ACCRETION AND THERMO-CHEMICAL EVOLUTION OF TERRESTRIAL PLANETARY BODIES

Abstract

The formation of terrestrial planets is thought to have occurred through accretion from an originally cold and chemically homogeneous cloud of gas and dust. The subsequent differentiation of a terrestrial planetary body along with the associated chemical evolution are typically the outcome of whole scale melting of the planetary body, or partial melting within the body. Understanding the accretion and thermo-chemical evolution of terrestrial planetary bodies requires the knowledge of various processes operating on a planet-scale, such as mineralogical and geochemical changes, chemical fractionation of elements and bulk chemistry, isotopic homogenization and, at times, resetting, silicate melting followed by metal-silicate segregation and differentiation, impacts, and aqueous alteration or thermal metamorphism. Different achondrite meteorites are a product of these early solar system processes and offer a window in investigating and understanding these early planetary processes. Achondrite meteorites represent the crucial intermediate stages during the physicochemical evolution of planetary bodies and result in varying degrees of planetary differentiation (Mittlefehldt et al., 1996). They include two types of resulting materials. The first group includes primitive achondrites that have retained their chondritic bulk compositions and formed through low degrees of partial melting during the earliest stages of asteroidal melting. The melting probably got arrested during this stage and did not differentiate further. These include the acapulcoites, the lodranites, and the winnonaites. The second group of meteorites results from extensive differentiation and fractionation of their chondritic precursors. They mainly comprise HED meteorites, angrites, silicate clasts in mesosiderites; metal-silicate meteorites formed at the core-mantle interface, such as pallasites, and iron meteorites (Wadhwa et al., 2006).