Geology Reference
In-Depth Information
Basaltic shergottite
Olivine-phyric shergottite
Lherzolitic shergottite
Nakhlite
Chassignite
Orthopyroxenite
LAR 06319
RBT 04261
QUE 94201
LEW 88516
ALH 77005
ALH84001
EET 79001
0.5
1
2
5
10
20
Mars ejection age (Ma)
Figure 7.2.
Mars ejection ages for SNC meteorites, calculated from cosmogenic nuclides. Only meteorites in the U. S. Antarctic
collection are labeled.
most groupings being composed of a single type of SNC
meteorite. These data suggest that as many as eight differ-
ent launch sites on Mars have provided the meteorites.
Many attempts have been made to restore some of the
geological context. Source craters within the young basaltic
Tharsis terrain were identified as possible shergottite
sources by
Mouginis-Mark et al.
[1992]. Later work specif-
ically suggested the crater Zunil as a potential source for
some shergottites based on a similar match between crater
dynamics and source materials [
Swindle et al.
, 2004].
Craters in the ancient (Noachian age) southern highlands
of Mars have been also been suggested as possible sources
for ALH 84001 [
Barlow
, 1997]. Spectral mapping from
Mars orbit has proven to be of relatively limited use in
identifying SNC source regions, primarily because their
igneous mineral assemblages (particularly the basaltic
lithologies) are hardly unique within the setting of
planetary crusts, and globally-distributed eolian dust com-
monly obscures bedrock mineralogy. Yet some potential
SNC launch sites have been identified. Similarities between
the relatively unique mineralogy of the nakhlites (Ca-rich
pyroxenes and Fe-rich olivines) and the Syrtis Major
volcanic complex (as determined from orbit) has been used
to suggest the latter as the source for both the nakhlites
and chassignites [
Harvey and Hamilton
, 2006].
Lang et al.
[2009] found a rayed crater on a volcanic flow in Tharsis
that had similar spectra to shergottites.
Despite the biases inherent in sampling by impacts and
the lack of geographical context, the martian meteorites
are an extremely important source of information that
complements what has been learned from spacecraft explo-
ration. Remote sensing spectroscopy can identify only a
few minerals, whereas complete mineral assemblages can
be determined from meteorites. These assemblages con-
strain the conditions of magmatic crystallization and
define subsequent alteration processes and shock history.
Although relative chronology of planetary surfaces can
be determined from crater counting and stratigraphic
mapping, absolute ages can only be measured using
radiogenic isotopes. Radiogenic and stable isotopes, as
well as most of the elements in trace abundances, in rocks
cannot presently be analyzed by remote sensing tech-
niques. These data can be obtained from laboratory mea-
surements on samples, however, and they carry important
information about the planet's composition, origin, and
differentiation, as well as alteration processes and the
cycling of volatile elements between the atmosphere and
lithosphere. Analyses of the isotopes of oxygen, the most
abundant element in SNCs, also provide a means of iden-
tifying these meteorites as SNCs and linking the different
SNC types to the same parent planet. The ratios of
17
O/
16
O
and
18
O/
16
O (Figure 7.3) define straight lines with slopes
of ~0.5 due to mass fractionation of the different isotopes.
Martian meteorites define the mass fractionation trend
