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Extreme oxygen and magnesium isotopic anomalies
in presolar spinel grains from the Murray carbonaceous meteorite.
Gyngard F., Morgand A., Nittler L. R., Stadermann F.
J., and Zinner E. (2009)
Lunar & Planet. Sci. 40, Abstract #1386.
ABSTRACT
Presolar spinel grains have been identified in several
meteorites by their anomalous O isotopic compositions and fall into the
four previously defined oxide groups. The majority of these grains, in
groups 1-3, are believed to originate in various types of red giant
(RGB) and asymptotic giant branch (AGB) stars, while the enigmatic
Group 4 grains, characterized by 18O excesses, have been
hypothesized to be of a supernova (SN) origin. Magnesium-Al spinel
grains allow for relatively precise determination of Mg isotopic
ratios, in contrast to previous Mg isotopic measurements in presolar
SiC, corundum, and graphite, where Mg is a minor element. Most of the
previously measured spinel grains' Mg isotopic compositions,
characterized by modest enrichments of 25Mg up to
~150‰, can be explained by nucleosynthetic processes. However,
several grains have been reported whose extreme Mg compositions are
difficult to explain with current AGB models. We report here O isotopic
data on 40 new spinel grains from the Murray carbonaceous (CM2)
chondrite and Mg isotopic data of the 28 grains remaining after O
isotopic analysis. Several grains have very large 25,26Mg
anomalies, and one grain has the largest 17O/16O
ratio ever observed.
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