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Origin of magnetite in oxidized CV chondrites: in situ measurement of oxygen isotope compositions of Allende magnetite and olivineMagnetite in the oxidized CV chondrite Allende mainly occurs as spherical nodules in porphyritic-olivine (PO) chondrules, where it is associated with Ni-rich metal and/or sulfides. To help constrain the origin of the magnetite, we measured oxygen isotopic compositions of magnetite and coexisting olivine grains in PO chondrules of Allende by an in situ ion microprobe technique. Five magnetite nodules form a relatively tight cluster in oxygen isotopic composition with delta 18O values from -4.8 to -7.1% and delta 17O values from -2.9 to -6.3%. Seven coexisting olivine grains have oxygen isotopic compositions from -0.9 to -6.3% in delta 18O and from -4.6 to -7.9% in delta 17O. The delta 17O values of the magnetite and coexisting olivine do not overlap; they range from -0.4 to -2.6%, and from -4.0 to -5.7%, respectively. Thus, the magnetite is not in isotopic equilibrium with the olivine in PO chondrules, implying that it formed after the chondrule formation. The delta 17O of the magnetite is somewhat more negative than estimates for the ambient solar nebula gas. We infer that the magnetite formed on the parent asteroid by oxidation of metal by H2O which had previously experienced minor O isotope exchange with fine-grained silicates.
Document ID
20040121339
Acquisition Source
Headquarters
Document Type
Reprint (Version printed in journal)
Authors
Choi, B. G.
(University of California Los Angeles 90095-1567, United States)
McKeegan, K. D.
Leshin, L. A.
Wasson, J. T.
Date Acquired
August 22, 2013
Publication Date
January 1, 1997
Publication Information
Publication: Earth and planetary science letters
Volume: 146
Issue: 2-Jan
ISSN: 0012-821X
Subject Category
Lunar And Planetary Science And Exploration
Funding Number(s)
CONTRACT_GRANT: NAGW-4112
CONTRACT_GRANT: EAR 95-09641
CONTRACT_GRANT: EAR 94-18520
Distribution Limits
Public
Copyright
Other

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