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NWA 3171



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 (An41.5Or3.7 - An54.4Or1.3). Accessory phases are ulvöspinel, 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 ulvöspinel. 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 ulvöspinel 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 δ18O = 4.56 ± 0.1, δ17O = 2.77 ± 0.1, Δ17O = 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), ulvöspinel+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 Hupé.

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 (Fs19.3Wo33.1, FeO/MnO = 26.4) and pigeonite (Fs29.9Wo12.1,, FeO/MnO = 28.2) to pigeonite rims as ferroan as Fs72.9Wo9.8 (FeO/MnO = 39.9). Plagioclase has been converted entirely to maskelynite and is compositionally inhomogeneous (An41.5Or3.7 - An54.4Or1.3). Accessory phases are ulvöspinel, 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 ulvöspinel 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 δ18O = +4.43 ± 0.06, δ17O = +2.72 ± 0.0, Δ17O = +0.42 ± 0.03 per mil; maskelynite δ18O = +4.69 ± 0.07, δ17O = +2.82 ± 0.1, Δ17O = +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.

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