Implications of presolar grains for galactic chemical evolution

We identify and analyze general, but direct, relationships between the laboratory study of meteoritic grains and Galactic chemical evolution. We illustrate their principles by application to the specific case of the silicon isotope ratios measured in meteoritic silicon carbide (SiC) grains. These include new methods for relating the solar abundance pattern to the mean interstellar medium (ISM) abundance pattern at the time of solar birth. The transformation formalism between these two reference frames is general, and can be applied to any element with one primary and two purely secondary isotopes. Galactic chemical evolution offers enlarged views on the meaning of the mainstream SiC grains. Although some aspects of the mean ISM are probably recorded in SiC grains, mean chemical evolution by itself cannot completely explain the measured silicon isotope ratios in mainstream SiC grains. Solar material appears to have had a chemical history that is different from that of the mean ISM; alternatively, the Sun may have migrated to its present location. Unique information from grains for Galactic chemical evolution justifies a vigorous search for an overarching paradigm.