[meteorite-list] New Martian Meteorite Found in Algeria: NWA 3171

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Aug 20 17:09:58 2004
Message-ID: <200408202109.OAA17999_at_zagami.jpl.nasa.gov>


Dr. Anthony Irving
University of Washington
August 20, 2004

Yet another Martian meteorite has been recovered from Northwest Africa,
and studied by an international team, including Canadian physician and
collector, Dr. David Gregory. The 506 gram basaltic shergottite was
confirmed by Drs. Anthony Irving and Scott Kuehner of the University of
Washington in Seattle in collaboration with Dr. Christopher Herd of the
University of Alberta and Drs. Toti Larson and Fred Longstaffe of the
University of Western Ontario. The rocky desert regions of Morocco,
Algeria and Libya - which themselves in places closely resemble a
Martian landscape - so far have produced 11 (over one-third) of the 32
known Martian meteorite specimens, with the remainder being found mostly
in Antarctica and Oman.

The new khaki-gray stone is partly coated by a thin black fusion crust
with flow orientation (see Image 1). The sample consists of
about equal amounts of prismatic, gray-brown pyroxene and sparkling,
glassy maskelynite (see Image 2). As is typical of basaltic
shergottites, the pyroxenes show complex compositional zoning from cores
of subcalcic augite and pigeonite to more ferroan pigeonite rims (see
Images 3 and 4). Plagioclase has been converted
entirely to maskelynite by shock, and is compositionally inhomogeneous
(An41.5 Or3.7 - An54.4 Or1.3). Accessory phases are ulvospinel, ilmenite,
chlorapatite, merrillite, pyrrhotite, Na-K-Al-Si-rich glass, silica,
rare baddeleyite, and rare barite and calcite (the last two probably
precipitated from groundwater while the sample lay in the western
Algerian desert). Grains of silica surrounded by radial cracks (see
Image 5) likely represent former stishovite, and some
mesostasis regions consist of quenched plagioclase needles with
interstitial silica (see Image 6).

The interior of the sample contains irregularly-distributed small
patches of dark brown, vesicular glass (probably produced by shock
induced melting), which also are visible as wart-like protrusions on
parts of the exterior of the specimen (see Image 7). These
glasses are being analyzed to determine whether they contain trapped
Martian atmospheric gases. Thin black fractures (possibly also produced
by shock) traverse the specimen, and evidently contain hydrous mineral
assemblages where they intersect grains of ulvospinel. Macroscopically,
this alteration appears as a rusty staining, but electron microprobe
analysis has identified a complex mixture of
Fe-Si-Al-Ca-Mg-Cl-K-Br(?)-bearing minerals, dominated by an iron
hydroxide mineral similar to terrestrial goethite. This type of
alteration is not typical of terrestrial hot desert weathering
environments, and it is possible that instead it represents a
hydrothermal weathering or alteration effect produced on Mars.

The relative levels of oxidation or reduction (expressed as oxygen
fugacity) during original crystallization of Martian shergottite magmas
can be estimated from chemical analyses of iron-titanium oxide minerals
and experimental laboratory calibrations. For NWA 3171, the compositions
of coexisting ulvospinel and ilmenite imply an oxygen fugacity of 1.3 to
1.4 log units below the temperature-dependent quartz-fayalite-magnetite
standard oxygen buffer curve, a value similar to that determined for
other basaltic shergottites such as Shergotty, Zagami and Los Angeles.

Replicate analyses of hand-picked pyroxene and maskelynite separates
from NWA 3171 at the University of Western Ontario gave a mean oxygen
isotopic composition of ?18O = 4.56 +/- 0.1, ?17O = 2.77 +/- 0.1, ?17O =
0.40 +/- 0.06 per mil, values very similar to those measured for other
Martian meteorites.

Based upon textures, mineral compositions and pattern of alteration, NWA
3171 is distinct from the other four African evolved basaltic
shergottites (Zagami, NWA 480/1460, NWA 856 and NWA 1669). Although it
is not strictly paired with these meteorites, NWA 3171 could have been
launched from the same target volume as some of these other specimens by
a single impact on Mars. Studies to determine the igneous formation age
and cosmic ray exposure age of this new sample are underway, and the
results will clarify the relationship of NWA 3171 to the other
shergottites. Irrespective of the times and sites of their discovery on
Earth, the number of launching impacts on Mars for the 32 presently
known unpaired Martian meteorites may be as few as five or six.

Image 1: Complete NWA 3171 stone showing flow lines in the black fusion
crust on the shield-like front face. Photo ? David Gregory.

