A portion of a fresh, fusion-crusted stone weighing 180 g was found in Northwest Africa and subsequently purchased by M. Cimala in Erfoud, Morocco. A type specimen was submitted for analysis to Northern Arizona University (J. Wittke and T. Bunch), and NWA 4969 was classified as a new brachinite. This brachinite may be paired with other brachinites found in Northwest Africa during a similar timeframe (e.g., NWA 4042, 4872, 4874, 4882, and 6349).
Northwest Africa 4969 is an olivine-rich (~89 vol%) dunite with a protogranular texture and has a bimodal grainsize, ranging from fine- to medium-grained (0.1 mm to 1.2 mm). It also contains minor clinopyroxene, K-poor sodic plagioclase, chromite, and FeNi-metal. Similar to some other brachinites, olivines contain very fine-grained assemblages of orthopyroxene and opaques lining various olivine grain boundaries which are thought to have formed during fluctuations in redox conditions (Rumble III et al, 2008).
In MAPS Vol. 41 (2006), #5288, A. Irving and D. Rumble III have argued that "if all these specimens (including Brachina) derive from the same parent body, then it must be isotopically quite heterogeneous." While this is plausible—and a wide isotopic dispersion is also observed among the winonaites and the acapulcoite/lodranite samples (Rumble III et al., 2008)—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 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 to 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. Notably, studies have found that partial melting of R chondrites at an oxygen fugacity of IW–1 could lead to the formation of brachinites (Gardner-Vandy and Lauretta, 2011).
However, 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 this concept that the brachinites all derive from a common parent body, the range of the group may be too narrowly defined, and perhaps some of the brachinite-like primitive achondrites such as Zag (b), Divnoe, and Tafassasset also share a genetic relationship.
Further details on this rare group of meteorites can be found on the other brachinite member pages of this website. The specimen of NWA 4969 shown above is a 0.93 g partial slice. The photo below shows the partial stone as found.
