CHEMICAL AND PHYSICAL STUDIES OF TYPE-3 CHONDRITES .12. THE METAMORPHIC HISTORY OF CV-CHONDRITES AND THEIR COMPONENTS

The induced thermoluminescence (TL) properties of 16 CV and CV-related chondrites, four CK chondrites and Renazzo (CR2) have been measured i n order to investigate their metamorphic history. The petrographic, mi neralogical and bulk compositional differences among the CV chondrites indicate that the TL sensitivity of the similar to 130 degrees C TL p eak is reflecting the abundance of ordered feldspar, especially in cho ndrule mesostasis, which in turn reflects parent-body metamorphism. Th e TL properties of 18 samples of homogenized Allende powder heated at a variety of times and temperatures, and cathodoluminescence mosaics o f Axtell and Coolidge, showed results consistent with this conclusion. Five refractory inclusions from Allende, and separates from those inc lusions, were also examined and yielded trends reflecting variations i n mineralogy indicative of high peak temperatures (either metamorphic or igneous) and fairly rapid cooling. The CK chondrites are unique amo ng metamorphosed chondrites in showing no detectable induced TL, which is consistent with literature data that suggests very unusual feldspa r in these meteorites. Using TL sensitivity and several mineral system s and allowing for the differences in the oxidized and reduced subgrou ps, the CV and CV-related meteorites can be divided into petrologic ty pes analogous to those of the ordinary and CO type 3 chondrites. Axtel l, Kaba, Leoville, Ball, Arch and ALHA81003 are type 3.0-3.1, while AL H84018, Efremovka, Grosnaja, Allende and Vigarano are type 3.2-3.3 and Coolidge and Loongana 001 are type 3.8. Mokoia is probably a breccia with regions ranging in petrologic type from 3.0 to 3.2. Renazzo often plots at the end of the reduced and oxidized CV chondrite trends, eve n when those trends diverge, suggesting that in many respects it resem bles the unmetamorphosed precursors of the CV chondrites. The low-petr ographic types and low-TL peak temperatures of all samples, including the CV3.8 chondrites, indicates metamorphism in the stability field of low feldspar (i.e., <800 degrees C) and a metamorphic history similar to that of the CO chondrites but unlike that of the ordinary chondrit es.