The most primitive material in meteorites

Abstract

The most primitive matter known to occur in meteorites are circumstellar condensates. They were originally found as result of the search for the carrier phases of isotopically anomalous noble gases and isolated from the bulk meteorites using harsh chemical treatments. Diamond was the first and may occur with an abundance 1‰, but it is not clear what fraction of the observed nanometer-sized diamonds is truly presolar. Others found by this approach consist of thermally and chemically highly resistant materials (graphite, silicon carbide, refractory oxides and silicon nitride) and occur on the sub-ppm to several ten ppm level. Identification of presolar minerals susceptible to chemical treatment became possible with technical developments in secondary ion mass spectrometry allowing searches in the meteorites in situ. Presolar silicates found by this approach occur on an abundance level up to ~200 ppm. Isotope abundance anomalies are the key feature based on which the presolar nature of a given grain is ascertained. They also allow identifying likely stellar sources for the grains. The major source of SiC, the oxide and the silicate grains are Red Giant/AGB stars, while some (~1%) of the SiC and all of the silicon nitride grains appear to be linked to supernovae, as are probably the presolar nanodiamonds. Graphite grains come from a number of different stellar sources. The detailed isotopic patterns also allow drawing conclusions about nucleosynthesis and mixing in stars as well as galactic chemical evolution. Astronomical detection around specific sources, such as in the case of SiC and silicates, would be helpful if it could be achieved also for other grains. Deriving an age for the grains and establishing a detailed history between time of formation in stellar outflows and arrival in what was to become the solar system is a task that remains to be accomplished. © 2010 Springer Berlin Heidelberg.