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(Figure  5.8). They are critically silica-undersaturated,
and hence with crystallization the silica activity in the
magma decreases, resulting in olivine and finally
olivine + kirschsteinite crystallizing in the groundmass.
This characteristic is unique among basaltic achondrites;
all others differentiated towards silica-saturated composi-
tions, crystallizing a silica polymorph in their mesostases.
Angrites are highly depleted in some moderately volatile
and volatile elements [ Mittlefehldt and Lindstrom , 1990;
Mittlefehldt et al. , 1998; Mittlefehldt et al. , 2002; Riches
et  al. , 2012]. Sodium abundances are ~0.02-0.04 × CI;
hence the nearly pure anorthite plagioclase they contain.
The refractory lithophile trace element contents of angrites
fall into three groups. Most have CI-normalized abundances
of 10-20; Angra dos Reis has abundances of ~30 × CI, while
NWA 2999 and NWA 4931 have abundances of ~5 × CI.
The latter two are thought to be paired. There is little frac-
tionation of one refractory lithophile trace element from
another, and all have relatively flat CI-normalized patterns,
with the exception of NWA 4931 which is poor in Nb and
Ta compared to other refractory lithophile elements [ Riches
et  al. , 2012]. Highly siderophile element contents [Ru, Pd,
Re, Os, Ir, and Pt) of angrites are very low in general and
highly variable [ Riches et  al. , 2012]. NWA 4931 is excep-
tional and has the highest abundances at ~0.2-0.3 × CI.
At the other extreme, NWA 1296 has CI-normalized
abundances of ~10 -5 . However, most angrites have highly
siderophile element abundances of between ~10 -4 and
~2 × 10 -2 . The highly siderophile elements are in roughly
chondritic relative ratios, although the most volatile of
them, Pd, is at slightly lower abundance in some angrites.
Prior to the first angrite recoveries from Antarctica, our
worldview of the differentiation history of the angrite par-
ent body was totally dependent on study of Angra dos
Reis, mostly derived from the 1977 consortium investiga-
tions. Angra dos Reis is old; the latest Pb isotopic data put
its formation at 4557.65 ± 0.13 Ma [ Amelin , 2008]. The
coarse grain size, unusual mineralogy, nearly monominer-
alic character, and preferred orientation of the ground-
mass calcic pyroxene led to the model that Angra dos Reis
is a cumulate from a highly silica-undersaturated magma
[ Prinz et  al. , 1977]. The large ion lithophile element con-
tents of bulk rock Angra dos Reis, at ~20-30 × CI, are high
for a cumulate. An acid-washed calcic pyroxene separate
was used to construct a trace-element-based model for the
petrogenesis of Angra dos Reis: the parent melt of Angra
dos Reis was formed by ~1% partial melting, and ~50% of
the parent melt had crystallized as calcic pyroxene prior
to formation of the Angra dos Reis cumulate [ Ma et al. ,
1977]. To generate a highly silica-undersaturated melt,
melting was envisioned to occur at >700 MPa pressure.
(For comparison, the central pressure of the largest
asteroid, 1 Ceres, is ~140-150 MPa.) An alternative
interpretation of the petrology of Angra dos Reis is that it
is nearly a melt composition, and that it is thus a metamor-
phosed volcanic rock [ Treiman , 1989].
One key aspect of angrites is that because they have not
been significantly shock-modified, and many still retain
extensive igneous zoning, the parent-daughter relation-
ships of radiogenic isotopic systems have not been
disturbed since crystallization for volcanic angrites, or
since metamorphism for the plutonic angrites. Because of
the very low volatile element contents, the parent asteroid
μ ( 238 U/ 204 Pb) is very high, and thus very precise 207 Pb- 206 Pb
absolute ages can be calculated from measured Pb isotopic
compositions. The very low highly siderophile element
contents of some indicate efficient separation of metal
from silicate during magma genesis, facilitating application
of the 182 Hf- 182 W short-lived chronometer. With the
recovery of the mineralogically varied angrites from
Antarctica, 26 Al- 26 Mg dating became practical because
anorthite and olivine provide a long lever arm, while spinel
and olivine provide the same for 53 Mn- 53 Cr dating. This
combination of very precise absolute Pb-Pb ages with pre-
cise dating by several short-lived chronometer systems
allowed for the first time pinning absolute ages to different
processes that occurred in the earliest history of the solar
system. The 207 Pb- 206 Pb, 182 Hf- 182 W, and 53 Mn- 53 Cr systems
are concordant [ Kleine et  al. , 2012], while the 26 Al- 26 Mg
suggests ages that are ~1 Myr younger for reasons that are
not clear [ Spivak-Birndorf et  al. , 2009]. The 182 Hf- 182 W
system indicates that core formation on the angrite parent
asteroid occurred within ~2 Myr of CAI formation. The
oldest angrite, D'Orbigny, crystallized ~4 Myr after CAI
formation and has a 207 Pb- 206 Pb age of 4563.4 ± 0.3 Ma.
Magmatism persisted for ~7 Myr, until 4556.6 ± 0.2 Ma,
the 207 Pb- 206 Pb age of Angra dos Reis.
Starting with the availability of the first two Antarctic
angrites, lEW 86010 and lEW 87051 (Plates 53 and 54),
more realistic and detailed petrologic models of angrite
formation could be constructed. Early results showed
that Angra dos Reis is not simply related to lEW 86010
and lEW 87051 in that a distinct source region is
required [ Mittlefehldt and Lindstrom , 1990; Longhi ,
1999]. However, the compositions of lEW 86010 and
lEW 87051 could be simply related via olivine control
acting on a common magma composition [ Mittlefehldt
and Lindstrom , 1990]. Further work on additional
angrites has shown that olivine control can explain major
and minor element compositions of many volcanic
angrites; addition of magnesian olivine xenocrysts to a
parent melt with a composition like that of D'Orbigny
and Sahara 99555 can replicate the composition of oliv-
ine-phyric angrites A-881371 and lEW 87051 [ Mikouchi
et  al. , 2004]. This model extends to lithophile trace
elements and includes NWA 1296, which is composition-
ally similar to D'Orbigny and Sahara 99555. However,
olivine control no longer seems a viable explanation for
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