B3
CAI in carbonaceous chondrite
the oldest materials in the solar system

Carbonaceous chondrites are meteorites comprising some of the oldest solid materials in the solar system; these include sphere-shaped particles (chondrules), as well as a variety of inclusions which contain Ca- and Al-rich inclusions (CAIs). Based on Pb isotope dating, CAIs are determined to be the oldest material in the solar system, formed about 4.57 billion years ago. CAIs also have a unique elemental-composition signature, believed to clearly record the chemical evolution of materials in the early solar system. The Allende chondrite is the largest carbonaceous chondrite ever found on Earth with a total mass of 2 t. Some large mass fragments, such as the display specimen, have been recovered composed of a homogeneous chemical composition; this meteorite is considered to be a geochemical standard. CAIs in Allende have been studied vigorously since the 1970s. With a decrease in temperature from about 2000 K in the solar nebula, gases precipitate to form more thermodynamically stable minerals. This physical process is called condensation. Rare earth elements (REEs) and actinides (e.g., Th and U) condense at high temperatures (~1500 K) along with other major elements and minerals. Among the major elements, Ca, Al, and Ti condense at about 1700-1500 K, forming silicate and oxide minerals. With a further decrease in temperature to about 1400 K, Mg, Si and Fe condense to form the major minerals and metals ubiquitously observed in meteorites. Researchers have revealed the presence of minerals formed at high temperatures in CAI as a result of the condensation process in the solar nebula, as well as the presence of super refractory elements which are considered to have formed at even higher temperatures. The elemental abundances of CAIs in the Allende meteorite are independent of their geochemical characteristics, suggesting that the CAIs are remnants of materials condensed at high temperatures which reflect the evolutional process of materials in the solar nebula. Since a wide variety of CAIs have been identified to date, they are considered to have experienced a more complex process. Even so, CAIs in the Allende meteorite contain many materials which still can be aptly explained by the condensation process from gas to solid in the solar nebula.　(Takafumi Niihara)