Lithium- and oxygen-isotope compositions of chondrule constituents in the Allende meteorite

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Abstract

We report in situ ion-microprobe analyses of Li- and O-isotope compositions for olivine, low-Ca pyroxene, high-Ca pyroxene, and chondrule mesostasis/plagioclase in nine chondrules from the Allende CV3 chondrite. Based on their mineralogy and O-isotope compositions, we infer that the chondrule mesostasis/plagioclase and ferroan olivine rims were extensively modified or formed during metasomatic alteration and metamorphism on the Allende parent asteroid. We excluded these minerals in order to determine the correlations between Li and both O and the chemical compositions of olivines and low-Ca pyroxenes in the chondrules and their igneous rims. Based on the O-isotope composition of the olivines, nine chondrules were divided into three groups. Average Δ 17 O of olivines (Fo >65 ) in group 1 and 2 chondrules are −5.3 ± 0.4 and −6.2 ± 0.4‰, respectively. Group 3 chondrules are characterized by the presence of 16 O-rich relict grains and the Δ 17 O of their olivines range from −23.7 to −6.2‰. In group 1 olivines, as Fa content increases, variation of δ 7 Li becomes smaller and δ 7 Li approaches the whole-rock value (2.4‰; Seitz et al., 2012), suggesting nearly complete Li-isotope equilibration. In group 2 and 3 olivines, variation of δ 7 Li is limited even with a significant range of Fa content. We conclude that Li-isotope compositions of olivine in group 1 chondrules were modified not by an asteroidal process but by an igneous-rim formation process, thus chondrule olivines retained Li-isotope compositions acquired in the protosolar nebula. In olivines of the group 3 chondrule PO-8, we observed a correlation between O and Li isotopes: In relict 16 O-rich olivine grains with Δ 17 O of ∼−25 to −20‰, δ 7 Li ranges from −23 to −3‰; in olivine grains with Δ 17 O > −20‰, δ 7 Li is nearly constant (−8 ± 4‰). Based on the Li-isotope composition of low-Ca pyroxenes, which formed from melt during the crystallization of host chondrules and igneous rims, the existence of a gaseous reservoir with a δ 7 Li ∼ −11‰ is inferred.

Original languageEnglish
Pages (from-to)107-125
Number of pages19
JournalGeochimica et Cosmochimica Acta
Volume252
DOIs
Publication statusPublished - May 1 2019

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Keywords

  • Allende
  • Asteroid
  • Chondrite
  • Chondrule
  • Igneous rim
  • Lithium
  • Oxygen
  • SIMS

ASJC Scopus subject areas

  • Geochemistry and Petrology

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