Radial transport and nebular thermal processing of millimeter-sized solids in the Solar protoplanetary disk inferred from Cr-Ti-O isotope systematics of chondrules

Abstract

Understanding the material transport and mixing processes in the Solar protoplanetary disk provides important constraints on the origin of chemical and isotopic diversities of our planets. The limited extent of radial transport and mixing between the inner and outer Solar System has been suggested based on a fundamental isotopic dichotomy between non-carbonaceous (NC) and carbonaceous (CC) meteorite groups. The limited transport and mixing could be further tested by tracing the formation regions of individual meteoritic components, such as Ca-Al-rich inclusions (CAIs) and chondrules. Here, we show further evidence for the outward transport of CAIs and chondrules from the inner and subsequent thermal processing in the outer region of the protoplanetary disk based on the petrography and combined Cr-Ti-O isotope systematics of chondrules from the Vigarano-like (CV) carbonaceous chondrite Allende. One chondrule studied consists of an olivine core that exhibits NC-like Ti and O, but CC-like Cr isotopic signatures, which is enclosed by a pyroxene igneous rim with CC-like O isotope ratios. These observations indicate that the olivine core formed in the inner Solar System. The olivine core then migrated into the outer Solar System and experienced nebular thermal processing that generated the pyroxene igneous rim. The nebular thermal processing would result in Cr isotope exchange between the olivine core and CC-like materials, but secondary alteration effects on the parent body are also responsible for the CC-like Cr isotope signature. By combining previously reported Cr-Ti-O isotope systematics of CV chondrules, we show that some CV chondrules larger than ~1 mm would have formed in the inner Solar System. The accretion of the millimeter-sized, inner Solar System solids onto the CV carbonaceous chondrite parent body would require their very early migration into the outer Solar System within the first 1 million years after the Solar System formation.