New Insights into the Origin of Calcium-Aluminum-Rich Inclusions in Non-Carbonaceous Chondrites

Meteorite NWA 869, an ordinary chondrite meteorite. Credit: H. Raab, Wikimedia Commons.

Before the planets formed, our solar system consisted of a rotating disk, with the young Sun surrounded by hot dust and gas. This disk is known as the solar nebular (or protoplanetary) disk. As this disk cooled, solids began to condense. The very first solids to form were rich in the elements calcium and aluminum and are called calcium-aluminum-rich inclusions (CAIs). The refractory nature (high melting/condensation temperatures) of the minerals in CAIs means that they must have formed at high temperatures (~1300–1700 K), presumably in the hottest, innermost part of the solar nebula.

Despite forming very near the Sun, CAIs are abundant in carbonaceous chondrite (CC) meteorites, which accreted in the cold, outer solar system. This constitutes a “classic” problem regarding the timing and transport of CAIs throughout the solar system. One way to test various transportation scenarios is to evaluate the populations of CAIs in CCs, compared with those in non-carbonaceous (NC) chondrites, which formed in the inner solar system. The CAIs in NC meteorites have not been studied to the same extent as their CC counterparts.

Emilie Dunham from the University of California, Los Angeles and colleagues recently studied a range of 76 NC chondrites. Their results show that CAIs in NC chondrites are smaller (~40 microns compared to the ~120 microns in CCs) and at least a factor of 10 less abundant than in CCs. These observations support astrophysical models in which, after the initial outward transport of CAIs, a gap was created in the disk by the formation of Jupiter, and most CAIs were trapped in the outer solar system. They also support more recent modeling suggesting that the gap was not absolute and that small particles (<50-100 microns) could have filtered through. Thus, small CAIs could have filtered back into the inner solar system and accreted into NC bodies. READ MORE