Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN 55455;
Department of Astronomy, University of Michigan, Ann Arbor, MI 48109.
Proc Natl Acad Sci U S A. 2021 Mar 30;118(13). doi: 10.1073/pnas.2026779118.
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 modeling. 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 two 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 are 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.
在类地行星的形成过程中，挥发物可能会通过星云中的处理、行星体的分化和行星的吸积而损失。我们研究铁陨石，将其作为行星体处理过程中挥发物损失的档案。通过热力学模型重建了岩浆铁陨石母体的碳含量。计算得出的 C 在固/液合金中的分配比随着液态 S 浓度的增加而大大增加，推断出母体 C 的浓度范围在 0.0004 到 0.11 重量%之间。母体分为两个成分群，以中等和低 C/S 的核心为特征。这两者都需要显著的行星体脱气，因为前体类似球粒陨石的变质脱挥发分不足以解释它们的 C 亏损。从封闭系统提取到完全熔融体的脱气的行星体核心形成模型表明，需要进行大量的开放系统硅酸盐熔融和挥发物损失，才能匹配中等和低 C/S 母体核心成分。与 S 相比，C 的消耗更大是硅酸盐脱气的标志，这表明母体核心成分记录了影响复合硅酸盐/铁行星体的过程。仅脱除其硅酸盐幔层的裸露核心的脱气会使 S 大量消耗而 C 损失可以忽略不计，并且不能解释推断出的母体核心成分。因此，小天体分化过程中的挥发分逸出是影响源自行星体和行星胚胎的类地行星挥发物含量的关键过程。
