Oxygen isotopes in forsterite grains from Julesburg and Allende: 16O rich material in an ordinary chondrite

Details Oxygen isotopes in forsterite grains from Julesburg and Allende: 16O rich material in an ordinary chondrite Oxygen isotopes in forsterite grains from Julesburg and Allende: 16O rich material in an ordinary chondrite

Sep 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33.1017s&link_type=abstract

Meteoritics & Planetary Science, vol. 33, no. 5, pp. 1017-1027

Computer Science

5

Scientific paper

We have used the Manchester ISOLAB 54 ion microprobe to make in-situ measurements of the 17O/16O and 18O/16O 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 16O 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 16O enriched. In an oxygen isotope plot, the Julesburg 16O-rich grains form a roughly linear array which 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 16O-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 forsterites do not have the thick Fa20 coats typical of those in Julesburg. These 16O-rich forsterites appear to be related to the 'blue olivine' of Steele (1986). Both cores and coats of 16O-rich grains in Julesburg are isotopically distinct from olivine in Semarkona group A and group B chondrules.

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