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.
These meteorites 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). In a similar way, the O-isotopic composition of brachinite LEW 88763 plots within the acapulcoite/lodranite field. 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 the winonaite group (0.3‰) and the acapulcoite/lodranite group (0.75‰) (Rumble III et al., 2008). Brachinites are considered to be primitive achondrites by some (Nehru et al. , 1992). They represent products of metamorphism and oxidation of chondritic material, possibly as a partial melt residue (restite). However, other scientists have determined that they are differentiated, ultramafic achondrites (Mittlefehldt et al., 2003) representing cumulates of high-temperature igneous melts. Many features of brachinites are most consistent with a cumulate origin rather than a metamorphic origin: 1) high olivine CaO content consistent with a high-temperature, igneous origin; 2) cumulate-textured plagioclase grains enclosing olivine grains; 3) preferred orientation (lineations/foliations) of olivine crystals; 4) element patterns inconsistent with the removal of a metal–sulfide 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.
The parent body composition of the brachinites has been compared to that of H chondrites but are highly oxidized (comparable to that of L-group chondrites), likely promoted by an environment rich in accreted ices. Alternatively, 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). It was 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, Reid 013, and LEW 88763), and those depleted in a basaltic component (DBRA) (e.g., ALH 84025, Eagles Nest, and Hughes 026). The primitive acapulcoite–lodranite group has a similar scenario with an undepleted member (ACA) and a depleted member (LOD), although this group is reduced instead of oxidized. The loss of an Fe–Ni–S partial melt in the lodranites is not analagous to the siderophile–chalcophile element pattern seen in brachinites.
Brachinites crystallized only a few m.y. after the formation of CAIs and were possibly derived from residues of low degrees of melting (1–10%) of average carbonaceous chondritic material, but with nebular fractionations ~1.5% higher in high-temperature condensates and ~0.7% lower in corundum. Alternative scenarios place the depletion of siderophile and chalcophile elements in a nebular or in an impact melt setting. Still, certain brachinites (ALH 84025 and EET 99402/99407) have chemical, mineral, and petrologic characteristics that are more consistent with an origin as an igneous cumulate. Finally, both subgroups were thermally metamorphosed to type 6 or 7 creating the highly equilibrated mineral compositions and recrystallized textures that are observed. A thermal event (large impact?) at ~4.13 b.y. ago is revealed through K–Ar 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 Fe–Ni–S 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 winonaite–IAB 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 brachinites, and therefore many or all of these meteorites may be considered likely members of the brachinite group. In a like manner, the acapulcoite–lodranite group 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.
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 asteroid 289 Nenetta.
In two recent consortia, analyses of the unique Antarctic alkalic igneous meteorite GRA 06128/06129 was conducted. It was suggested that this thermally metamorphosed meteorite containing cumulate sodic plagioclase may represent very low degrees of fractional melting (~10–15%) 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 Fe–Ni–S melt phase. GRA 06128/06129 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.
A very ancient crystallization age of the GRA achondrites based on Sm–Nd and Al–Mg systematics is 4,564.9 (±0.2) m.y., consistent with Brachina 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 36–100 km in diameter. As with brachinites and the other inner Solar System objects, the Sm–Nd age of the GRA meteorite was reset ~3.4 b.y. ago, corresponding to the late heavy bombardment period. Some investigators from this consortium argue that it is most likely a member of the brachinite group (Zeigler et al., 2008), while other investigators come to the same conclusion based on studies of highly siderophile element (HSE) abundances, and upon examining the metal-sulfide segregation processes (Day et al., 2012). Still others speculate that Venus may possibly be its origin (Shearer et al., 2008). In light of its isotopic, geochemical, and mineralogical similarities to IAB complex irons such as Caddo County, it is also plausible that this could be its parent body of origin (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.
