Accretion of differentiated achondritic and aqueously-altered chondritic materials in the Early Solar System - significance of an igneous fragment in the CM chondrite NWA 12651.

Ebert S., Patzek M., Lentfort S., and Bischoff A.

Abstract

One approach to decipher the dynamics of material transport and planetary accretion in the early Solar System is to investigate xenolithic fragments in meteorites. In this work, we examined an igneous fragment from the NWA 12651 meteorite – the first igneous fragment found in any CM chondrite – by analyzing its mineralogy, rare earth elements, and O-isotopes. The study shows that the exsolution lamellae of the igneous fragment consist of Fe-rich and Ca-rich pyroxene. Thus, the fragment was part of a progressive crystallization in a closed system, such as in a depleted magma reservoir or mantle. In this environment, the pyroxene co-crystalized with plagioclase, resulting in a negative Eu anomaly and enrichment of the heavy rare earth elements compared to the light rare earth elements. The O-isotopes of the fragment are more 16O-enriched than the mafic minerals in the matrix or in other bulk CM chondrites; therefore, the fragment was formed in a different region than the NWA 12651 parent body. The iron meteorites Tucson and Deep Springs, the pallasite Milton, and the CB chondrites have similar O-isotopes as the igneous fragment. However, no direct connection can be drawn and it is questionable, if the fragment shares a same parent body with one of these meteorites. The close formation region to the CB chondrites may suggest a formation of the fragment in the carbonaceous chondrite region. Thus, a wide transport through the nebula of the early solar system may not have been necessary to move the fragment to the CM chondrite formation region.