Primitive Achondrite, ungrouped
(Brachinite-like; Brachinite in MetBull 86)
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 follow-up 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, 69th 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.
On the basis of a number of published studies, there are several factors that indicate 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 an orthopyroxene abundance (1015 vol%) significantly higher than in typical brachinites; notably, this is 10× the next highest abundance, measured for 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.
Many of the known brachinites have disparate cosmic-ray exposure ages, indicating that they represent numerous separate ejection events. As shown in a study by Patzer et al. (2003), the CRE ages of EET 99402/407, Hughes 026, and Eagles Nest form a cluster at ~48 m.y., and those of Reid 013 and ALH 84025 coincide at ~10 m.y. In a separate study by Ma et al. (2003), the cosmogenic nuclide calculations establish a range of CRE ages from 4 m.y. for Brachina to ~25.5 m.y. for Eagles Nest. From their noble gas analyses of 15 brachinite and brachinite-like meteorites, together with the literature values for seven others, Beard et al. (2018) identified three potential CRE age clusters. The intermediate cluster reflects a possible ejection event that occurred ~25.0 (±3.4) m.y. ago, comprising the five brachinites LEW 88763, NWA 3151, NWA 4874, NWA 7297, and RaS 309, and the three brachinite-like meteorites NWA 595, NWA 6077, and NWA 8777. It should be noted that although the FeO-rich LEW 88763 is currently classified as a brachinite, new analyses by Day et al. (2015) led them to propose a reclassification as anomalous achondrite, with a possible relationship to the ungrouped achondrite NWA 6704 pairing group, now considered a member of the newly proposed ténéréite group (Agee et al., 2020). Importantly, two of the resolved CRE age clusters include both brachinite and brachinite-like meteorites, which attests to a common parent body for all of these meteorites (see diagram below).
click on image for a magnified view
Diagram credit: Beard et al., 81st MetSoc, #6170 (2018)
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.