05 Classification of.. - Department of Earth and Planetary Sciences

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pyroxene, calcium-rich olivine, and anorthitic
plagioclase (Table 6 and Figure 18); spinel,
troilite, kirschsteinite, whitlockite, titanomagnetite,
and FeNi-metal are accessory (e.g., Prinz et al.,
1977; McKay et al., 1988, 1990; Yanai, 1994;
Mittlefehldt et al., 1998). Although oxygen
isotopic compositions of angrites are indistinguishable
from those of the HED meteorites,
brachinites, and mesosiderites (Figure 17), the
unusual mineralogies and compositions of
the angrites suggest that they are not related to
any of these other meteorite groups. Angrites are
the most highly alkali-depleted basalts in
the solar system and have low abundances of the
moderately volatile element gallium. However,
they are not notably depleted in the highly volatile
elements—bromium, selenium, zinc, indium,
and cadmium—compared to lunar basalts and
basaltic eucrites. All angrites have superchondritic
Ca/Al ratios, which contrasts with the
chondritic Ca/Al ratios of eucritic basalts
(Mittlefehldt et al., 1998).
1.05.4.2.2 Aubrites
The aubrites (or enstatite achondrites) are
highly reduced enstatite pyroxenites. Except
Shallowater, which has an igneous texture
(Keil et al., 1989), all are fragmental (Figure 19)
or, less commonly, regolith breccias, as indicated
by their high contents of solar-wind-implanted
noble gases (e.g., Keil, 1989). They consist mostly
(,75–95 vol.%) of nearly FeO-free enstatite, but
also contain lesser amounts of albitic plagioclase
and nearly FeO-free diopside and forsterite
(e.g., Watters and Prinz, 1979). Clasts of both
igneous and impact-melt origin are common,
and the precursors to the breccias were mostly
coarse-grained (probably plutonic) orthopyroxenites
(Okada et al., 1988). Other phases
include accessory silicon-bearing FeNi-metal
Classification of Meteorites
(e.g., Casanova et al., 1993) and a host of unusual
sulfides in which elements that are normally lithophile
are chalcophile, such as titanium in heideite
(FeTi 2 S 4 )(Keil and Brett, 1974) and calcium in
oldhamite (CaS) (Wheelock et al., 1994).
Similarities in mineralogy and oxygen isotopic
compositions (Figures 4 and 17) suggest that
aubrites are related to enstatite chondrites,
although they are not thought to have formed on
the same parent bodies. Arguments have been
presented that the brecciated aubrites, the igneous
Shallowater aubrite, and the H- and L-group
enstatite chondrites represent samples from
four different asteroidal parent bodies (Keil, 1989).
1.05.4.2.3 Brachinites
The brachinites are medium- to coarse-grained
(0.1–2.7 mm) equigranular dunitic wehrlites,
consisting dominantly of olivine (74–98%); they
also contain augite (4–15%), plagioclase
(0–10%), orthopyroxene (traces), chromite
(0.5–2%), Fe-sulfides (3–7%), minor phosphates,
and FeNi-metal (Table 6 and Figure 20).
Lithophile element abundances in Brachina are
close to chondritic and are unfractionated,
whereas other brachinites show depletions in
aluminum, calcium, rubidium, potassium, and
sodium (Mittlefehldt et al., 1998). Siderophile
element abundances in brachinites are nearchondritic
(,(0.1–1) £ CI). Trapped noble gases
show planetary-type patterns. Oxygen isotopic
compositions are indistinguishable from those of
HED meteorites (Figure 17), but brachinites are
clearly not from the HED parent body.
The question of whether brachinites are primitive,
partial melt residues, or igneous cumulates
(Warren and Kallemeyn, 1989; Nehru et al., 1996)
remains open. Petrologic and geochemical differences
between the different brachinites suggest
that they may not have all formed by the same
process (Mittlefehldt et al., 1998; Mittlefehldt
and Berkley, 2002).
Figure 19 Hand-specimen photograph of the Bustee
aubrite showing the brecciated texture of the rock (9 cm
in horizontal dimension) (photograph courtesy of the
Smithsonian Institution).
1.05.4.2.4 Ureilites
The ureilites (Figure 21) are carbon-bearing
ultramafic rocks composed of olivine, pyroxene,
and ,10% dark interstitial material consisting of
varying amounts of carbon, metal, sulfides, and
minor fine-grained silicates (Mittlefehldt et al.,
1998). The characteristic features of ureilites that
distinguish them from other achondrite groups
include: (i) high CaO contents in olivine and
pigeonite; (ii) high Cr 2 O 3 contents in olivine;
(iii) high (up to ,5 wt.%) carbon contents;
(iv) presence of reduced rims on olivine (and
sometimes pyroxene) where in contact with
graphite; and (v) large range in oxygen isotopic
compositions (Figure 17) which plot along