Martian Meteorite Guidelines   Chart
Mars Meteorites
Mars pdf file
Missclassification article
JPL Mars website  http://www2.jpl.nasa.gov/snc/  Excellent information
 

S  N  C
There are now 4 major groups of Martian Meteorites

1.Shergottite  have 4 subgrops 
A Basalt    Example is Zagami
An olivine-poor basalt subgroup comprising those meteorites with a volcanic origin derived from a
fractionated magma and consisting primarily of the clinopyroxenes pigeonite and augite, in addition
to having a high abundance of feldspathic glass (actually, most basaltic shergottites are more 
accurately termed komatiites based on their low plagioclase content and depleted trace element composition).

B. Olivine-phyric Example  NWA 1068
The establishment of a new shergottite subgroup has been proposed, one which comprises those similar
shergottites with olivine-porphyritic textures. The name picritic shergottite was suggested for this new
subgroup by Barrat 2002, while the name olivine-phyric shergottite was suggested by
Goodrich, 2002. Goodrich suggests that the term picritic shergottite implies certain petrogenetic characteristics,
such as mixing of two compositionally distinct magma sources, which is not necessarily the
case for all members of this new subgroup; therefore the purely descriptive term olivine-phyric is favored.

C. Ol-opx-pyric Example  NWA 2046
Although similar in many respects to the primitive olivine-orthopyroxene-phyric shergottite NWA 1195,
NWA 2046 has olivine megacrysts that are more strongly zoned and have more magnesian cores (Fo84.3)
and more ferroan rims. It is the second most primitive martian meteorite after the olivine-phyric shergottite Y-980459
(containing olivine cores with a more magnesian composition of Fo86, and lacking plagioclase [Mikouchi et al., 2003]).
These zoning features of NWA 2046 suggest that it was derived from a very primitive source magma and
experienced rapid cooling. The crystal faces of the megacrysts in NWA 2046 are better defined than those in NWA 1195.

Using spectral data obtained from the Viking and Pathfinder Mars landers, it was proposed that the
olivine-orthopyroxene-phyric meteorites are compositionally similar to certain rocks analyzed on the
martian surface - the high-silica andesites. These silica-rich rocks are analogous to terrestrial boninites
(SiO2 >;53%, MgO >;8%, and TiO2 <;0.5%), and both of these rock types may have experienced
a similar petrogenesis. They suggested a possible process for the formation of olivine-phyric shergottite magmas:
partial melting and dehydration of hydrous harzburgitic peridotite as a result of the heat from rising mantle plumes.

D  Lherzolite  Example is NWA 1950
A subgroup with olivine-porphyritic textures. The name picritic shergottite was suggested for this
new subgroup by Barrat et al., 2002, while the name olivine-phyric shergottite was suggested by
Goodrich, 2002. Goodrich suggests that the term picritic shergottite implies certain petrogenetic
characteristics, such as mixing of two compositionally distinct magma sources, which is not necessarily the
case for all members of this new subgroup; therefore the purely descriptive term olivine-phyric is favored.

2. Nakhlite  Example is Nakhla
 
The green cumulate crystals found in Nakhla are composed largely (~80%) of the high-Ca clinopyroxene augite,
with minor amounts (5-10%) of Fe-rich olivine, plagioclase, K-feldspar (~1%), Fe-Ti oxides, pyrrhotite, pyrite,
chalcopyrite, gypsum, a reddish-brown aqueous alteration phase similar to iddingsite (consisting of smectite clays,
 ferrihydrides, and iron oxides), and a halite-siderite-anhydrite-chlorapatite  assemblage derived from
evaporite deposits and incorporated into silicate melt. In addition, a halite-clay assemblage was identified that was
probably formed by percolating fluids (Rost <i>et al.</i>, 2005). The water-soluble ions of Cl, K, Na, S, C, Ca, and
some others that are found within Nakhla match those that would be expected to precipitate through the
low-temperature evaporation of an acidic brine.

3. Chassignite  Example is Chassigny
Chassigny contains noble gases different from the martian atmosphere, and is presumed to originate from the martian mantle.
Chassigny contains 90 vol% Mg-rich olivine (Fo68), resembling a terrestrial cumulate dunite (although more FeO-rich),
with the remaining constituents comprising the pyroxenes pigeonite and augite (5%), plagioclase feldspar (2%),
and chromite (1.4%), along with minor pyrite and both fluorapatite (in melt inclusions) and chlor-fluorapatite
(in interstitial maskelynite). McCubbin and Lindsley (2006) inferred from the apatite structural formulas that
the water content was likely very low during crystallization of Chassigny.

Three different types of glass-bearing inclusions (up to 0.2 mm) are present in Chassigny: polyphase,
pure glass, and glass + crystal (Monkawa 2003). Polyphase inclusions typically contain low- and high-Ca pyroxene,
feldspar- and Si-rich glasses, sometimes Ti-rich kaersutite, and more rarely, chlorapatite, FeS, chromite, anorthite
and albite. Water contained within hydrous minerals accounts for over 0.1 wt%.
Similar magmatic inclusions are present in nakhlites, shergottites, and lherzolites,
which all typically contain Al-Ti augite. Monkawa  (2003) argue that both the reverse zoning present in some augites
in Chassigny inclusions, and the absence of Ti-rich phases other than kaersutite, provide evidence for a late impact
shock event on Chassigny. This impact created more reducing conditions (causing
reverse zoning) and higher temperatures and pressures (causing Ti-rich phases other than kaersutite to melt).

4. Orthopyroxenite  Example is ALH 84001 PDF Document
ALH84001 is a coarse-grained, cataclastic orthopyroxenite (97% orthopyroxene, 2% chromite,~1% maskelynite,
 0.15% phosphate) with minor augite, olivine, pyrite and secondary Fe-Mg-Mn-Ca carbonate.
The igneous minerals are essentially unzoned whereas the carbonate is highly zoned in composition. Mason
et al.
(1992) described a thin section with
“orthopyroxene crystals up to 5 mm long forming a polygonal-granular mosaic”

 

Evidence supporting a Martian origin in meteorites includes the following

1. A young crystallization age of only 180 m.y. (however, this age may
instead represent a shock event accompanied by maskelynite production
[Jagoutz and Dreibus, 2002; Eugster , 1997
2. A mineral composition consisting of 0.043 wt% water (kaersutite),
which contains high D/H ratios consistent with the martian atmosphere.
3. The presence of trapped gases, with isotopic percentages matching those 
measured by the Viking and Pathfinder missions
4. Features of a weak gravity field acting on the crystallizing minerals.
5. Residual magnetic properties.
6. Unique Mn/Fe ratios in pyroxene.
7. A unique O-isotopic signature common to all SNC meteorites, but distinct 
from other meteorite classes.
8. A positive match between known basaltic shergottites and a rockfrom the 
plains at Meridiani Planum, named Bounce Rock, from analyses of a suite of 
instruments employed by the Mars Exploration Rover.




O/I