Showing papers in "Antarctic meteorite research in 1999"

New petrographic and trace element data on thermally metamorphosed carbonaceous chondrites

Abstract: Mineral grains and matrix of heated chondrites Yamato (Y)-82162, Belgica (B)-7904, Y-86720, and Asuka (A)-881655 were examined for major elements and, where appropriate, phyllosilicate and matrix samples were characterized by TEM. CM chondrites A-881655 and B-7904 were only partially aqueously altered before thermal metamorphism initiated dehydration and recrystallization. Tochilinite is absent in both A-881655 and B-7904 probably due to mild thermal metamorphism. Phyllosilicates in B-7904 are dehydrated but not completely recrystallized to olivine and pyroxene. Y-86720 experienced a history very different from other CM chondrites: its chondrules and other coarse-grained components were completely altered by aqueous fluids. Fine-grained olivine was subsequently replaced during an episode of thermal metamorphism producing compositions near Fo10 and complete destruction of serpentine occurred. Y-82162 comes from a CI parent and is characterized by complete destruction of preexisting anhydrous silicates during aque­ ous alteration. Subsequent heating at 600-700° C resulted in dehydration of phyllosilicates. The matrix contains abundant fine-grained olivine which would not likely survive the original aqueous alteration and therefore must have been formed by phyllosilicate metamorphism. Alternating episodes of oxidation and sulfidization following aqueous alteration are evident in all but Y-86720. Either Y-86720 escaped an oxidizing event or effects of such an event were obliterated by later sulfidization. Based on RNAA data for thermally mobile trace elements in them and in Murchison CM2 samples heated for one week at 500° , 600° , and 700° C under conditions reasonable for interiors of primitive parent bodies, the four thermally metamorphosed carbonaceous chondrites can be ordered by the severity of open-system heating as 500° ::::::A-88165S

UV-Vis-NIR absorption features of heated phyllosilicates as remote-sensing clues of thermal histories of primitive asteroids

Abstract: Powder samples of five kinds of phyllosilicates (antigorite, lizardite, clinochrysotile, chlorite, and saponite) have been heated in vacuum at 200, 300, 400, 500, and 600 ° C, and their UV-Vis-NIR reflectance spectra have been measured to test the hypothesis that the empirical correlation between the UV and 3-μm absorption strengths of primitive asteroids and CI/CM chondrites are due to the presence of phyllosilicates (T. HIROI et al., Meteorit. Planet. Sci., 31, 321, 1996). The chlorite sample showed virtually no change in absorption strengths. Only the saponite sample showed a clear positive correlation between the UV and 3-μm absorption strengths, whose reason being yet to be studied. Heating experiments with coexisting carbon (graphite rod) at 600 ° C have given no systematic trend. Although an apparent conclusion would be that saponite is the cause of the correlation between the UV and 3-μm absorption strengths, it may not be true because saponite is not as abundant as serpentines in CM chondrites (M.E. ZoLENSKY et al., Geochim. Cosmochim. Acta, 57, 3123, 1993).

Noble gases and K-Ar ages of five Rumuruti chondrites Yamato(Y)-75302, Y-791827, Y-793575, Y-82002, and Asuka-881988

Abstract: Noble gases and K concentrations have been determined for aliquot samples prepared from the five Antarctic R-chondrites Yamato (Y)-75302, Y-791827, Y-793575, Y-82002, and Asuka (A)-881988. K-Ar ages of about 4.2Ga were ob­ tained for Y-75302, Y-791827, Y-793575 and A-881988, while Y-82002 showed a slightly younger age of 3.9 Ga. The Y-75302, Y-791827 and Y-82002 are enriched in solar light noble gases. Four meteorites Y-75302, Y-791827, Y-82002 and A-881988 have cosmic-ray exposure ages of about 20 Ma, while the age of Y-793575 is about 7 Ma. Based on the noble gas compositions and K-Ar ages, Y-75302 and Y-791827 are probably paired and Y-82002 may belong to this pair. Relatively high and variable 12Xe/ 132Xe ratios between 2-3. 7 as well as enrichments of heavier Xe isotopes were observed in all R-chondrites.