Much of this mass arrives in larger bodies (planetesimals or planetary embryos) that differentiated soon after formation ( 5). Addition of volatiles to nascent planets varies during accretion as protoplanetary systems become dynamically excited, contributing material originating from different heliocentric distances ( 3) and with different thermal histories. Formation of volatile-poor planets from a volatile-rich protoplanetary disk is a result of processes in the solar nebula, in accretion of precursor solids, and in interior differentiation. Major volatiles (H, C, N, and S) are inherently plentiful in the interstellar medium and abundant in primitive carbonaceous chondrites (CCs) ( 1, 2), but are scarce in terrestrial planets, which gained most of their mass from the inner parts of the solar nebula ( 3, 4). Devolatilization during small-body differentiation is thus a key process in shaping the volatile inventory of terrestrial planets derived from planetesimals and planetary embryos. 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. 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. 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 are required to match medium and low C/S parent body core compositions.
Both of these require significant planetesimal degassing, as metamorphic devolatilization on chondrite-like precursors is insufficient to account for their C depletions. Parent bodies fall into two compositional clusters characterized by cores with medium and low C/S. 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%. The carbon contents of the parent bodies of magmatic iron meteorites are reconstructed by thermodynamic modeling.
#Iron meteorite archive
We investigate iron meteorites as an archive of volatile loss during planetesimal processing. During the formation of terrestrial planets, volatile loss may occur through nebular processing, planetesimal differentiation, and planetary accretion.
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