Thermally metamorphosed carbonaceous chondrites from data for thermally mobile trace elements

We report radiochemical neutron activation analysis (RNAA) data for U, Co, Au, Sb, Ga, Rb, Cs, Se, Ag, Te, Zn, In, Bi, Tl, and Cd (ordered by increasi ng ease of vaporization and loss from the Murchison CM2 chondrite during op en-system heating) in nine Antarctic C2 and C3 chondrites. These meteorites exhibit properties (obtained by reflectance spectroscopy, O isotopic mass spectrometry and/or mineralogy-petrology) suggesting thermal metamorphism i n their parent bodies. Five of these meteorites (Asuka (A) 881655, Yamato (Y) 793495, Y-790992, Pe cora Escarpment (PCA) 91008, and Y-86789-paired with Y-86720) exhibit signi ficant depletion of the most thermally mobile 1-5 trace elements, which is consistent with open-system loss during extended parent-body heating (under conditions duplicated by week-long heating of the Murchison C2 chondrite, heated at 500-700 degrees C in a low-pressure (initially 10(-5) atm) H-2 at mosphere). From earlier data, three other C3 chondrites-Allan Hills (ALH) 8 1003, ALH 85003, and Lewis Cliffs (LEW) 85332-show significant Cd depletion . Nine additional C2 and C3 chondrites show no evidence of mobile trace ele ment depletion-including Y-793321, which by all other criteria was mildly m etamorphosed thermally. Either metamorphism of these nine meteorites occurr ed under closed conditions and/or alteration took place under such mild con ditions that even Cd could not be lost. The RNAA data suggest that 10 of the 46 Antarctic carbonaceous chondrites ( including 4 of 37 from Victoria Land and 6 of 9 from Queen Maud Land) exhib it open-system loss of at least some thermally mobile trace elements by hea ting in their parent bodies, whereas none of the 25 non-Antarctic falls exp erienced this. These results are consistent with the idea that the Antarcti c sampling of near-Earth material differs from that being sampled today.