Image 2: Thin section image (width 1.5 cm) of NWA 3171 showing prismatic
pyroxene (clove brown), maskelynite (white), opaque Fe-Ti oxides (with
associated secondary, hydrous(?) orange stains), and thin shock
veinlets. Photo ? Anthony Irving and Scott Kuehner.

Image 3: False-color, back-scattered electron image of NWA 3171 showing
irregular compositional zoning in clinopyroxene (blue, green and red),
maskelynite (black), ulvospinel+ilmenite (white). Photo ? Anthony Irving
and Scott Kuehner.

Image 4: Pyroxene compositions in NWA 3171. Diagram ? Anthony Irving,
Christopher Herd and Scott Kuehner.

Image 5: Scanning electron microscope image of former stishovite grain
(50 microns across) in NWA 3171 showing characteristic radial expansion
cracks formed during shock decompression. Photo ? David Joswiak and
Anthony Irving.

Image 6: False-color, back-scattered electron image of a mesostasis
region in NWA 3171 showing needles of plagioclase or maskelynite (green
and blue) with minor ilmenite and chlorapatite (both yellow) in a matrix
of silica (black). Photo ? Anthony Irving and Scott Kuehner.

Image 7: Broken interior cut slab of NWA 3171 showing dark glass pockets
(especially at upper left). Photo ? Greg Hupe.


Description Submitted for Publication in the Meteoritical Bulletin

Northwest Africa 3171
    Purchased February 2004
    Martian meteorite (basaltic shergottite)

A 506 g broken, khaki-gray, ellipsoidal stone, believed to have been
found at an unspecified site in western Algeria, was purchased by A.
Aaronson for D. Gregory (Gregory) in 2004 February. One shield-like face
of the stone is coated by fresh, black fusion crust with flow lines
radiating away from a small dimple-like central depression, and in
addition there are some small, black, wart-like protrusions on other
sides of the stone. Classification and mineralogy (A. Irving and S.
Kuehner, UWS; C. Herd, UAlberta): The sample consists mainly of about
equal amounts of intergrown prismatic, grayish-brown pyroxene and
sparkling, glassy maskelynite. Very thin, subparallel, black glass veins
and irregularly-distributed small pockets of vesicular, dark brown glass
occur within the interior of the specimen. The latter evidently are
responsible for the wart-like, exterior protrusions, and probably were
produced by shock-induced melting. Pyroxenes are zoned from cores of
subcalcic augite (Fs19.3 Wo33.1, FeO/MnO = 26.4) and pigeonite
(Fs29.9 Wo12.1,, FeO/MnO = 28.2) to pigeonite rims as ferroan as
Fs72.9 Wo9.8 (FeO/MnO = 39.9). Plagioclase has been converted entirely to
maskelynite and is compositionally inhomogeneous (An41.5 Or3.7 -
An54.4 Or1.3). Accessory phases are ulvospinel, ilmenite, chlorapatite,
Na-K-Al-Si-rich glass, silica (formerly stishovite, judging from radial
cracks around some grains), merrillite, pyrrhotite and baddeleyite.
Coexisting Fe-Ti oxide compositions imply oxygen fugacities 1.3 to 1.4
log units below QFM at temperatures of 761-782 degrees C. Rare barite
and calcite probably are products of minor desert weathering. Minor
rusty staining around ulvospinel grains and along thin, black shock
veinlets appears to be a complex mixture of fine-grained iron hydroxide
and Si-Al-Ca-Mg-Cl-K-bearing phases. This specimen is not obviously
paired with any of the other four African olivine-free basaltic
shergottites (Zagami, NWA 480/1460, NWA 856 or NWA 1669). Oxygen
isotopes (T. Larson and F. Longstaffe, UWO): replicate analyses of
hand-picked minerals by laser fluorination gave the following results:
pyroxene ?18O = +4.43 +/- 0.06, ?17O = +2.72 +/- 0.0, ?17O = +0.42 +/- 0.03
per mil; maskelynite ?18O = +4.69 +/- 0.07, ?17O = +2.82 +/- 0.1, ?17O =
+0.38 +/- 0.07 per mil. Specimens: type specimen, 22 g and four polished
thin sections, UWS; main mass, Gregory.

Irving, A. J., Herd, C. D. K., Kuehner, S. M., Gregory, D. A. and
Aaronson, A. A. (2004) Petrology and redox state of basaltic shergottite
NWA 3171. Abstr. 67th Met. Soc. Mtg., #5196.
Received on Fri 20 Aug 2004 05:09:43 PM PDT

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