When investigating genetic relationships among meteorites and planets, nucleosynthetic isotopes, which formed in the stellar sources that contributed material to the early solar system, can be a powerful tool. On the basis of nucleosynthetic isotopes of several elements, all solar system materials exhibit a dichotomy between carbonaceous (CC) and non-carbonaceous (NC) materials, with the CCs being enriched in neutron-rich isotopes. The NC reservoir is commonly associated with the inner solar system and the CC reservoir with the outer solar system beyond Jupiter. The mechanism responsible for creating this dichotomy, however, is debated.
The element zirconium (Zr) may provide new insights into the origin of this dichotomy. Zr has five isotopes, with the neutron-rich 96Zr being nucleosynthetic. Zirconium is refractory, meaning that it condenses into solids at high temperatures. It is also lithophile (rock-loving), meaning that it is concentrated into silicate minerals rather than metal. Thus, Zr is not affected by either volatile loss or core formation and is entirely hosted in the silicate portion of a planet. This makes it possible to directly compare Zr isotopes of the bulk silicate portions of Earth, Mars, and meteorite parent bodies to provide information on the nature and origin of their building blocks.
Jan Render of the University of Münster and coauthors present new high-precision Zr isotope data for a range of NC and CC meteorites. Their work reveals the same NC-CC dichotomy observed for other elements, with 96Zr being enriched in CC materials. The authors showed that CC compositions could be produced as a mixture between NC-like materials and calcium-aluminum-rich inclusions (CAIs). The latter, which are the earliest solids that formed in the solar system and are enriched in the neutron-rich isotopes, are more abundant in CCs than in NCs. This result supports a previously proposed model in which the isotopic compositions of material falling into the protoplanetary molecular cloud varied over time. READ MORE