A single fusion-crusted stone weighing 1,500 g was found in Morocco or Algeria. The meteorite was subsequently purchased by G. Hupé while visiting in Morocco. A type specimen was submitted for analysis to the University of Washington in Seattle (A. Irving and S. Kuehner), and NWA 3151 was classified as a brachinite, the largest member of the group found to date. A separate 26.5 g stone, NWA 5191, has been determined to be a possible pairing to NWA 3151 (A. Ibhi, LPMM, and M. vanGinneken and L. Folco, MNA-SI).
Northwest Africa 3151 is a coarse-grained (0.71.6 mm), olivine-rich (~95 vol%) dunite (Irving et al., 2005). It contains minor clinopyroxene, FeNi-metal, chromite, and troilite, and rare sodic plagioclase and orthopyroxene. Silicates in the brachinites are FeO-rich (~Fa3236) and have granoblastic, recrystallized textures and equilibrated, homogeneous mineral compositions. Terrestrial oxidation of the metal phase has formed hydroxides among the grains, but the silicates remain unaltered. Northwest Africa 3151 is most similar to the paired brachinites EET 99402/407.
Initial O-isotopic data for NWA 3151 plot within the brachinite field (D. Rumble III, Carnegie Institution, Washington D.C.), but additional measurements have established more positive values than those for Brachina. It is notable that two other Saharan stones, the brachinite-like achondrites NWA 595 and NWA 4042, share very similar O-isotopic values with NWA 3151 and the other brachinites (with the exception of Brachina).
In their study of a number of petrologically and isotopically similar meteorites, including Brachina, NWA 595, NWA 3151, and NWA 4042, A. Irving and D. Rumble III concluded ". . . if all these specimens (including Brachina) derive from the same parent body, then it must be isotopically quite heterogeneous." (69th MetSoc, #5288 ). Given the wide isotopic dispersion observed among meteorite samples from both the winonaite and the acapulcoite/lodranite clan (Rumble III et al., 2008), a single-body scenario is plausible. However, the disparity in O-isotopes that exists between Brachina and the Saharan brachinites revealed in this study is consistent with an origin for the Saharan brachinites on one or more parent bodies distinct from that of Brachina. Other evidence supports such a multiple parent body scenario, in that some members exhibit characteristics of primitive achondrites with near-chondritic compositions (e.g. Brachina), while others appear to have experienced igneous fractionation through partial melting (e.g. NWA 3151); the latter exhibit siderophile, chalcophile, and incompatible element depletions, and can contain melt inclusions in olivine. It could be inferred that this varied group represents a diversity of petrogenetic models, possibly originating from more than one parent body.
Petrologic and isotopic analyses, thermodynamic modeling, and experimental trials have been conducted for a number of brachinites and brachinite-like achondrites (Gardner-Vandy and Lauretta, 2011; Gardner-Vandy et al., 2012, 2013). Evidence from these studies led to the conclusion that residual phases matching the mineralogy and mineral compositions of the brachinites could be attained through significant partial melting (1431% at ~1250°C) and melt removal on an FeO-rich (~Fa3740), R chondrite-like parent body (but not genetically related to known R chondrites). It was demonstrated that most of the brachinites formed under such a scenario at an oxygen fugacity of ~IW1, while that for Brachina was slightly more oxidizing at ~IW. Reduction of highly oxidized R-chondrite-like precursor material to brachinite compositions likely proceeded through depletion of primary water from hydrated minerals (e.g. hornblende and phlogopite) during the melting process (Gardner-Vandy et al., 2013).
In a similar manner, Sosa et al. (2017) employed multiple modeling techniques and conducted melting experiments utilizing R4 chondrite LAP 03639 in an effort to attain the composition of the GRA 06128/9 meteorite, considered to be a likely representative of the brachinite parent body feldspathic crust. Their results demonstrate that an R chondrite-like precursor asteroid can undergo low-degree partial melting (~1620%) at 1140°C at an oxygen fugacity of ~IW to produce a brachinite-like residue and a complementary evolved melt with a composition like that of GRA 06128/9. Additional experimental data and modeling results attained by Lunning et al. (2017) has further constrained the conditions of formation for GRA 06128/9. Their investigation indicates that both equilibrium and non-equilibrium partial melting (the latter condition corresponding to lower temperatures and degrees of melting) on an oxidized parent body similar to R chondrites, in which 1422% melt is generated at a temperature of 11201140°C and a redox state of IWIW+1, reproduces most closely the whole rock composition of the GRA 06128/9 meteorite. The authors also posit that unsampled lithologies containing higher silica abundances may have been produced on the GRA 06128/9 (or the brachinite) parent body, in association with very low degrees of non-equilibrium partial melting. These potential lithologies might be akin to the Almahata Sitta trachyandesite samples MS-MU-011/035, which are thought to represent the primary crust of the ureilite parent body.
If the isotopically and petrographically diverse suite of brachinites did originate on a common ancient heterogeneous parent body, then its size should be commensurate with the low degree of heating exhibited by some of the known samples. Given a scenario in which the brachinites all derive from a common parent body, the range of the group might be too narrowly defined, and perhaps some of the brachinite-like achondrites such as Zag (b) and Divnoe also share a genetic relationship.
Many of the known brachinites have disparate cosmic-ray exposure ages, indicating that they represent numerous separate ejection events. According to 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. 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 diagram for a magnified view
Diagram credit: Beard et al., 81st MetSoc, #6170 (2018)
Although the brachinite parent body still comprises a relatively small number of representative samples in our collections, a significant number of new samples have recently been found in Northwest Africa, some of which may eventually be determined to be paired. Despite the fact that the Austalian brachinite Reid 027 was found in close proximity to Reid 013 (a portion of which is classified under the synonyms Nova 003 and Window Butte) establishing a definite likelihood for their pairing, the two brachinites exhibit differences in both grain size and plagioclase composition; for some investigators, these variations support the decision to maintain their independent status until more thorough comparisons can be made.
Further details about this meteorite group can be found on the other brachinite member pages on this website. The specimen of NWA 3151 shown above is a 0.498 g thin partial slice. The photo below shows the complete uncut mass, and the bottom image is an excellent petrographic thin section micrograph of NWA 3151, shown courtesy of Peter Marmet.
Photo courtesy of Greg Hupé
click on photo for a magnified view
Photo courtesy of Peter Marmet