The origin and significance of the CCAM line: Evidence from chondrules and dark inclusions in Allende (CV3)

For guidance on citations see FAQs. c [not recorded] Version: Version of Record Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online's data policy on reuse of materials please consult the policies page. Introduction: The process responsible for the mass independent oxygen isotope variation observed in Solar System materials remains poorly understood. While self-shielding of CO, either in the early solar nebula [1,2], or precursor molecular cloud [3], appears to be a viable mechanism, alternative models have also been proposed [e.g. 4]. An important aspect of this problem relates to the interpretation of various reference lines on oxygen three-isotope diagrams. The Carbonaceous Chondrite Anhydrous Mineral (CCAM) line, derived predominantly , but not exclusively, from analyses of components in the Allende (CV3) meteorite, is the most widely used reference and has a slope of 0.94±0.01(2) [5,6]. However, the fundamental significance of the CCAM line has been questioned by [7]. Based on the results of a UV laser ablation study of an Allende CAI, it was suggested that a line of exactly slope 1 was of more fundamental significance. As pointed out by [7], almost all Solar System materials, with the exception of the R chondrites, plot to the right of the slope 1 line. This variation could be explained if the primitive oxygen isotope composition of the Solar System was represented by a line of exactly slope 1, with subsequent mass fractionation, or isotopic exchange, shifting compositions to the right [7]. The fact that a highly 17,18 O-enriched phase (