Silicate melting and volatile loss during differentiation in planetesimals

Introduction Planetesimals ranging between tens and hundreds of kilometers in size populated the inner solar system during its first several million years (Wood, 1964; Tang and Dauphas, 2012). Direct observations of asteroids and the meteoritic record provide abundant evidence that radiogenic heating of such bodies drove the production of aqueous fluids from ice and, for some objects, the melting of metals and silicates (Castillo-Rogez and Young, Chapter 5, this volume; Kleine and Wadhwa, Chapter 11, this volume). Progressive internal heating led to a diverse array of physical and chemical changes in the host planetesimal. The potential for pore fluids to migrate over length scales greater than hand-sample size potentially led to the net transport of labile elements and isotopes in meteoritic samples (Young et al., 1999; Young et al., 2003; Bland et al., 2009; Palguta et al., 2010). Subsequent advection of silicate melts may have been an efficient mode of heat transport and holds strong implications for the survival of chondritic material in both meteorites and modern asteroid surfaces (Wilson and Keil, 2012; Fu and Elkins-Tanton, 2014; Neumann et al., 2014). Understanding the prevalence and effects of both volatile and silicate melt migration during radiogenic heating is therefore key to interpreting both meteoritic and remote-sensing data. Despite its importance, the extent of both volatile and silicate melt migration remains poorly constrained. Although the mineralogy of primitive chondrites implies the past existence of aqueous fluids, predictions about the mobility of such fluids must cope with large uncertainties in the material permeabilities, the interplay of forces driving fluid migration, and the unknown effect of large-scale fracturing. Because the volatile content of silicate melt strongly affects buoyancy, these uncertainties regarding aqueous fluid flow are propagated into models of silicate melt mobility. In this chapter, we consider in chronological sequence the behavior of volatiles and, ultimately, silicate melts during progressive internal heating of planetesimals. We review the expected thermal and volatile budget of early-forming planetesimals (Section 6.2) and summarize theoretical and observational constraints on the transport and evolution of aqueous fluids (Section 6.3), emphasizing the potential implications of fluid-rock reactions and transport for the interpretation of parent-body and nebular processes based on meteorite observations.