Eagles Nest

Brachinite

Found Summer of 1960
no coordinates recorded A well-oriented, fusion-crusted meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and 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. Eagles Nest shows some differences to most brachinites including its lack of fine-grained assemblages of orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur and opaques lining various olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion grain boundaries. Goodrich (2010) described reductionOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants 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 bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today..

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 troctoliticTroctolite is an intrusive igneous rock consisting of plagioclase feldspar and olivine. It is a member of gabbroic rocks family. It is compositionally similar to gabbro. The main difference is that it does not contain pyroxene or contains very little while it is a major mineral in gabbro. It can anorthositeA phaneritic, intrusive igneous rock made with a modal composition (i.e. volume%) > 90% plagioclase feldspar of undefined composition (anorthitic to albitic, or combination thereof), and a small mafic component between 0 - 10% such as pyroxene, ilmenite, magnetite, and olivine 1. The name anorthosite is derived from the calcium-rich (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 plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite (oligoclase: Ab85An15 mol%); this is an extraordinarily high abundance of plagioclase compared to other meteorite magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava. 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 pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. are indistinguishable from that of other brachinites (this ratio remains constant regardless of differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities.   When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including processes and is diagnostic for the origin of each planetary body). Moreover, the major, minor, and trace elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of chemistry of GRA 06128/9 is very similar to that of brachinites. Studies of highly siderophile elementLiterally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, Mo, Re, Au, and PGE. These elements are relatively common in undifferentiated meteorites, and, in differentiated asteroids and planets, are found in the metal-rich cores and, consequently, extremely rare on (HSE) abundances and examination of the metal–sulfide 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 crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. age of brachinites, the GRA 06128/9 samples have a Sm–Nd crystallization age of ~4.55 b.y. (Nyquist et al., 2008, 2009) and an Al–Mg age relative to D’Orbigny angriteType of evolved achondrite meteorite that represent some of the earliest stages of asteroidal differentiation and magmatism in our solar system. Angrites are named for the Angra dos Reis meteorite, which fell in Rio de Janeiro, Brazil, in early 1869. They are basaltic (mafic) rocks, often containing porous areas, and 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 CAISub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they formation. As with brachinites and the other inner Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. objects, the Sm–Nd age of the GRA meteorite was reset ~3.4 b.y. ago, close to the Late Heavy BombardmentPeriod between ~4.0 to 3.8 Ga ago when the Moon and other objects in the Solar System were pounded heavily by wayward asteroids. The evidence for the Late Heavy Bombardment (LHB) includes the lunar maria basins and similar structures elsewhere, such as the Caloris Basin on Mercury and the great period. The CRE age calculated for GRA 06128/9 is 2.9–3.0 m.y., which is within the range of CRE ages (2.0–3.5 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 (44–49 m.y.).

Many of the known brachinites have disparate cosmic-ray exposure ages, indicating that they represent numerous separate ejection events. 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 nuclideA nuclear species characterized by Z protons and N neutrons. calculations establish a range of CRE ages from 4 m.y. for Brachina to ~25.5 m.y. for Eagles Nest. From their noble gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. 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 attests to 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 cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. developed after 10–30% partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting of a chondritic source on the brachinite parent body. An estimated cooling rate of 10–20°C/year was derived (University of Tokyo; Miyamoto), consistent with a near-surface burial at a depth of 15–20 m. This is contrasted with the deep, ultramaficTerm used for silicate minerals with cations predominantly Mg and/or Fe. Mafic minerals are dominated by plagioclase and pyroxene, and also contain smaller amounts of olivine. mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in 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 isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: ¹H, ²H (deuterium), and ³H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. anomalies (Mo systematics) shows them to be unrelated.
Diagrams credit:
(left) Kruijer et al., PNAS, vol. 114, #26, p. 6713 (2017), ‘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 (oxidizedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants) R chondrite-like precursor asteroid can undergo significant partial melting (14–31% 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 to be a likely representative of the brachinite parent body feldspathic crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the, Sosa et al. (2017) employed multiple modeling techniques and conducted melting experiments utilizing R4 chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such LAP 03639. Their results demonstrate that an R chondrite-like precursor asteroid can undergo low-degree partial melting (~16–20%) at 1140°C with an oxygen fugacityUsed to express the idealized partial pressure of a gas, in this case oxygen, in a nonideal mixture. Oxygen fugacity (ƒO2) is a measure of the partial pressure of gaseous oxygen that is available to react in a particular environment (e.g. protoplanetary disk, Earth's magma, an asteroid's regolith, etc.) and near the iron–wï ¿ ½stite 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 equilibriumTerm used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A system in static and non-equilibrium partial melting (the latter condition corresponding to lower temperatures and degrees of melting) on an oxidized parent body similar to R chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such, in which 14–22% melt is generated at a temperature of 1120–1140°C and a redoxOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants state of IW–IW+1, reproduces most closely the whole rock composition of the GRA 06128/9 meteorite. The authors also posit that unsampled lithologies containing higher silicaSilicon dioxide, SiO2. 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 fusionProcess in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion. 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.

click on image for a magnified view

Photos courtesy of Ken Regelman

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For additional information on GRA 06128/9, read the PSRD article by G. Jeffrey Taylor: ‘More Evidence for Multiple Meteorite Magmas‘, Feb 2009.

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