Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..240k&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 240
Computer Science
2
Scientific paper
Three carbonaceous chondrites from the MacAlpine Hills region of Antarctica (MAC87300, MAC87301, and MAC88107) are very similar compositionally and petrographically, and are somewhat more loosely related to Allan Hills A77307 and Colony. Mason (1988, 1989) described the three MacAlpine chondrites as petrographically similar to CM2 chondrites. Sears (1990) suggested that they are related to Colony and ALHA77307 because they all have similar induced thermoluminescence (TL) glow curves. The natural TL values for MAC87300 and MAC87301 are also identical suggesting that those two are paired. Colony and ALHA77307, though, are petrographically similar to the CO chondrites, not CM. All of the chondrites were analyzed for 28 elements by INAA. Compositionally, MAC87300 and MAC87301 are very similar. Their Mg-normalized abundances for lithophile and siderophile elements are nearly identical. Their compositional and petrographic similarities, and the close proximity of their collection points support the conclusion that they are paired. Henceforth, both will be discussed together simply as MAC87300. The Mg-normalized lithophile abundance pattern for MAC88107 is similar to that of MAC87300, with only a small difference for K. The siderophile pattern is also similar for most elements, but shows noticeable differences (up to 20%) for Ni, Co, Ir, Os, and Br. The first four of those elements are likely affected by weathering loss of metal, while the latter is easily redistributed by weathering processes. ALHA77307 has abundance patterns similar to MAC87300 and MAC88107, but the Colony specimen was so badly weathered that a meaningful comparison cannot be made with its abundance patterns. The established carbonaceous chondrite groups are easily delineated by their Zn/Mn and Al/Mn abundance ratios. The MAC87300 and MAC88107 ratios are nearly identical and fall in the hiatus between CM and CO, as do the ALHA77307 and Colony ratios. CI-normalized refractory lithophile element abundances for MAC87300 and MAC88107 are similar to the CM-CO clan levels. Refractory and common siderophile element abundances are also similar to CM-CO levels. Differences are apparent with the more volatile elements. Moderately volatile lithophile element abundances tend to be closer to CO chondrites, distinctly lower than CM chondrites. Siderophile and chalcophile elements more volatile than Au have abundances intermediate between CM and CO. Although MAC87300 and MAC88107 are petrographically similar to CM2 chondrites, their compositional data seem to preclude their placement in the CM group. Their TL and compositional data are similar to Colony and ALHA77307, but the latter two chondrites are petrographically similar to CO3 chondrites. It seems best that MAC87300 and MAC88107 be treated as an independent grouplet. Their nearest relatives are ALHA77307 and Colony, but presently available data are too ambiguous to justify placing these in the same grouplet. References: Mason B. (1988) Ant. Met. Newsl. 11(2), 34. Mason B. (1989) Ant. Met. Newsl. 12(3), 20. Sears D.W.G., Sears H. and Myers B.M. (1990) Lunar Planet. Sci (abstract) 21st, 1121-1122.
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