Brachinite
Found Summer of 1960
no coordinates recorded
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).
A consortium of institutions was recently 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, which is composed predomanantly (75 vol%) of albitic plagioclase (oligoclase: Ab85An15 mol%)—an extraordinarily high abundance 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, its major, minor, and trace element chemistry is very similar to that of brachinites. Studies of highly siderophile element (HSE) abundances, and examination of the metal-sulfide segregation processes, it was determined by Day et al. (2012) that GRA 06128/9 and were likely genetically related (i.e. from the same parent body) to brachinites.
Consistent with the ancient crystallization 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 angrite of ~4.566 b.y. (Shearer et al., 2009), consistent with the beginning 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 Sm–Nd age of the GRA meteorite was reset ~3.4 b.y. ago, corresponding to the late heavy bombardment period.
It has been argued that GRA 06128/9 possibly represents a lower crustal cumulate developed after 10–30% partial 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, ultramafic mantle region from which the 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 from a large, partially differentiated planetary body distinct from the Earth, Moon, or Mars—speculation has surfaced about a possible origin on Venus (Shearer et al., 2008). However, 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 is plausible that this could be its body of origin (Nyquist et al., 2009). ∗ Additional information concerning petrogenetic scenarios for GRA 06128/9 can be found on the Reid 013 page.
Eagles Nest has a CRE age of 44–49 m.y.; that calculated for GRA 06128/9 is 2.9–3.0 m.y., which is within the range of CRE ages (2.0–3.5) calculated for Brachina (Mittlefehldt et al., 1998, and references therein; Matsuda et al., 2008). Eagles Nest shows some differences to most brachinites such as a lack of fine-grained assemblages of orthopyroxene and opaques lining various olivine grain boundaries. Goodrich (2010) described reduction features that exist on such assemblages in some brachinites, which are necessarily absent in Eagles Nest. It was concluded that Eagles Nest may have originated from a different brachinite-like parent body. Further information about the diverse brachinite group can be found on the other brachinite member pages of this website.
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.
Click the photo above for an enlarged view
Photo courtesy of K. Regelman
