Fig. 8.

Thermal evolution for the heating stage for an ordinary chondrite-type of source rock showing the influence of varying the ambient disk temperature, T₀, on the onset of melting. The subchondritic Al/Mg ratio of the lodranite (Al/Mg = 0.007 and μ²⁶Mg* = −9.7; Table 3), which experienced 5–20% melting (McCoy et al., 1997), suggests that melt extraction (and thus retarded ²⁶Mg* ingrowth) will occur once the source rock has experienced only a few % melting. In accordance, with models for the temperature evolution of accretion disks (e.g. Ciesla, 2008), the onset of silicate differentiation inferred from the lowest μ²⁶Mg* (gray shades) value for Main Group Pallasite olivines is best explained if the planetesimal formed from an initially hot protoplanetary disk at T₀ > 550 K.