Harry Y. McSween

Cosmochemistry: Meteorites: a record of nebular and planetary processes

Overview Most meteorites (~86%) are chondrites, which are primitive rocks that have elemental compositions similar to that of the Sun. They are named for the millimeter-sized droplets of quenched silicate melt, called chondrules, that are particularly abundant in these meteorites. Chondrites come from the asteroid belt. They are physical mixtures of accreted components (chondrules, refractory inclusions, metal and sulfide, dust) whose characteristics and mineralogy we will describe. The classification of chondrites utilizes a combination of primary features (bulk chemical composition, oxygen isotopes) and secondary features (thermal metamorphism or aqueous alteration). About 14% of meteorites arriving on Earth consist of differentiated materials. Their compositions have been affected by melting and crystallization, and they include various kinds of achondritic meteorites, irons, and stony irons from the asteroid belt, along with lunar samples and Martian meteorites. Achondrites, so-named because they do not contain chondrules, are divided into two types. Primitive achondrites are the solid residues left behind after extraction of partial melts from chondritic material. Magmatic achondrites crystallized from the extracted melts or from completely melted and differentiated chondritic material. Irons and stony irons formed by separation of molten metal/sulfide from silicates. Lunar rocks include cumulates from the ancient highlands and mare basalts that flooded huge craters. Martian meteorites include basalts and cumulates formed from basaltic magmas. Primitive versus differentiated Cosmochemistry is the study of the chemical compositions of various solar system materials. Chondrites are the most abundant primitive samples.