NORTHWEST AFRICA 595


Primitive Achondrite, ungrouped
(brachinite-like)
standby for northwest africa 595 photo
Purchased November 9, 2000
no coordinates recorded

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 primitive achondrite with subchondritic chemistry and mineralogy, and despite some anomalous features like more magnesian silicates, has been grouped by some investigators with 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. The Fe-Mn-Mg relations of NWA 595 olivine show correlations to other brachinites, and as found in many brachinites, fine-grained assemblages of orthopyroxene and opaques lining olivine grain boundaries are present throughout (Goodrich, 2010). At the same time, these three Saharan meteorites have more positive O-isotopic values than Brachina. In their concluding statement (Irving and Rumble III, 2006 MetSoc #5288) 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 some Saharan brachinites, which is consistent with an origin for these 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 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 might be too narrowly defined, and perhaps some of the brachinite-like primitive achondrites are also genetically related. Goodrich et al. (2017) determined that brachinites and brachinite-like achondrites have a distinct redox trend and a higher Fe/Mg ratio compared to all other primitive achondrites, consistent with formation in a similar nebula reservoir; therefore, they suggest that brachinites and brachinite-like achondrites be called the brachinite clan.

According to published studies, several factors indicate that NWA 595 and similar brachinite-like achondrites may not be members of the brachinite group. Despite its similarities to the brachinites in chemical composition and Xe-isotopic ratios, the O-isotopic ratios of NWA 595 plot outside of the brachinite field towards the TFL. In addition, FeO/MnO ratios of both olivine and clinopyroxene are lower than for typical brachinites and plot outside of the brachinite field. Moreover, NWA 595 contains more magnesian olivine, lacks plagioclase (along with ALH 84025 and Eagles Nest), and contains more orthopyroxene than is typically found in brachinites, 10 × the next highest abundance in Hughes 026. The brachinite-like MIL 090206 has similar abundances of opx and magnesian olivine to NWA 595 (Goodrich et al., 2012). Moreover, through studies of highly siderophile element (HSE) abundances, and upon examining the metal-sulfide segregation processes, it was determined by Day et al. (2012) that NWA 595 and similar brachinite-like achondrites were not likely genetically related (i.e. from the same parent body) to brachinites, but rather, originated on similar volatile-rich, oxidized, chondritic precursor asteroids while experiencing similar petrologic processes during their formation history.

A transmitted light view of a petrographic thin section of NWA 595 can be seen on John Kashuba's page. The specimen of NWA 595 shown above is a 0.81 g partial slice.