Geology Reference
In-Depth Information
Trachy-
andesite
Basaltic
trachy-
andesite
Tephrite
Trachy-
basalt
6
Gusev
Rock RAT
Rock brush
Soil
Basalt
Foidite
Basaltic
andesite
4
Andesite
GRS
NWA 7034
Picrobasalt
Bounce rock
2
Martian meteorites
0
35
45
55
65
SiO
2
(wt. %)
Figure 7.4.
Geochemical classification diagram for volcanic rocks, comparing the different compositions for martian meteorites
and Noachian-age rocks and soils in Gusev crater analyzed by the Spirit rover on Mars. A box encloses analyses of large areas of
the martian surface analyzed by an orbiting GRS.
and Keil
, 1981] and unusual brown coloration due to
oxidation of iron during shock [
Ostertag et al.
, 1984].
Olivines and pyroxenes are more magnesian than in
other shergottites, as appropriate for a cumulate rock.
The compositions of spinels in ALH A77005 were
reported by
Goodrich et al.
[2003], who inferred an
oxygen state, defined in terms of oxygen fugacity,
equivalent to QFM - 2.6 (where QFM is the quartz-
fayalite-magnetite buffer) during crystallization. ALH
A77005 is heavily shocked, accounting for the transfor-
mation of plagioclase to maskelynite and the occur-
rence of pockets of impact melt (now glass with skeletal
olivine crystals).
Smith and Steele
[1984] and
Ikeda
[1994] published additional petrographic descriptions
of the meteorite.
The mineralogy of ALH A77005 is consistent with
early accumulation of crystals from a magma having the
crystallization sequence of the parent magmas of olivine-
phyric shergottites [
McSween et al.
, 1979a;
Goodrich
et al.
, 2003]. The crystallization history was as follows:
• Crystallization and accumulation of olivine and
chromite
• Crystallization of pyroxenes, in some cases enclosing
olivine and chromite
• Crystallization of plagioclase and other minerals
from intercumulus melt
• Equilibration of previously formed phases, including
pyroxene exsolution, formation of iron-rich olivine, reac-
tion of chromite to form ulvöspinel
• Shock metamorphism, producing maskelynite and
impact melt pockets
Trace element abundances in ALH A77005 minerals
[
Lundberg et al.
, 1990] confirm the crystallization
sequence inferred from mineral compositions and rock
texture.
The meteorite is coarse grained and heterogenous,
showing variation in texture and modal abundances on a
centimeter scale. Local areas can vary from cumulate
olivine-phyric gabbro to lherzolite composed of olivine
poikilitically enclosed by pyroxene. Consequently, small
samples of lherzolitic shergottites may not be represen-
tative, and bulk chemical analyses should be viewed with
caution. Bulk analyses for major, minor, and trace
elements in ALH A77005, with original sources, were
summarized by
Lodders
[1998].
Shih et al.
[1982],
Jagoutz
[1989], and
Nyquist et al.
[2001] reported its crystallization
age as approximately 179 Ma. The name
lherzolitic sher-
gottite
(first applied by
Jagoutz
[1989]) is not ideal, as it
implies modal abundances seen only in some portions of
large specimens. Harzburgite is a more appropriate term
in some regions, and other specimens might be more
properly termed gabbroic shergottites.
