Geology Reference
In-Depth Information
originally classified as an unusual carbonaceous chon-
drite similar to Coolidge, with mafic silicate composi-
tions that are more magnesian than typical for ordinary
chondrites (olivine of Fa
10-13
vs. Fa
16-30
for ordinary
chondrites). Subsequent bulk chemical analyses by
Kallemeyn et al
. [1998] revealed that QUE 94570 is
chemically similar to L chondrites, although with much
more magnesian silicate compositions, similar to the
non-Antarctic meteorites Moorabie and Suwahib
Buwah.
In contrast, the paired meteorites LAP 04757/04773
are compositionally and isotopically indistinguishable
from H chondrites but have mafic silicate compositions
more reduced than H chondrites (olivine of Fa
13
vs. Fa
16-20
for H chondrites) [
Troiano et al
., 2011].
Troiano et al.
[2011] review models for formation of chondrites similar
to ordinary chondrites but with lower FeO concentra-
tions in the mafic silicates. These authors favor a parent
body model in which less accretion of reduced compo-
nents (e.g., metal) occurred relative to oxidized compo-
nents (e.g., ice) within the H chondrite parent body,
producing more mafic silicates. Alternatively, these mete-
orites could sample materials from the solar nebula that
were more reduced initially. While these theories may well
be correct, they have greater difficulty accounting for the
formation of L chondrites with FeO-poor mafic silicate
compositions. Further, they should predict a continuum
of compositions within the H chondrite population, from
that of typical H chondrites down to that of the low-FeO
chondrites. A detailed examination of H chondrite mafic
silicate compositions might further our understanding of
the number of ordinary chondrite parent bodies and their
formation.
remarkably deuterium-rich nature, the proximity of hot
and cold materials may have required a role for impact,
either through crater formation on the parent asteroid or
disruption and reassembly. Although further insights
into understanding these scenarios could come from
additional studies of LAP 04840, this meteorite illus-
trates the value of additional sample recovery. Subsequent
field parties have recovered a number of similar meteor-
ites from the LaPaz Icefield. In this case, these are
thought to be fragments of the same meteorite which
either broke apart during atmospheric entry, creating a
shower of stones that fell, or during transport in the ice.
These fragments, essentially identical to LAP 04840,
offer few new insights. In contrast, a recent recovery
from a geographically (and glacially) distinct region,
Miller Range (MIL) 11207 is very different in degree of
metamorphism and is certainly not paired with LAP
04840 [
Gross et al
., 2013]. Future studies of this mete-
orite will undoubtedly further elucidate our under-
standing of the role of water in the alteration of
chondritic bodies.
8.5. IMPACT PRODUCTS
Study of Antarctic meteorites has made significant
contributions to understanding the impact history of our
solar system.
Cohen et al
. [2000] reported
40
Ar-
39
Ar ages
for impact melt clasts from lunar meteorites, including
MacAlpine Hills (MAC) 88105 (Plate 68) and Queen
Alexandra Range (QUE) 93069. These authors found no
clasts older than 4.0 billion years, supporting the idea
that an increase in the impact flux produced most of the
large basins on the Moon, the so-called lunar cataclysm.
Bogard and Garrison
[2003] reported
39
Ar-
40
Ar ages for
eucrites that suggest the same cataclysm might have
occurred on the parent body of these meteorites, thought
to be asteroid 4 Vesta. Few detailed studies of other types
of breccias from the Antarctic collection have been con-
ducted. In particular, substantial numbers of large
Antarctic ordinary chondrites have never been adequately
examined for evidence of brecciation [
Corrigan et al
.,
2012].
Welzenbach et al
. [2005] report a variety of clasts in
MAC 87302 (Plate 5) and
Corrigan et al
. [2012] in a
variety of ordinary chondrite breccias, including impact
melt clasts that could be dated to further elucidate
whether the impact events recorded on the Moon and 4
Vesta are a solar system-wide phenomenon. Coupled
with other interesting phases recorded in ordinary chon-
drite breccias (e.g., halides), this largely unstudied
population deserves closer scrutiny.
Some of the more enigmatic and poorly studied groups
of Antarctic meteorites are those that exhibit igneous
textures but mineralogies that are essentially chond-
ritic. Some of these meteorites, such as the acapulcoites
8.4. PARENT BODY AQUEOUS ALTERATION
The phyllosilicate-bearing matrix of CI, CM, and CR
and some low petrologic type ordinary chondrites have
long testified to the importance of water in parent body
alteration. However, recent studies have revealed an
increasingly diverse set of aqueous alteration products
beyond the hydrated carbonaceous chondrites. Rare salt
crystals, some of which contain fluid inclusions, have
been identified in two ordinary chondrite breccias
[
Zolensky et al
., 1999], although none to date in Antarctic
meteorites. An occurrence of hydrated phases first iden-
tified in the Antarctic collection was in LAP 04840
(Plate 36), where biotite and amphibole were observed
[
McCanta et al
., 2008]. Containing ~13% ferri-magnesio-
hornblende and ~0.4% phlogopite by volume,
McCanta
et al.
[2008] suggest that LAP 04840 formed when a hot,
dry R chondrite was juxtaposed to a cooler, hydrogen-
bearing rock. While the hydrogen could have been liber-
ated from insoluble organic matter, explaining its
