A well-oriented, fusion-crusted meteorite weighing 154 g was found in Central Australia lying next to an eagle's nest. Although the type specimen Brachina was found in central South Australia, differences between it and Eagles Nest in mineralogy, chemistry, and average grain size indicate that they are not fall-related. Analysis and classification of Eagles Nest was conducted at the University of Arizona, Lunar and Planetary Laboratory (W. V. Boynton), and a classification of brachinite was proposed.
The modal composition of Eagles Nest is olivine (82%), clinopyroxene as augite (8%), troilite (1%), chromite (1%), chlorapatite (trace), along with secondary Fe-rich alteration products (8%) (Swindle et al., 1998; Crossley et al., 2020). Eagles Nest shows some differences to most brachinites including its lack of fine-grained assemblages of orthopyroxene and opaques lining various olivine grain boundaries. A study of Eagles Nest conducted by Swindle et al. (1998) determined that it is mineralogically and petrographically very similar to the cumulate brachinite Brachina, and that it also represents a fractionated cumulus phase rather than a partial melt residue. Goodrich (2010) described reduction features that exist on such assemblages in some brachinites, but which are absent in Eagles Nest. It was conjectured that Eagles Nest might have originated from a different brachinite-like parent body. Eagles Nest is a member of the oxidized subgroup of Crossley et al. (2020); see the NWA 3151 and NWA 11756 pages for more details about their classification scheme).
To further resolve the petrogenesis of the brachinite group, a consortium of institutions was established to make an in-depth study of the unique Antarctic meteorite GRA 06128/9. This is a high-phosphate, high-Na, coarse-grained troctolitic anorthosite (Zeigler et al., 2008), perhaps also properly called an alkalic leucodiorite (Treiman et al., 2008) or a basaltic trachyandesite/trachyandesite (mugearite/benmoreite). The meteorite is composed predominantly (~75 vol%) of albitic plagioclase (oligoclase: Ab85An15 mol%); this is an extraordinarily high abundance of plagioclase compared to other meteorite magma types. It has an O-isotopic composition that plots close to the TFL and which is indistinguishable from the plot of brachinites. In a similar manner, the Fe/Mn ratio measured for both olivine and pyroxene are indistinguishable from that of other brachinites (this ratio remains constant regardless of differentiation processes and is diagnostic for the origin of each planetary body). Moreover, the major, minor, and trace element chemistry of GRA 06128/9 is very similar to that of brachinites. Studies of highly siderophile element (HSE) abundances and examination of the metalsulfide segregation processes led to the determination by Day et al. (2012) that GRA 06128/9 was likely genetically related (i.e., from the same parent body) to the brachinites.
Consistent with the ancient crystallization age of brachinites, the GRA 06128/9 samples have a SmNd crystallization age of ~4.55 b.y. (Nyquist et al., 2008, 2009) and an AlMg age relative to D'Orbigny angrite of ~4.566 b.y. (Shearer et al., 2009), attesting to commencement of magmatism on the parent body within a couple of m.y. after CAI formation. 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. The CRE age calculated for GRA 06128/9 is 2.93.0 m.y., which is within the range of CRE ages (2.03.5 m.y.) calculated for Brachina (Mittlefehldt et al., 1998 and references therein; Matsuda et al., 2008), but is significantly different from that of Eagles Nest (4449 m.y.).
Many of the known brachinites have disparate cosmic-ray exposure ages indicating they represent numerous separate ejection events. As shown in 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 oldest cluster reflects a possible ejection event that occurred ~49.9 (±4.1) m.y. ago comprising the six brachinites Eagles Nest, EET 99402, Hughes 026, NWA 4969, NWA 7605, and NWA 10637. Importantly, two of these CRE age clusters include both brachinite and brachinite-like meteorites, which suggests there was a common parent body for all of these meteorites (see diagram below).
click on image for a magnified view
Diagram credit: Beard et al., 81st MetSoc, #6170 (2018)
It has been argued that GRA 06128/9 possibly represents a lower crustal cumulate developed after 1030% partial melting of a chondritic source on the brachinite parent body. An estimated cooling rate of 1020°C/year was derived (University of Tokyo; Miyamoto) consistent with a near-surface burial at a depth of 1520 m. This is contrasted with the deep, ultramafic mantle region from which typical brachinites are thought to have formed as residues of partial melting (Ash et al., 2008). It can be inferred from petrological, geochemical, and mineralogical data that GRA 06128/9, and thus all brachinites, originated on a large partially differentiated planetary body distinct from the Earth, Moon, or Mars; interestingly, speculation has surfaced about a possible origin on Venus (Shearer et al., 2008). Nevertheless, in light of its isotopic, chemical, and mineralogical similarities to IAB complex irons such as Caddo County, which similarly contains inclusions of albitic plagioclase (Ab84 mol%), it was considered plausible that this iron asteroid could be the parental source of the brachinites (Nyquist et al., 2009). However, a comparison between brachinites and IAB irons utilizing nucleosynthetic isotope anomalies (Mo systematics) shows them to be unrelated (see diagrams below).
(left) Kruijer et al., PNAS, vol. 114, #26, p. 6713 (2017open accesslink),
'Age of Jupiter inferred from the distinct genetics and formation times of meteorites' (http://dx.doi.org/10.1073/pnas.1704461114)
(right) Budde et al., 49th LPSC, #2353 (2018)
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. In their continued effort to attain the composition of the GRA 06128/9 meteorite, considered likely to represent the feldspathic crust of the brachinite parent body, Sosa et al. (2017) employed multiple modeling techniques and conducted 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 with an oxygen fugacity near the ironwüstite buffer (~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. Additional information concerning the origin and petrogenesis of brachinites and GRA 06128/9 can be found on the NWA 3151 and Reid 013 pages.
The specimen of Eagles Nest shown above is a 1.2 g fusion-crusted partial slice (photo courtesy of K. Regelman). The top photo below shows a complete slice of Eagles Nest, while the bottom photo shows a small preserved portion of the oriented face of this meteorite exhibiting radial flowlines.