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EET 99402 has the lowest abundances of highly siderophile
elements with Ru, Pd, Re, Os, Ir, and Pt in the range of
0.01-0.08 × CI. Cobalt and Ni abundances are less vari-
able (0.3-0.9 and 0.06-0.5) and do not correlate with
highly siderophile element abundances.
The oxygen isotopic compositions of brachinites show
a substantial range in Δ 17 O from ~0‰ to -0.31‰ using
modern, high-precision analysis methods [ Day et  al. ,
2012a]. This contrasts with the narrow range in Δ 17 O for
HED meteorites of −0.21‰ to −0.27‰ using these anal-
ysis methods [e.g., Greenwood et al. , 2005]. Brachina and
the possible brachinite Divnoe have the lowest Δ 17 O
(−0.31‰ and −0.30‰), while the two possible brachinites
NWA 5400 and NWA 6077 have the highest (0.00
and −0.02‰). Excluding these four, the brachinite range
would be −0.11‰ to −0.24‰.
Brachina was the only brachinite known prior to the onset
of systematic collection of meteorites in Antarctica. Because
Brachina is a fine-grained ultramafic rock with igneous tex-
ture, unfractionated incompatible lithophile trace elements,
and relatively high abundances of siderophile elements, it
was first thought to be an igneous rock that crystallized
from a melt of its own composition [ Johnson et  al. , 1977;
Floran et  al. , 1978], possibly formed by impact melting
[ Ryder , 1982]. In contrast, Nehru et  al. [1983] included
Brachina with their broad category of primitive achon-
drites, a characterization they found fit three subsequent
finds [ Nehru et al. , 1996]. In their view, brachinites had a for-
mational history somewhat similar to that of the acapulco-
ite-lodranite clan of meteorites discussed above (section 2),
although different in some details. Specifically, Nehru et al.
[1996] suggest that brachinites evolved from CI-like material
that was oxidized during planetary heating, converting
orthopyroxene to olivine as FeO increased. Some brachi-
nites were heated to the point of partial melting, allowing
removal of metal-sulfide and basaltic melts.
The first Antarctic brachinite find, AlH 84025
(Plate 48), prompted a rethinking of the petrogenesis of
the group. Warren and Kallemeyn [1989] found that the
textures and compositions of AlH 84025 and Brachina
are more compatible with an origin as igneous cumulates.
Mittlefehldt et al. [2003] and Swindle et al. [1998] similarly
argued for a cumulate origin for EET 99402/7 and Eagles
Nest based on mineralogy, texture, and composition.
Mittlefehldt et al. [2003] argued that the parent asteroid
of the brachinites was differentiated and had undergone
high degrees of melting. Mafic brachinite gRA 06128/9
is considered to be a melt composition slightly modified
by crystal accumulation and possibly metasomatism [ Day
et  al. , 2009, 2012a; Shearer et  al. , 2010]. Recent studies
have suggested that many of the ultramafic brachinites
may be anatectic residues and not cumulates. There is yet
no consensus on the petrogenesis of the brachinite parent
asteroid, but it appears that brachinites represent the
products of a range of magmatic processes. Because of
the compositional and isotopic variations among brachi-
nites, the membership of the group is not settled, making
it difficult to arrive at a consensus regarding the
differentiation history of their parent asteroid.
5.6. ASTEROIDAl CRUSTS
5.6.1. The Howardite-Eucrite-Diogenite (HED) Clan
The howardite-eucrite-diogenite clan of meteorites pro-
vides us with the most diverse suite of crustal rocks from
any asteroid. Though not universally accepted, a very
strong case can be made that HEDs are derived from the
only intact, differentiated asteroid, 4 Vesta [ Drake , 2001].
The HED meteorites sample a range of depths of their par-
ent asteroid from the surface down to the lower crust.
geochemical modeling of the differentiation of a Vesta-
sized asteroid based on HED data suggests formation at
depths of ~20-40 km may be represented by some diogen-
ites [ Righter and Drake , 1997; Ruzicka et al. , 1997]. This is
the same range of crustal exposure as seen on Vesta by the
Dawn mission where the large Rheasilvia basin floor is
found to be ~22 km below the best-fit spheroid [ Schenk
et  al. , 2012]. The HED suite thus provides a detailed
window into the crustal structure of large, differentiated
asteroids. Information derived from HEDs informs inter-
pretation of data returned by the Dawn mission [e.g., De
Sanctis et al. , 2012; Prettyman et al. , 2012], and conversely,
Dawn observations influence interpretation of HED data
[e.g., McSween et al. , 2013]. The discussion here is largely
derived from Mittlefehldt et al. [1998] and references therein,
and updated with recent results.
Eucrites are mafic rocks of rather simple mineralogy.
Major phases are pyroxene and plagioclase. Most eucrites
are coarse- to fine-grained hypabyssal or volcanic rocks,
and in these pyroxene makes up ~51 vol.% and plagio-
clase  ~43 vol.% [ Delaney et  al. , 1984]. Pyroxene originally
crystallized as ferroan pigeonite, but metamorphism has
rendered it variable mixtures of orthopyroxene, pigeonite,
and augite. Some eucrites are plutonic rocks, and these have
varied pyroxene:plagioclase ratios from 68:30 to 32:65
[ Delaney et al. , 1984]. Some of this variation is a reflection
of heterogeneity in these coarse-grained rocks, making
accurate determinations of modes from thin sections diffi-
cult, but there is nevertheless genuine variation in mineral
proportions. Minor and accessory phases are silica poly-
morphs, ilmenite, chromite, apatite, merrillite, zircon, bad-
deleyite, troilite, and metal. Diogenites are coarse-grained
ultramafic rocks composed dominantly of magnesian
orthopyroxene with minor chromite and variable amounts
of olivine from 0 to ~35 vol.%. One HED meteorite is a
dunite that is classified as a diogenite because of similarities
in mineral compositions to those in diogenites [ Beck et al. ,
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