Supernova graphite in the NanoSIMS: Carbon, oxygen and titanium isotopic compositions of a spherule and its TiC sub-components

摘要

Presolar graphite spherules from the Murchison low-density separate KE3 contain a large number of internal TiC crystals that range in size from 15 to 500 run. We have studied one such graphite grain in great detail by successive analyses with SEM, ims3f SIMS, TEM and NanoSIMS. Isotopic measurements of the 'bulk' particle in the ims3f indicate a supernova origin for this graphite spherule. The NanoSIMS measurements of C, N, O and Ti isotopes were performed directly on TEM ultramicrotome sections of the spherule, allowing correlated studies of the isotopic and mineralogical properties of the graphite grain and its internal crystals. We found isotopic gradients in C-12/C-13 and O-16/O-18 from the core of the graphite spherule to its perimeter, with the most anomalous compositions being present in the center. These gradients may be the result of isotopic exchange with isotopically normal material, either in the laboratory or during the particle's history. No similar isotopic gradients were found in the O-16/O-17 and N-14/N-15 ratios, which are normal within analytical uncertainty throughout the graphite spherule. Due to an unusually high O signal, internal TiC crystals were easily located during NanoSIMS imaging measurements. It was thus possible to determine isotopic compositions of several internal TiC grains independent of the surrounding graphite matrix. These TiC crystals are significantly more anomalous in their O isotopes than the graphite, with O-16/O-18 ratios ranging from 14 to 250 (compared to a terrestrial value of 499). Even the most centrally located TiC grains show significant variations in their 0 isotopic compositions from crystal to crystal. Measurement of the Ti isotopes in three TiC grains found no variations among them and no large differences between the compositions of the different crystals and the 'bulk' graphite spherule. However, the same three TiC crystals vary by a factor of 3 in their O-16/O-18 ratios. It is not clear in what form the O is associated with the TiC grains and whether it is cogenetic or the result of surface reactions on the TiC grains before they accreted onto the growing graphite spherule. The presence of Ca-44 from short-lived Ti-44 (t(1/2) = 60y) in one of the TiC subgrains confirms the identification of this graphite spherule as a supernova condensate. Copyright (C) 2005 Elsevier Ltd.