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Oxygen isotopes in forsterite grains from Julesburg and Allende: Oxygen‐16‐rich material in an ordinary chondrite
Author(s) -
SAXTON J. M.,
LYON I. C.,
TURNER G.
Publication year - 1998
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1998.tb01708.x
Subject(s) - allende meteorite , forsterite , oxygen , isotopes of oxygen , chondrite , geology , geochemistry , olivine , meteorite , carbonaceous chondrite , mineralogy , isotope , astrobiology , chemistry , physics , nuclear physics , organic chemistry
— We have used the Manchester ISOLAB 54 ion microprobe to make in situ measurements of the 17 O/ 16 O and 18 O/ 16 O ratios of olivine grains in the Julesburg (L3.6) and Allende (CV3) chondrites. We have discovered a population of olivines in Julesburg characterised by (1) the most 16 O‐rich compositions yet reported for olivine from an ordinary chondrite; (2) cores of low‐Fa olivine, which frequently shows blue cathodoluminesce; (3) thick coats of more Fa‐rich (Fa ∼20) olivine, which is also 16 O‐enriched. In an O isotopic plot, the Julesburg 16 O‐rich grains form a roughly linear array that is offset from the Allende mixing line. The presence of very low Fa olivine and, sometimes, well‐defined Fa‐rich coats indicates that these grains experienced significantly less thermal metamorphism than most of the olivine in the meteorite. Some 16 O‐rich Julesburg grains are associated with minor feldspar or pyroxene and are probably chondrule fragments. They are isotopically indistinguishable from forsterite in Allende; however, Allende forsterite grains do not have the thick Fa20 coats typical of those in Julesburg. These 16 O‐rich forsterite grains appear to be related to the “blue olivine” of Steele (1986). Both cores and coats of 16 O‐rich grains in Julesburg are isotopically distinct from olivine in Semarkona group A and group B chondrules.