Geology Reference
In-Depth Information
that sample a range of regions in their parent asteroids,
that represent a range of processes, that have sufficient
numbers to allow for detailed understanding of the
processes that occurred on strongly heated asteroids, and
for which Antarctic meteorites have been especially impor-
tant for advancing our understanding. We cover the
acapulcoite-lodranite clan, ungrouped iron meteorites,
ureilites, brachinites, the howardite-eucrite-diogenite clan,
and angrites.
Antarctic meteorites than the acapulcoite-lodranite clan
(Plates 39 to 42). Even with the relatively recent influx of
hot desert meteorites from Northwest Africa, Antarctic
meteorites comprise more than half of the acapulcoite-
lodranite clan and those in the U.S. collection more
than a quarter of the members. Papers by
Floss
[2000],
McCoy et al.
[1996, 1997a, b],
Mittlefehldt et al.
[1996],
Nagahara
[1992],
Patzer et al.
[2004] and
Takeda et al.
[1994] all centered on Antarctic meteorites. The publica-
tions by
McCoy et al.
refined the petrogenesis of acapul-
coites-lodranites, arguing that rather than the end
members represented by Acapulco and lodran, these
meteorites represented a continuum in partial melting,
melt migration, and melt removal. While lodranites are,
in general, the residues from more substantial partial
melting and melt removal, melt removal was not complete.
Elephant Moraine (EET) 84302 experienced removal of
troilite without removal of plagioclase. lewis Cliff
(lEW) 86220 samples two distinct lithologies, one of
acapulcoite modal mineralogy and grain size and the
other of Fe,Ni metal, troilite, and plagioclase-augite
(basaltic-gabbroic) mineralogy with a very coarse grain
size (Figure 5.2). Plagioclase grains reach almost a centi-
meter in length in this lithology. This meteorite appears
to sample the melt complementary to the residual lodra-
nites, intruding a typical acapulcoite host rock. One of
the more interesting members of the clan is graves
Nunataks (gRA) 95209, which samples a physically,
5.2. UlTRAMETAMORPHOSED CHONDRITIC
MATERIAl: THE ACAPUlCOITE-lODRANITE
ClAN OF PRIMITIVE ACHONDRITES
Acapulcoites and lodranites are fine- to medium-
grained equigranular rocks composed of orthopyroxene,
olivine, chromian diopside, sodic plagioclase, Fe,Ni metal,
schreibersite, troilite, whitlockite, chlorapatite, chromite,
and graphite. The mineral assemblage is broadly similar to
that found in ordinary chondrites, although mineral com-
positions, abundances, and textures differ from ordi-
nary chondrites. Mineral compositions are substantially
more magnesian than observed in ordinary chondrites
(olivine mg# 87-97 in aca-lod vs. 70-84 in OCs, where
mg# is defined as 100 × molar MgO/(MgO + FeO)).
Commensurate with the more magnesian mafic mineral
compositions, orthopyroxene is more abundant than
olivine. Recrystallization is evident from abundant 120°
triple junctions.
Acapulcoites are finer-grained and the modal miner-
alogy is approximately that of ordinary chondrites,
containing roughly 10 wt.% plagioclase and 5-6 wt.%
troilite. Acapulcoites also contain metal, troilite, and
phosphate veins that range in size from a few microm-
eters in width and hundreds of micrometers in length
to several mm in width and cm in length. In contrast,
lodranites are coarser-grained and typically exhibit
marked depletions in plagioclase and/or troilite relative
to ordinary chondrites. A comprehensive description
of acapulcoites and lodranites known at that time is
given by
Mittlefehldt et al.
[1998] and references
therein.
Prior to the collection of Antarctic meteorites, the
acapulcoite-lodranite clan was represented by only the
type meteorites, Acapulco and lodran.
Bild and Wasson
[1976] published the first modern, comprehensive study
of the petrology of lodran, concluding that it experi-
enced a multistage formation with a significant role for
partial melting and melt removal. Acapulco, which fell in
1976 and was first comprehensively described by
Palme
et al.
[1981], was ascribed as representing the early stage
of incipient partial melting of a chondritic parent body.
Perhaps no group of meteorites has been more defined
(and our understanding of their petrogenesis refined) by
Figure 5.2.
lewis Cliff (lEW) 86220 is an unusual member of
the acapulcoite-lodranite clan. While the bulk of the meteorite
consists of a finer-grained equigranular host similar to acapul-
coites (lower left side), coarse-grained enclaves of plagioclase,
calcic pyroxene, metal, and troilite suggest infiltration of a
basaltic-Fe,Ni-FeS melt that crystallized in situ (top and right
side). The lodranites are residues complementary to that melt.
Scale bar is 1 mm.
