Chronology of planetesimal differentiation

Introduction The differentiation of planetesimals from a loose aggregate of diverse solar nebula components into a compact and compositionally layered body consisting of core, mantle, and crust is one of the most fundamental processes in the early solar system. Differentiation involves the physical separation of metal and silicate through the segregation of dense metal melts towards the center, forming a core, and the upward migration of silicate melts, forming a crust. Direct evidence for these processes comes from differentiated meteorites as they include samples derived from the core, mantle, or crust of extensively melted bodies (McCoy et al., 2006; Mittlefehldt et al., 1998). For example, while magmatic iron meteorites formed by fractional crystallization inside a metal core (Scott, 1972; Scott and Wasson, 1975), eucrites and angrites are basaltic magmas that crystallized on or near the surface of differentiated bodies (Mittlefehldt et al., 1998). Dating such samples, therefore, makes it possible to determine the timescales of differentiation. This information is key for understanding the physical and chemical conditions of melting and melt segregation, the identification of the heat source(s) for melting, and constraining the timescales of planetesimal accretion. Obtaining such constraints is important not only for understanding the formation and evolution of planetesimals, but may also help to assess the fundamental question of why some planetesimals are differentiated while others - such as the parent bodies of chondritic meteorites - are not. In this chapter, we review the chronology of the accretion and chemical differentiation of planetesimals as derived from isotopic studies of differentiated meteorites. We do not intend to provide a comprehensive summary of available chronological data for meteorites, but rather focus on age information most relevant for understanding the differentiation of planetesimals. To this end, this review is almost entirely based on constraints from magmatic iron meteorites, eucrites and angrites, which in terms of chronology are by far the best studied differentiated meteorites. We will first summarize and explain some of the dating methods used to investigate the timescales of differentiation and will then discuss the chronology of core formation and mantle-crust differentiation on meteorite parent bodies.