  During the formation of terrestrial planets, volatile loss may occur through
nebular processing, planetesimal differentiation, and planetary accretion. We
investigate iron meteorites as an archive of volatile loss during planetesimal
processing. The carbon contents of the parent bodies of magmatic iron
meteorites are reconstructed by thermodynamic modelling. Calculated
solid/molten alloy partitioning of C increases greatly with liquid S
concentration and inferred parent body C concentrations range from 0.0004 to
0.11 wt.%. Parent bodies fall into 2 compositional clusters characterized by
cores with medium, and low C/S. Both of these require significant planetesimal
degassing, as metamorphic devolatilization on chondrite-like precursors is
insufficient to account for their C depletions. Planetesimal core formation
models, ranging from closed system extraction to degassing of a wholly molten
body, show that significant open system silicate melting and volatile loss is
required to match medium and low C/S parent body core compositions. Greater
depletion in C relative to S is the hallmark of silicate degassing, indicating
that parent body core compositions record processes that affect composite
silicate/iron planetesimals. Degassing of bare cores stripped of their silicate
mantles would deplete S with negligible C loss, and could not account for
inferred parent body core compositions. Devolatilization during small-body
differentiation is thus a key process in shaping the volatile inventory of
terrestrial planets derived from planetesimals and planetary embryos.
