Fig. 7.

Thermal evolution for the heating stage for an ordinary chondrite-type of planetesimal that forms with a reduced ²⁶Al/²⁷Al ratio of 1.6 × 10⁻⁵ and accretes to a final radius of 100 km in 3 Myr. Heating profiles are shown as a function of distance from the planetesimal centre. Black curves represent temperatures, whereas green curves represent the time at which a given melting temperature is reached as a function of distance. The ambient disk temperature was assumed constant at 250 K. The green area represents the region within the planetesimal that experiences only partial melting of the source rock. This region progresses outwards as accretion proceeds. The heating profiles are not significantly influenced by heat loss through convection up to about 50% melting (McCoy et al., 2006). As such, the liquidus temperature curve may progress towards the planetesimal centre when melt migration and convection becomes faster than the heating rate. The top sketch shows a cross-section through the final onion-shell structure of the partially differentiated planetesimal, which consists of an inner Fe-Ni core overlaid bya partial melting zone comprised of olivine-rich residues produced through basaltic/pyroxenitic melt extraction. The partial melting zone is dominated by progressively more olivine-rich silicate residues towards the interior and could be represented by meteorites such as Main Group Pallasites (>50% melting) overlain by pyroxene-type pallasites (30–50% melting), followed by lodranites (5–20% melting) and acapulcoites (<5% melting). Through continuous accretion, this partially differentiated interior becomes capped by undifferentiated chondritic crust, which experiences higher degrees of metamorphism towards the inner layers.