An oriented, fusion-crusted stone weighing 300 g was found near the site of the Zag meteorite fall in Morocco (see photo of main mass with flow lines at the RSPD website). Zag (b) is an unbrecciated achondrite rich in forsteritic olivine (68 vol%) that incorporates mm-sized orthopyroxene channels containing metal or sulfide or weathered metal inclusions. These orthopyroxene and opaque assemblages exhibit several features that indicate the occurrence of a late reduction process, and the FeMnMg relations are also consistent with reduction processes on the brachinite parent body. Several methods for the reduction of primary olivine were reviewed by Goodrich et al., 2017), including its reaction with methane to form orthopyroxene + metal (Irving et al., 2013) and through its sulfurization by a S-rich fluid or gas to form orthopyroxene + sulfide (e.g., Singerling et al., 2013). Olivine and orthopyroxene exhibit variability in their distribution (Day et al., 2012). The oxygen isotope ratios plot close to those for Divnoe, NWA 4042, and the brachinites, although Zag (b) has undergone a more thorough reduction process.
Zag (b) shares very close similarities with Divnoe and the brachinites in Fe ratios and oxygen isotopic systematics. Moreover, the acapulcoites and lodranites fall on the same oxygen mixing line, and they have FeMnMg features consistent with a close relationship to Zag (b), Divnoe, and the brachinites. All of these different meteorites likely formed in the same nebular region from common precursor material, with Zag (b) having an intermediate position between Divnoe and the ACALOD cluster. Because of its close relationship to Divnoe, and by extension, the brachinites, Zag (b) is sometimes conveniently included with the brachinites in classification studies. Further information about Zag (b) can be found in the abstract 'Zag (b): A Ferroan Achondrite Intermediate Between Brachinites And
Lodranites' by Delaney et al., 31st LPSC, #1745 (2000).
On a newly compiled O-isotope diagram for brachinites and other planetary achondrites, published by Rumble III et al. (2008), Zag (b) has a Δ17O value that plots within a select grouping of brachinites including NWA 3151, NWA 595, and the ungrouped brachinite-like achondrite NWA 4042, and these investigators believe that Zag (b) should probably be lumped with the brachinites. However, 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 Zag (b) 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. 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.
The measured HSE abundances are consistent with a partially melted parent body in which heating from short-lived radionuclides came to a halt before a core was fully formed. Studies of the fractionation trends for Zag (b) led Day and Warren (2015) to conclude that this meteorite might not have been a residue after partial melting, but instead represents a cumulate that incorporated a residual metallic melt with a high Pt/Os ratio; the ungrouped cumulate achondrite NWA 6704 and the brachinite-like cumulate achondrite MIL 090206 (and pairings) exhibit similar elevated Pt/Os ratios.
The specimen of Zag (b) shown above is a 2.2 g partial slice. A microscopic examination reveals the yellow-green olivine enclaves scattered throughout this meteorite.