On the significance of oxygen-isotope variations in chondrules from carbonaceous chondrites

Libourel, G.; Nagashima, K.; Portail, Marc; Krot, Alexander N.

doi:10.1016/j.gca.2023.01.026

Cited by 6 publications

(1 citation statement)

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“…Ferroan or previously ferroan relicts, while isotopically related to enstatite chondrites (as found, e.g., as FeO‐bearing pyroxenes reported by Weisberg et al., 2011) could not have formed during melting at the forsterite–enstatite peritectic under reducing conditions but must be inherited from a more oxidized, FeO‐bearing reservoir (e.g., Jacquet et al., 2015). Such a reservoir may have become oxidized during a yet earlier period of chondrule formation followed by cooling at constant p O 2 , a principle well understood in thermodynamic and experimental petrology (e.g., Kilinc et al., 1983) and recently suggested for chondrules in CCs (Libourel et al., 2023). In chondrites more broadly, forsteritic olivines occur either in type I chondrules, as relict grains in type II chondrules, as isolated grains in the matrix, or in less common objects such as amoeboid olivine aggregates (AOAs; e.g., Frank et al., 2014).…”

Section: Discussionmentioning

confidence: 89%

Forsteritic olivine in EH (enstatite) chondrite meteorites: A record of nebular, metamorphic, and crystal‐lattice diffusion effects

McKibbin,

Hecht,

Makarona

et al. 2024

Meteorit & Planetary Scien

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The occurrence of forsteritic olivine in EH enstatite chondrites is indicative of bulk disequilibrium. In MgO‐rich magmatic systems, forsterite can either crystallize as a liquidus phase or be produced during peritectic melting of enstatite. Because diffusion of divalent cations through forsterite is relatively rapid, it records peak melting (i.e., chondrule‐forming events) and is also sensitive to subsequent metamorphism in the EH chondrite parent body. Here, we report the major and minor element geochemistry of olivine in EH chondrites across petrologic types 3 and 4. In all cases, olivine meets the technical definition of forsterite (>90 mole% Mg2SiO4). For unequilibrated EH chondrites, minor elements identify CaO‐Al2O3‐TiO2‐rich (refractory forsterite), MnO‐rich (“LIME” forsterite), and FeO‐bearing (forsteritic olivine) endmember components, the latter with Cr2O3‐rich and Cr2O3‐poor varieties. At higher petrologic type, minor element concentrations become restricted and compositions approach pure forsterite, while grain sizes reduce strongly with peak metamorphic temperatures. These changes reflect diffusive equilibration with enstatitic groundmass and dissolution reaction with free silica. The global geochemical distribution of forsteritic olivine in EH chondrites is, perhaps unexpectedly, more similar to those in low‐FeO type I chondrules and associated objects in carbonaceous chondrites (CCs), rather than equivalent objects in ordinary (H, L, LL), low‐FeO (or HH), or Kakangari (K) chondrites. Among achondrites, there is similarity between pure forsterite in aubrites and EH4 chondrites arising due to subsolidus equilibration in both settings, while Cr2O3‐poor forsteritic olivine in EH3 and CCs is similar to magnesian xenocrystic olivine in angrites. This might reflect CaO‐rich and SiO2‐poor magmatic sources across multiple early solar system reservoirs.

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