The Most Primitive Material in Meteorites
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 chemica
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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.
U. Ott (B) Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany e-mail: [email protected] Ott, U.: The Most Primitive Material in Meteorites. Lect. Notes Phys. 815, 277–311 (2010) c Springer-Verlag Berlin Heidelberg 2010 DOI 10.1007/978-3-642-13259-9_7
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1 The Most Primitive Material in Meteorites What is primitive? In a sense, all the material making up the major group of meteorites – the chondrites (∼85% of all falls) – is primitive. All their constituents, including thermally and/or aqueously metamorphosed ones, formed within the first few tens of millions of years after formation of the solar system (e.g., [1, 2]). They thus constitute old material formed 4.5–4.6 Ga ago and essentially unaltered since then – they are primitive. In fact chondritic meteorites, and in particular the subgroup of the carbonaceous chondrites represent the most pristine material that is available for detailed laboratory study in large amounts. Others, less abundantly available materials include interplanetary dust picked up by aircraft in the stratosphere, micrometeorites collected in deep sea sediments/ polar ice (cf. chapter “The
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