Fig. 4.

Modeled core formation effects on planetesimals from chondritic precursors compared to inferred compositions of parent body cores (SI Appendix, Table S2 and Fig. 3). Three chondritic precursor compositions, VRC (volatile-rich chondrite), PDC (partially depleted chondrite), and VDC (volatile depleted chondrite) are used (SI Appendix, Table S1 and Fig. 1). For labels to individual iron meteorite groups, see Fig. 3. In A, two different core-forming processes are modeled. “No silicate melting” (cartoon A1) shows removal of alloy+C+S from each bulk composition planetesimals ranging from 5% (more C-rich cores) to 30% metal (less C-rich). “Closed magma ocean” (cartoon A2) shows removal of core alloy from a planetesimal with a solid carapace and interior magma ocean (63), with no degassing to the surface. Calculated core compositions are for planetesimals with 5 to 30% metal, partition coefficients given in SI Appendix, Table S3, and extent of metal–silicate equilibration (“Q” in SI Appendix, Text, Eq. S1) ranging from 50 to 100%. Partition coefficients are appropriate for core formation at oxygen fugacities of IW-3 to IW-1, as well as for formation of S-rich (>15 wt%) cores. The latter are not calculated for the VDC composition, which is too S-poor to generate S-rich cores. Some of the calculated core compositions plot within the stability fields of graphite or of two liquids (SI Appendix, Text). B models a “Partially degassed magma ocean” (calculated only for PDC and VDC compositions), which is similar to the “Closed magma ocean” calculations except that the molten silicate is assumed to have partially degassed to the planetesimal surface (cartoon B). Inset illustrates this, which is assumed to be loss of 0 to 90% of either the C (“Δ = 0”, diagonal bound on each trend) or both the C and S (“ΔS = ΔC”, horizontal bound), as further described in SI Appendix, Text. (C) Modeled cores produced by segregation from a planetesimal in which alloy was segregated from a magma ocean that had equilibrated with an atmosphere produced by whole-planetesimal degassing (cartoon C). Larger planetesimals produce atmospheres with greater partial pressures, thereby enhancing volatile retention in silicate and alloy portions. For each planetesimal bulk composition (VRC, PDC, and VDC), calculations are conducted for planetesimals with diameters of 500, 400, 300, 200, 100, 50, 20, and 10 km (with symbols in plot moving from C-rich to more C-poor as diameters diminish) and for partition coefficients appropriate for IW-3 (highest C/S for each bulk composition), IW-2, and IW-1 (lowest C/S). The vertical arrow shows the putative effect of secondary degassing of bare iron cores after differentiation and fragmentation of the parent meteorite. This results in extensive loss of S and negligible C loss, increasing the C/S ratio at constant C concentration.