Geology Reference
In-Depth Information
3.5
Martian meteorites
3.0
2.5
Terrestrial
mass fractionation
line
2.0
1.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
δ
18
O (permil)
Figure 7.3.
Oxygen isotope compositions of SNC meteorites define a mass fractionation line for Mars. The δ notation refers to the
ratio of
17
O or
18
O to
16
O, compared to a terrestrial standard.
for Mars, which is distinct from those of the Earth and
other bodies. Reviews of the properties and compositions
of martian meteorites and what has been learned from
them have been published by
McSween
[1994, 2002,
2008],
McSween and Treiman
[1998],
Treiman
[2005], and
Bridges and Warren
[2006].
With the exception of ALH 84001 and NWA 7034,
martian meteorites represent Amazonian-age volca-
nism, and their compositions can be compared with
those of older martian igneous rocks analyzed by
remote sensing to explore magmatic evolution through
time. For example, the Spirit rover APXS analyzed
hundreds of Noachian igneous rocks and soils. Their
compositions are compared with shergottites and
nakhlites in Figure 7.4 [
McSween et al.
, 2009]. Also
included in this figure is a box enclosing the composi-
tions of large areas of the martian surface analyzed by
a gamma-ray spectrometer on an orbiting spacecraft
[
McSween et al.
, 2009]. This plot demonstrates that
these martian meteorites are not compositionally repre-
sentative of the whole martian crust or the mantle
source regions that partially melted to produce magmas.
However, the composition of the older NWA 7034 brec-
cia [
Agee et al.
, 2013] is similar to the rocks analyzed by
Spirit (Figure 7.4). There is also at least one rock on
Mars that is similar to SNCs. The Opportunity rover
analyzed Bounce Rock, an ejecta block lofted onto
Meridiani Planum from elsewhere. This rock was found
to be mineralogically and chemically similar (Figure 7.4)
to basaltic shergottites [
Zipfel et al.
, 2011], further sup-
porting their martian origin. Similarly, the Spirit rover
identified an outcrop in the Columbia Hills as mineral-
ogically and chemically similar to ALH 84001, including
significant carbonates [
Morris et al.
, 2010].
We now consider specific martian meteorites in the
U.S. Antarctic collection that have been especially influ-
ential in understanding Mars and its rocks.
7.3. LHERZOLITIC SHERGOTTITE: ALH A77005
The importance of Allan Hills A77005 (480 g) is as the
first known sample of its kind, and as the first “new”
kind of SNC discovered since the group was defined in
the mid-seventies (Plate 72). ALH A77005, a cumulate
rock obviously related to the shergottites but formed
from a less fractionated magma, became the first recog-
nized lherzolitic shergottite, although that classification
term was not applied in the original descriptions
[
McSween et al.
, 1979a, 1979b;
Ishii et al.
, 1979]. The
descriptor “lherzolitic” is not ideal, as it implies modal
mineral abundances seen only in some portions of large
specimens; ALH A77005 is more properly characterized
as a heterogeneous olivine gabbro, containing ultramafic
segregations of olivine and pyroxene. The meteorite was
initially described at the same time that shergottites were
first postulated to be martian rocks, and its unique prop-
erties expanded the known geologic complexity of the
shergottite parent planet.
Mineralogically ALH A77005 consists of cumulus
olivine and chromite, with low- and high-calcium
pyroxenes, maskelynite, ilmenite, troilite, and whitlock-
ite. The olivines show preferred orientation [
Berkley
