The effects of bulk composition on planetesimal core sulfur content and size

This study explores the compositions and sizes of metallic cores that result from planetesimals forming from a range of chondritic bulk compositions. Our models examine the influence of starting bulk composition on core size and composition, how oxygen fugacity (fO₂), temperature, pressure, and bulk composition affect sulfur partitioning between the core and silicate mantle of planetesimals, and the formation and fate of immiscible sulfur-rich liquid during core solidification. We apply experimentally-derived equations for the sulfur distribution coefficient to the bulk compositions of ordinary chondrites (H,L,LL) and carbonaceous chondrites (CM, CI, CO, CK, CV) under conditions appropriate for melting planetesimals. The sulfur content of all modeled cores is above 6 wt% S, which is greater than the amount of sulfur needed to form an immiscible sulfide liquid in the presence of other light elements (e.g., C, Si, and/or P). We concluded that early planetesimal cores likely formed either an immiscible sulfide liquid, a eutectic sulfide liquid, or most surprisingly, were composed of mostly monosulfide solid solution, [(Fe, Ni)_1-xS].