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

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Figure 28 Bulk oxygen isotopic compositions of SNC
meteorites, lunar meteorites, and HED meteorites
(source Clayton and Mayeda, 1996).
Classification of Meteorites
of finer-grained FeO-rich olivine, pigeonite,
augite, maskelynite, and other late phases
(McSween et al., 1979a,b; Treiman et al., 1994;
Ikeda, 1997). McSween et al. (1979a,b)
noted that the dominant mineralogy of these
rocks is consistent with early crystallization
from magmas having the crystallization sequence
inferred for basaltic shergottites such as
Shergotty. This led to their classification as
shergottites. Some authors, however, feel that
they should be referred to simply as martian
lherzolites (e.g., Treiman et al., 1994).
EET79001 is a unique shergottite consisting
of two lithologies (designated A and B)
separated by an obvious contact (Figure 30(a)).
Lithology B is a clinopyroxene–plagioclase
rock resembling the basaltic shergottites. Lithology
A, however, is distinct from either the
basaltic or lherzolitic shergottites. It has a
porphyritic texture consisting of megacrysts of
olivine, orthopyroxene, and chromite, in a finergrained
pigeonite–plagioclase groundmass. Fournew
shergottites (e.g., Dar al Gani 476 and Sayh
al Uhaymir 005 (Figure30(b))) consist entirely of
olivine–porphyritic lithologies resembling
EET79001 lithology A. These meteorites have
been referred to by various terms, including
“basaltic shergottite” (on the basis of plagioclase
content), “transitional shergottite,” or “mixed
shergottite.” We recommend the term “olivinephyric
shergottite,” which emphasizes their
differences from basaltic shergottites such as
Shergotty or QUE94201, and does not have
genetic implications.
Figure 29 Weight percentages of FeO and MnO in
solar-system basalts and martian meteorites. Filled
circles are SNC basalts, open circles are other martian
meteorites, and filled squares are Mars’ surface
materials. Martian meteorites have essentially the
same FeO/MnO ratio as do Mars surface materials
and eucrite basalts. Only Chassigny has a comparable
FeO/MnO ratio to terrestrial basalts (source Treiman
et al., 2000).
the type shergottite, Shergotty) contain cumulus
pyroxene, and have strongly foliated textures
suggesting crystal accumulation in near-surface
dikes or lava flows (Stolper and McSween, 1979;
McCoy et al., 1992b). However, some
(e.g., QUE94201 and Los Angeles) have higher
plagioclase contents and may represent magma
compositions (McSween et al., 1996; McKay
et al., 2002; Rubin et al., 2000).
The lherzolitic shergottites are magnesian
(Mg# , 70) olivine–clinopyroxene–chromite
cumulates, dominated by coarse-grained
poikilitic pigeonite enclosing rounded olivine
(and chromite) crystals, with interstitial areas
1.05.5.2 Nakhlites (Clinopyroxenites/Wehrlites)
Nakhlites are clinopyroxenites or wehrlites,
dominated by augite with lesser olivine. Their
coarse-grained textures (Figure 30(c)) and the
presence of exsolution lamellae in augite (which
require slow cooling) suggest that they are
cumulates. They also contain interstitial material
consisting of a microcrystalline groundmass
of radiating crystalline plagioclase, pigeonite,
ferroaugite, titanomagnetite, pyrite, troilite, chlorapatite,
and rare silica-rich glass (e.g., Berkley
et al., 1980; Treiman, 1986, 1990, 1993).
1.05.5.3 Chassignite (Dunite)
Chassigny is the only known martian dunite.
It is an olivine–chromite cumulate consisting
of 90% olivine (Fa ,32 ), 5% pyroxene, 2%
feldspar (An ,20 ) that has been transformed into
maskelynite, and 3% accessory minerals including
chromite (Figure 30(d)). Melt inclusions
in olivine contain hydrous amphibole,