NWA 595

A small, highly weathered (W3/4), but complete stone, weighing 196 g was purchased in M'hamid, Morocco and given the name Northwest Africa 595. Northwest Africa 595 is an olivine-rich achondrite with subchondritic chemistry and mineralogy and has been grouped among the brachinites. Its matrix constituents include minor troilite and chromite.

The brachinite group consists of chemically and mineralogically diverse members. A study of NWA 595 by Irving et al. (2005) revealed an anomalous mineral composition and an O-isotopic composition that plots away from the brachinite group. However, in a followup O-isotopic analysis and petrographic study incorporating a more thorough acid-washing procedure, Irving and Rumble III (2006) did show that NWA 595 is both isotopically and petrologically similar to the brachinite NWA 3151, as well as to the primitive achondrite NWA 4042. At the same time, these three Saharan meteorites have more positive O-isotopic values than Brachina. In their concluding statement (Abstract #5288, MAPS Vol. 41, Suppl., 2006) they suggest that "if all these specimens (including Brachina) derive from the same parent body, then it must be isotopically quite heterogeneous."

This study and a study by Greenwood et al. (2007) both revealed a disparity in O-isotopes between Brachina and the Saharan brachinites, which is consistent with an origin for the Saharan brachinites on a parent body separate from that of Brachina. Other evidence supports such a multiple parent body scenario: some brachinite members exhibit characteristics of primitive achondrites, i.e., have near-chondritic compositions, while others appear to have experienced igneous fractionation with element depletions, and to contain melt inclusions in olivine. Evidence is also ambiguous among brachinites as to whether they represent cumulates or metamorphic processes. It could be inferred that this varied group represents a diversity of petrogenetic models representing more than a single parent body.

However, it is also plausible that the isotopically and petrologically diverse suite of brachinites originated on a very heterogeneous common parent body. If the latter viewpoint is true, then the range of the brachinite group may be too narrowly defined, and perhaps some of the brachinite-like primitive achondrites such as Zag (b), Divnoe, NWA 1500, and Tafassasset are also genetically related. A transmitted light view of a petrographic thin section of NWA 595 can be seen on J. Kashuba's page. The specimen of NWA 595 shown above is a 0.81 g partial slice.

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