A mass of 330 g was found in the Nullarbor region of Western Australia. Its O-isotopic composition and bulk chemistry matched that of four previously identified olivine-rich achondrites. Interestingly, another brachinite, Reid 027, was recovered in the same region as Reid 013 (~59 km away), but it has been argued that it is unpaired to Reid 013 in light of grain size and plagioclase compositional differences.
Reid 013 is composed primarily of olivine (<90 vol%) forming 120° triple junctions, minor clinopyroxene (~6 vol%) and chromite (~1 vol%), along with trace amounts of orthopyroxene, FeNi-metal, and sulfides (Hasegawa and Mikouchi, 2016). Brachinites are resolved into a small, unique, but diverse group, which was initially identified in 1983 with the discovery of the Brachina meteorite. Interestingly, on an oxygen three-isotope diagram Brachina plots well away from the other known brachinites and is close to the winonaite field (Greenwood et al., 2007). Another feature of Brachina that is contrary to other brachinite members is its high abundance of plagioclase, measured to be 9.9 vol%other brachinites contain none or only trace amounts (Mittlefehldt et al., 1998).
Brachinites exhibit significant O-isotopic heterogeneity with a dispersion of 0.17, a value which lies between that of HEDs (0.12) on one hand, and both the winonaite group (0.3) and the acapulcoitelodranite clan (0.75) on the other (Rumble, III et al., 2008). It is inferred that the brachinites either formed on a single heterogeneous parent body, or on multiple bodies that underwent similar petrogenetic processes (Gardner-Vandy et al., 2012). This relatively high O-isotopic heterogeneity of brachinites has persuaded some that the meteorites should be considered primitive achondrites (Nehru et al. , 1992). However, other investigators have argued that brachinites are differentiated, ultramafic achondrites representing cumulates (Mittlefehldt et al., 2003), or partial melt residues (Gardner-Vandy et al., 2013) of high-temperature igneous melts.
Brachinites represent products of metamorphism and oxidation of chondritic material, some likely formed as a partial melt residue (restite) and others as a cumulate. Some brachinites and brachinite-like meteorites, including Reid 013, Hughes 026, NWA 7388, and NWA 7605 contain secondary reduction features possibly formed during parent body metasomatism by methane (Irving et al., 2013). Brachinites such as ALH 84025 and EET 99402/07 exhibit several features that are consistent with a cumulate origin: 1) high olivine CaO content consistent with a high-temperature, igneous origin; 2) cumulate-textured plagioclase grains enclosing olivine grains; 3) crystallographic preferred orientation (CPO) comprising foliation [planar alignment] and lineation [much longer in one dimension than in the other two] of olivine; 4) element patterns inconsistent with the removal of a metalsulfide melt; 5) lack of tectonic strain features; 6) mineralogy exhibiting igneous textures rather than metamorphic recrystallization; and 7) REE patterns and elemental ratios more consistent with igneous rocks containing cumulate plagioclase. A petrographic analysis of Reid 013 conducted by Hasegawa and Mikouchi (2016) revealed no obvious preferred orientation of olivine crystals, which is consistent with a partial melt residue rather than a cumulus origin. However, in another investigation of olivine CPO in brachinite EET 99402/07, Hasegawa et al. (2017) found a CPO pattern (preferential alignment along the b axis) similar to that observed in Reid 013 and the brachinite-like MIL 090206 pairing group, one which is indicative of high-degree melting and olivine accumulation; further study is needed to distinguish between a CPO pattern associated with compaction of residual grains and one associated with accumulation from a melt.
Alternatively, studies have demonstrated that partial melting and melt removal of a chondritic source similar to R chondrites at an oxygen fugacity of IW1 could lead to the formation of FeO-rich brachinites (Gardner-Vandy and Lauretta, 2011; Gardner-Vandy et al., 2012, 2013). It has been proposed that the brachinites could be divided into two subgroups (Nehru, et al., 1996): those which are near-chondritic and undepleted (UBRA) (e.g., Brachina and Reid 013), and those depleted in a basaltic component (DBRA) (e.g., ALH 84025, Eagles Nest, and Hughes 026). The primitive acapulcoitelodranite clan has a similar scenario with an undepleted member (ACA) and a depleted member (LOD), although this group is reduced instead of oxidized. Moreover, the loss of an FeNiS partial melt in the lodranites is not analagous to the siderophilechalcophile element pattern seen in brachinites. Further evidence for a formation of brachinites as a partial melt residue is provided on the NWA 3151 page. Ultimately, brachinites were thermally metamorphosed to type 6 or 7 creating the highly equilibrated mineral compositions and recrystallized textures that are observed. A thermal event at ~4.13 b.y. ago is revealed through KAr isotopic chronometry.
