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

Research output: Contribution to journalArticle

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

Fingerprint

Oxygen Isotopes
Allende meteorite
Meteorites
chondrule
lithium
Lithium
oxygen isotope
olivine
Isotopes
Chemical analysis
isotope
pyroxene
plagioclase
Asteroids
Mineralogy
ion microprobe
Crystallization

Keywords

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

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

Lithium- and oxygen-isotope compositions of chondrule constituents in the Allende meteorite. / Kunihiro, Takuya; Ota, Tsutomu; Nakamura, Eizou.

In: Geochimica et Cosmochimica Acta, Vol. 252, 01.05.2019, p. 107-125.

Research output: Contribution to journalArticle

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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.",
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T1 - Lithium- and oxygen-isotope compositions of chondrule constituents in the Allende meteorite

AU - Kunihiro, Takuya

AU - Ota, Tsutomu

AU - Nakamura, Eizou

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N2 - 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.

AB - 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.

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KW - Asteroid

KW - Chondrite

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KW - Oxygen

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