<scp>LIME</scp> silicates in amoeboid olivine aggregates in carbonaceous chondrites: Indicator of nebular and asteroidal processes

<jats:title>Abstract</jats:title><jats:p>MnO/FeO ratios in olivine from amoeboid olivine aggregates (<jats:styled-content style="fixed-case">AOA</jats:styled-content>s) reflect conditions of nebular condensation and can be used in concert with matrix textures to compare metamorphic conditions in carbonaceous chondrites. <jats:styled-content style="fixed-case">LIME</jats:styled-content> (low‐iron, Mn‐enriched) olivine was identified in <jats:styled-content style="fixed-case">AOA</jats:styled-content>s from Y‐81020 (<jats:styled-content style="fixed-case">CO</jats:styled-content>3.05), Kaba (<jats:styled-content style="fixed-case">CV</jats:styled-content>~3.1), and in Y‐86009 (<jats:styled-content style="fixed-case">CV</jats:styled-content>3), Y‐86751 (<jats:styled-content style="fixed-case">CV</jats:styled-content>3), <jats:styled-content style="fixed-case">NWA</jats:styled-content> 1152 (<jats:styled-content style="fixed-case">CR</jats:styled-content>/<jats:styled-content style="fixed-case">CV</jats:styled-content>3), but was not identified in <jats:styled-content style="fixed-case">AOA</jats:styled-content>s from Efremovka (<jats:styled-content style="fixed-case">CV</jats:styled-content>3.1–3.4) or Allende (<jats:styled-content style="fixed-case">CV</jats:styled-content>>3.6). According to thermodynamic models of nebular condensation, <jats:styled-content style="fixed-case">LIME</jats:styled-content> olivine is stable at lower temperatures than Mn‐poor olivine and at low oxygen fugacities (dust enrichment <10× solar). Although this set of samples does not represent a single metamorphic sequence, the higher subtypes tend to have <jats:styled-content style="fixed-case">AOA</jats:styled-content> olivine with lower Mn/Fe, suggesting that Mn/Fe decreases during parent body metamorphism. Y‐81020 has the lowest subtype and most forsteritic <jats:styled-content style="fixed-case">AOA</jats:styled-content> olivine (Fo<jats:sub>>95</jats:sub>) in our study, whereas Efremovka <jats:styled-content style="fixed-case">AOA</jats:styled-content>s are slightly Fe‐rich (Fo<jats:sub>>92</jats:sub>). <jats:styled-content style="fixed-case">AOA</jats:styled-content> olivines from Kaba are mostly forsteritic, but rare Fe‐rich olivine precipitated from an aqueous fluid. A combination of precipitation of Fe‐rich olivine and diffusion of Fe into primary olivine grains resulted in iron‐rich compositions (Fo<jats:sub>97–59</jats:sub>) in Allende <jats:styled-content style="fixed-case">AOA</jats:styled-content>s. Variations from fine‐grained, nonporous matrix toward higher porosity and coarser lath‐like matrix olivine can be divided into six stages represented by (1) Y‐81020, Efremovka, <jats:styled-content style="fixed-case">NWA</jats:styled-content> 1152; (2) Y‐86751 lithology B; (3) Y‐86009; (4) Kaba; (5) Y‐86751 lithology A; (6) Allende. These stages are inferred to represent general degree of metamorphism, although the specific roles of thermally driven grain growth and diffusion versus aqueous dissolution and precipitation remain uncertain.</jats:p>