NORTHWEST AFRICA 3151


Brachinite
standby for nwa 3151 photo
Purchased April 2005
no coordinates recorded

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 (0.7–1.6 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 [2006]). 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 (14–31% 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 ~IW–1, 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 (~16–20%) 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.

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), Divnoe, and NWA 5400 also share a genetic relationship.

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); these include NWA 595, NWA 3151, NWA 4042, NWA 4872, NWA 4874, NWA 4876, NWA 4882, NWA 4969, NWA 5191, NWA 5471, and NWA 6349. 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. 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, possibly related to the ungrouped achondrite NWA 6704 pairing group.

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.

standby for nwa 3151 photo
Photo courtesy of Greg Hupé

standby for nwa 3151 ts photo
click on photo for a magnified view
Photo courtesy of Peter Marmet