Auger Nanoprobe analysis of presolar ferromagnesian
silicate grains from primitive CR chondrites QUE 99177 and MET 00426.
Floss C. and Stadermann F. J. (2008)
Geochim. Cosmochim. Acta, 73, 2415-2440.
investigated the presolar grain
inventories of two CR chondrites, QUE 99177 and MET 00426, which are
less altered than most members of this meteorite group. Both
meteorites contain high abundances of O-anomalous presolar grains, with
concentrations of 220 ± 40 and 160 ± 30 ppm for QUE 99177
and MET 00426, respectively. The presolar grain inventories are
dominated by ferromagnesian silicates with group 1
oxygen isotopic compositions, indicative of origins in low mass red
giant or asymptotic giant branch stars. Grains with pyroxene-like
compositions are somewhat more common than those with olivine-like
compositions, but most grains are non-stoichiometric with compositions
intermediate between these two phases, consistent with recent work
suggesting that amorphous interstellar silicates have stoichiometries
between olivine and pyroxene type silicates. Although structural
data are not available, one grain contains only Si and O, and has a
stoichiometry consistent with SiO2.
Our presolar grains are much more Fe-rich
than predicted by astronomical observations. Although secondary
alteration may play a role in enhancing the Fe contents of presolar
grains, it seems unlikely that the large and ubiquitous Fe enrichments
observed in the grains from this study can be due only to secondary
processing, particularly given the highly primitive nature of these two
meteorites. Grain condensation in the stellar outflows where
these grains formed likely proceeded under rapidly changing kinetic
conditions that may have enhanced the incorporation of Fe into the
grains over that expected based on equilibrium condensation theory.
Both QUE 99177 and MET 00426 appear to contain
unusually low abundances of oxide grains and have higher silicate/oxide
ratios than other primitive meteorites analyzed to date. We
explore various possibilities for this discrepancy, but note that most
scenarios are not likely to result in the preferential destruction of
oxides relative to silicates. Thus, the highest silicate/oxide ratios,
such as those observed in the CR chondrites, should reflect the true
initial proportions of presolar silicate and oxide grains in the parent
molecular cloud from which the solar nebula evolved.