Diamonds presolar grains

Meteorites contain a variety of trapped noble gas components other than the solar wind. In many cases their compositions have not been measured in pure form, but have been determined from measured (i.e., not pure) compositions based correlation / mixing lines involving assumptions about one isotopic ratio in the end-member. Some of the components established in this way are of nucleosynthetic origin and carried by presolar grains (diamond, SiC, graphite). Their isotopic compositions are testament to the nuclear processes by which elements are made in the interior of the stars around which those carrier grains formed. 

Presolar stardust grains diamond, graphite, carbides and oxides... 

The discovery happened by accident. Lewis and Anders were frustrated by their failure to find the carrier of anomalous xenon in carbonaceous chondrites. They decided to try an extreme treatment to see if they could dissolve the carrier. They treated a sample of the colloidal fraction of an Allende residue with the harshest chemical oxidant known, hot perchloric acid. The black residue turned white, and to their surprise, when they measured it, the anomalous xenon was still there The residue consisted entirely of carbon, and when they performed electron diffraction measurements on it, they found that it consisted of tiny (nanometer sized) diamonds. After a detailed characterization that included chemical, structural, and isotopic studies, they reported the discovery of presolar diamond in early 1987 (Lewis et al., 1987). The 23-year search for the carrier of CCFXe (Xe-HL) was over, and the study of presolar grains had begun. 

Lewis, R. S., Tang, M., Wacker, J. F., Anders, E. and Steel, E. (1987) Interstellar diamonds in meteorites. Nature, 326, 160-162. The paper describing the discovery of the first presolar grains. 

One of the enormous benefits of seeking diamond, or for that matter SiC, as presolar grain material is the incomparable resistance of the mineral to chemical attack. Other species are far more difficult but not impossible to isolate. 

Chondrite matrices are also the carriers of presolar grains. These phases cover a broad mineralogy and include diamonds and graphite, silicon carbide (SiC), titanium carbide (TiC), silicon nitride (SisN, ), corundum, spinel, silicates, and even rare metal grains. 

Figure 1 Exotic noble gas components present in presolar carbonaceous grains. Diamond is the carrier of Xe-HL, SiC the carrier of Xe-S and Ne-E(H), and graphite the carrier of Ne-E(L) (source Anders and Zinner, 1993).

Much the same can be said about the diamonds, in that most minor and trace elements are radically anomalous. But the major element, carbon, has an isotopic composition within the range of solar-system normal. Moreover, the individual diamond grains are extremely small, characteristically only a few nanometers, and too small to support analysis even of carbon in individual grains. It may be that diamond carbon appears isotopically normal only because any isotopic analysis is an average over many grains. But because of the normal carbon there is persistent suspicion that most the diamonds are not really circumstellar or even presolar after all, and that the real presolar grain carrier is a small subset of the diamonds or some other phase entirely, less abundant than the diamonds but which follows them in the separation procedures. 

Figure 7. Kr isotopic compositions in gas components hosted by presolar grains P3, P6, HL in presolar diamonds, G and N in presolar SiC and graphite (Table 4). Ratios are normalized to Kr, and shown are deviations in per mill of the Kr/ Kr ratios from the corresponding ratios measured for the solar wind (Table 4). Derived compositions generally involve assumption of a value for one characteristic ratio (see text). Two compositions are shown for the P6 component, which involve different assumptions about Kr/ Kr in P6. For Kr/ Kr in these no error has been given (and hence no error is shown). No experimental data have been reported for Kr/ Kr in Kr-G and Kr-N, and for the G component the theoretically expected value of -1000 per mill is plotted. Ratios Kr/ Kr and r/ Kr in Kr-G are variable and not shown. For discussion see text.

Ott U (1996) Interstellar diamond xenon and timescales of snpemova ejecta. Astrophys J 463 344-348 Ott U, Begemann F (2000) Spallation recoil and age of presolar grains. Meteoritics Planet Sci 35 53-63 Ott U, Begemann F, Yang J, Epstein S (1988) S-process krypton of variable isotopic composition in the Mmchison meteorite. Natine 332 700-702... 

The discovery of presolar grains was made possible by the development of chemical procedures in which carbonaceous meteorites were subjected to a stringent acid digestion regime. Carbon compounds such as diamonds, SiC and graphite were isolated in this manner and identified through their distinctive pattern and anomalous noble gas component. These carbonaceous phases are samples of interstellar matter which provide a window into the prehistory of the Solar System. 

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