Compositional comparisons of metamorphosed carbonaceous chondrites

Five carbonaceous chondrites (Allan Hills 82135,Coolidge, Karoonda, Pecora Escarpment 82500 and Yamato-693) thought to be metamorphosed chondrites of petrologic types 4 or 5 were analyzed by instrumental and radiochemical neutron activation analysis for 28 elements : Na, Mg, Al, K, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, As, Se, Br, Ru, Cd, Sb, La, Sm, Eu, Yb, Lu, Os, Ir and Au. Coolidge and Y-693 are members of the CV clan based on refractory lithophile abundances. Coolidge has significantly depleted volatile element abundances relative to CV3 chondrites and is tentatively classified as CV4. Y-693 is compositionally indistinguishable from the oxidized subgroup CV3 chondrites, showing no significant depletion of volatile element abundances, in conflict with the petrographic data indicating type 5; the chondrite may be heterogeneous. Compositional and petrographic data for Karoonda and ALH82135 suggest they form a grouplet, part of a clan with refractory lithophile abundances intermediate between those of the CM-CO and CV clans. Compositional classification of the severely weathered PCA82500 is difficult because of the scatter of data. It is probably closely related to Karoonda and ALH82135.