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 increasing ease of vaporization and loss from the Murchison CM2 chondrite during open-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 in their parent bodies. Five of these meteorites (Asuka (A) 881655, Yamato (Y) 793495, Y-790992, Pecora Escarpment (PCA) 91008, and Y-86789-paired with Y-86720) exhibit significant 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 atmosphere). From earlier data, three other C3 chondrites-Allan Hills (ALH) 81003, ALH 85003, and Lewis Cliffs (LEW) 85332-show significant Cd depletion. Nine additional C2 and C3 chondrites show no evidence of mobile trace element depletion-including Y-793321, which by all other criteria was mildly metamorphosed thermally. Either metamorphism of these nine meteorites occurred under closed conditions and/or alteration took place under such mild conditions 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) exhibit open-system loss of at least some thermally mobile trace elements by heating in their parent bodies, whereas none of the 25 non-Antarctic falls experienced this. These results are consistent with the idea that the Antarctic sampling of near-Earth material differs from that being sampled today.