Brachinites have trace element compositions similar to those of winonaites and silicate inclusions in IAB-IIICD irons: a high ratio of refractory metals to nonrefractory metals (Ir/Au), depletion of chalcophiles (through loss of FeNiS melt), and high volatile abundances (Zn). If the highly oxidized brachinites were to undergo redox exchange processes, the resulting chemistry would be very similar to that of the pyroxene-rich winonaites. On an oxygen 3-isotope diagram, the brachinites overlap parts of the winonaiteIAB complex field, as well as parts of the HED and angrite fields. Moreover, O-isotopes and mineralogy for several additional olivine-rich achondrites, including Zag (b), Divnoe, and NWA 4042, plot very close to the brachinite field, and therefore many or all of these meteorites might be members of the brachinite group, or could reflect formation by analogous processes on different parent bodies. In a like manner, the acapulcoitelodranite clan also falls on the same O-mixing line, suggesting that each of these diverse groups may be related to a common chondritic precursor through variable redox processes.
In two recent consortia, analyses of the unique Antarctic alkalic igneous meteorite GRA 06128/9 was conducted. It was suggested that this thermally metamorphosed meteorite containing cumulate sodic plagioclase may represent moderate degrees of fractional melting (1330%) at intermediate temperatures (<1200°C) on an L-like chondrite parent body having chondritic abundances of moderately-volatile elements (Shearer et al., 2009). This alkali-rich partial melt phase was subsequently extracted under volatility-enhanced (possibly water) conditions, after incorporation of a low-temperature FeNiS melt phase. GRA 06128/9 has Δ17O-isotopic values, Fe/Mn ratios, and olivine Ni contents which are similar to those of brachinites (and with similar trends to those of angrites, pallasites, mesosiderites, and Vesta), although its δ18O values are higher than those of brachinites and other meteorite groups. It also has major, minor, and trace element chemistry similar to brachinites, and it was formed under similar redox conditions.
Recent laboratory melting experiments conducted by Gardner-Vandy et al. (2013) have demonstrated that an FeO-rich (oxidized) R chondrite-like precursor asteroid can undergo significant partial melting (1431% at ~1250°C) and melt removal to produce a brachinite-like residue, and possibly also a low degree partial melting (<10% at <1250°C) and melt removal to produce a complementary evolved melt having a composition like that of the brachinite-related GRA 06128/9. Sosa et al. (2017) employed multiple modeling techniques and conducted further melting experiments utilizing R4 chondrite LAP 03639. 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.
A very ancient crystallization age of the GRA achondrites based on SmNd and AlMg systematics is 4.5649 (±0.0002) b.y., consistent with that of Brachina (MnCr age of 4.5648 [± 0.0005] b.y.; Dunlap et al., 2016) and other brachinites, and consistent with ancient magmatism and crustal formation on the brachinite parent body just 2.65 m.y. after CAI formation (Wimpenny et al., 2011). Dating single feldspar grains in GRA 06128 gave an age of 4.34 (±0.02) b.y. (Lindsay et al., 2011). A model developed by Senshu and Usui (2011) predicts that formation occurred at a depth of <12 km on a parent body 36100 km in diameter. As with brachinites and the other inner Solar System objects, the SmNd age of the GRA meteorite was reset ~3.4 b.y. ago, close to the Late Heavy Bombardment period. Some investigators argue that GRA 06128/9 is most likely a member of the brachinite group based on an examination of the metal-sulfide segregation processes and the observed highly siderophile element (HSE) abundances (Zeigler et al., 2008; Day et al., 2012).
Many of the known brachinites have different cosmic-ray exposure ages, indicating different ejection events for each. 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. Current research to determine the olivine compositions of asteroids has established a strong link between brachinites and the A-type asteroids 289 Nenetta and 246 Asporina (Sunshine et al., 2007).
As demonstrated by Sanborn et al. (2014), a coupled Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships between meteorites. Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. The diagram below includes the brachinites Brachina, NWA 3151, and the GRA 06128/9 paired stones, and it is apparent that they all plot very near each other in Δ17Oε54Cr space, consistent with formation in a common isotopic reservoir.
Diagram credit: Sanborn and Yin, 46th LPSC, #2241 (2015)
While there has been some speculation that Venus may possibly be the source for the brachinites (Shearer et al., 2008), others have recognized the isotopic, geochemical, and mineralogical similarities that exist between GRA 06128/9 and the IAB complex irons such as Caddo County and consider this iron asteroid to be a plausible source (Nyquist et al., 2009). Reid 013 is among the least weathered brachinites discovered to date. The photo above shows a 1.3 g slice of Reid 013, with the reverse shown below.