a whole range of planetary materials, from completely unmelted and unfractionated stony chondrites to highly fractionated and differentiated iron meteorites. Meteorites, and components within them, carry records of all stages of Solar System history.

1. INTRODUCTION

Meteorites are pieces of rock and metal that fall to the Earth. Almost all are fragments broken from asteroids during collisions, taking between around 0.2 - 100 million years to journey from the asteroid belt to the Earth. There are also currently about 20 meteorites that come from the Moon, and a similar number from Mars. Meteorites are the only physical materials available on the Earth that allow direct study of the original dust from which the Solar System formed. The relationship between meteorites and the primordial material from which they aggregated as asteroids is shown in Figure 1.

Different components within different types of meteorite can place a reasonably accurate relative chronology on many of the processes that have affected Solar System bodies; study of rare, exotic grains within meteorites can also assist in tracing the evolution of the Sun's galactic neighbourhood.

2. METEORITE CLASSIFICATION

Before discussing what can be learnt from meteorites, I will give an overview of the different types of meteorites and their classification. Meteorites exhibit a compositional variation that spans a whole range of planetary materials, from completely unmelted and unfractionated stony chondrites to highly fractionated and differentiated iron meteorites. Classification of meteorites into groups is one way of identifying materials that might be associated in space and time, e.g., through accretion in closely neighbouring regions of the solar nebula, or having suffered similar processes of heating, melting, differentiation and/or hydrothermal alteration. However, the classification scheme is incomplete, and there are many meteorites that do not fit comfortably into the framework. Clasts and inclusions within meteorites also frequently defy ready assignation to recognised meteorite groups.

Asteroidal meteorites are classified according to their gross composition, into stones (dominantly silicate rocks), irons (almost completely metal) and stony-irons (as the name suggests, an approximate mixture of silicate and metal). Sub-classification is then on the basis of the processing (melting and differentiation) that the meteorites experienced on their parent bodies (Figure 2). Unmelted stones are chondrites. Melted meteorites are subdivided into the stony achondrites, stony-irons and iron meteorites. There is then a further complicated

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