Correlated XANES, TEM, and NanoSIMS of Presolar Graphite Grains

1,2,3Evan E. Groopman, 4Larry R. Nittler
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.02.011]
1Laboratory for Space Science, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
2National Research Council Postdoctoral Fellow at the U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA
3Materials Science and Technology Division, U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA
4Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5251 Broad Branch Rd NW, Washington, DC 20015, USA
Copyright Elsevier

We report correlated XANES, TEM, and NanoSIMS measurements of twelve presolar graphite grains extracted from primitive meteorites and for which isotopic data indicate predominantly Type-II supernovae origins. We find continued evidence for isotopic heterogeneities in presolar graphite grains, including the first observation of a radial gradient in the inferred initial 26Al/27Al within a presolar graphite grain. The XANES spectra of these samples show a variety of minor absorbances near the C K-edge, attributable to vinyl-keto, aliphatic, carboxyl, and carbonate molecules, as well as possible damage during sample preparation. Each sample exhibits homogeneous C K-edge XANES spectra within the graphite, however, showing no correlation with isotopic heterogeneities. Gradients in the isotope ratios of C, N, O, and Al could be due to both processes during condensation, e.g., mixing in stellar ejecta and granular transport, and post-condensation effects, such as isotope dilution and exchange with isotopically normal material in the early Solar System or laboratory, the latter of which is a significant issue for high-density presolar graphite grains. It remains unknown whether the mechanisms behind isotope exchange would also affect the local chemistry and therefore the XANES spectra. Ti L-edge XANES from most Ti-rich subgrains match standard spectra for TiC and potentially TiCN. A rare rutile (TiO2) subgrain has been identified, though it lacks the lowest energy L3 peak typically seen in standard spectra. Ca has also been identified by EDXS in TiC subgrains, likely due to the decay of live 44Ti at the time of formation. Future NanoSIMS measurements will determine the variability of initial 44Ti in TiC subgrains, an important constraint on mixing in the ejecta of the grains’ parent supernovae.

Discuss

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s