Metamorphism of CO and CO-like chondrites and comparisons with type 3 ordinary chondrites

In order to explore their metamorphic history, thermoluminescence data have been obtained for 10 CO or CO-related chondrites from the Antarctic. Six have TL properties indicating low to intermediate levels of metamorphism, while Lewis Cliff 85332 and three paired meteorites from MacAlpine Hills (87300,87301 and 88107) have unusual TL properties similar to those of the very primitive Colony and Allan Hills A77307 CO-related chondrites. Cathodoluminescence photomosaics of nine well-studied CO chondrites are also presented and compared with similar data for the type 3 ordinary chondrites in which CL properties vary systematically with metamorphism. It is concluded that the CO chondrites, like the ordinary chondrites, form a metamorphic sequence and may be subdivided in an analogous manner using TL, CL and other petrographic and compositional data. Definitions for CO chondrites of the petrologic types 3.0-3.9 are proposed. However, it is stressed that the thermal history of the CO and ordinary chondrites is quite different, the range of equilibration for the CO chondrites is similar to the ordinary chondrites, but the former have not experienced temperatures above those experienced by type 3.5 ordinary chondrites (probably around 600℃). Presumably the CO chondrites spent longer times at lower temperatures. A CL photomosaic of Murchison is also presented, which has two features in common with the type 3.0-3.1 CO and ordinary chondrites; type I chondrules whose mesostases produce yellow CL (due to an unidentified but highly metamorphism-sensitive phase) and fine-grained matrix with red CL due to forsterite. Haloes of matrix material around chondrules and other objects in Murchison are thought to be due to aqueous destruction of those objects, and Fezoning in olivines in chondrules with broad haloes is also throught to be due to aqueous processes