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NWA 2046

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30th MARTIAN METEORITE FOUND IN ALGERIA: NWA 2046


Dr. Anthony Irving
University of Washington
January 26, 2004

The total number of unpaired Martian meteorites now stands at 30 following the recovery in September 2003 and subsequent confirmation of a new, very primitive type of shergottite. The 63 gram stone, found near Lakhbi in Algeria, was obtained by veteran collectors Mike Farmer and Jim Strope, and studied collaboratively by former NASA scientists Drs. Theodore Bunch and Anthony Irving and their colleagues Drs. James Wittke and Scott Kuehner at Northern Arizona University and the University of Washington in Seattle. This sample is very distinctive in containing large crystals of both olivine and orthopyroxene which exhibit a preferred alignment, probably as a result of crystal growth and transport by flow within a magma conduit beneath the surface of Mars. Additional minerals in the finer grained matrix of the rock include pigeonite, maskelynitized plagioclase, smaller olivine grains, chromite, ilmenite, pyrrhotite, merrillite and apatite. Renewed study of another Martian meteorite, NWA 1195, now confirms that it is of the same petrologic type as NWA 2046, and also has a strong preferred crystal orientation, which can account for its puzzling elongated shape. Both stones can be termed olivine-orthopyroxene-phyric shergottites, and rival the Antarctic olivine-phyric shergottite Yamato 980459 as the most magnesium-rich samples known from Mars, and therefore those most likely to give direct information about the nature of the Martian mantle, from which they presumably were derived by partial melting hundreds of millions of years ago. Further information on NWA 2046 will be presented at the 35th Lunar and Planetary Conference in Houston in March 2004, which promises to be a major venue for scientific discussion in light of the successful landings of NASA's Spirit and Opportunity rovers on Mars. Scientists are eager to compare the compositions of the surface rocks to be analyzed at both Gusev Crater and Meridiani Planum with those of all the Martian meteorites, which represent samples excavated from various depths beneath the surface of Mars.

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Image 1: Complete NWA 2046 stone. Photo © Michael Farmer and Jim Strope.
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Image 2: Cross section of NWA 2046 main mass. Photo © Michael Farmer and Jim Strope.
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Image 3: Thin section image (width 5 mm) in cross-polarized light of NWA 2046 showing olivine phenocrysts (green with dark brown mantles) and two parallel prismatic orthopyroxene phenocrysts (pale yellow, at lower left and right) in a finer grained groundmass of twinned pigeonite (blue, magenta and yellow), maskelynite (black) and opaque oxides. Photo © Anthony Irving and Scott Kuehner.
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Image 4: False-color backscattered electron image of NWA 2046 showing strong compositional zoning in olivine phenocrysts (blue to yellow), orthopyroxene microphenocrysts (blue), and groundmass pigeonite (green and red), maskelynite (black), olivine, chromite and ilmenite (all light yellow). Photo © Anthony Irving and Scott Kuehner.
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Image 5: False-color backscattered electron image of NWA 2046 showing prismatic orthopyroxene phenocryst and microphenocrysts (blue) mantled by pigeonite (green), with groundmass pigeonite (orange and yellow), olivine (white), maskelynite (black), chromite and ilmenite (both white). Photo © Anthony Irving and Scott Kuehner.
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Image 6: False-color backscattered electron image of a trapped melt inclusion within an olivine phenocryst in NWA 2046. The once totally molten inclusion engulfed by the growing olivine crystal now contains crystals of aluminous diopside, possible amphibole, and skeletal merrillite, as well as glass. Photo © Anthony Irving and Scott Kuehner.

Description Submitted for Publication in the Meteoritical Bulletin


Northwest Africa 2046 
    Algeria     
    Purchased 2003 September     
    Martian meteorite (basaltic shergottite)

A 63g complete and partially crusted stone found near Lakhbi, Algeria was purchased from a Moroccan dealer in 2003 September by M. Farmer (Farmer). The ellipsoidal stone has an average width of 30 mm, with a 1 to 3 mm thick weathering rind; the interior is very fresh and unweathered. Classification and mineralogy (J. Wittke and T. Bunch, NAU; A. Irving and S. Kuehner, UWS): olivine-orthopyroxene-phyric basaltic rock. Subhedral to euhedral, dark brown olivine phenocrysts (up to 2.2 mm long) are strongly zoned from cores of Fa15.7 to rims of Fa47.9, and subhedral to euhedral, prismatic orthopyroxene phenocrysts (up to 2.1 mm long) have cores as magnesian as Fs17.7Wo2.5, surrounded by irregularly zoned mantles with both pigeonite and augite, and pigeonite rims as ferroan as Fs39.0Wo12.5. The groundmass consists mainly of zoned pigeonite (Fs30Wo6.5 to Fs40Wo13) intergrown with maskelynite (zoned from cores of Ab25.5Or0.1 to rims of Ab36.5Or1.1) and small grains of relatively ferroan olivine (Fa47.6-58.1). Accessory minerals include Ti-chromite (Al/(Al+Cr) = 13.8-28.3), chromite, ilmenite, Cr-ulvospinel, pyrrhotite (commonly in parallel growth with ilmenite), merrillite, rare chlorapatite, and rare fayalite (which occurs as a reaction rim on groundmass pigeonite in contact with ilmenite or pyrrhotite). Trapped melt inclusions in olivine contain aluminous diopside, amphibole(?), pleonaste, chromite, merrillite and glass. Large, prismatic orthopyroxene phenocrysts exhibit preferred orientation suggestive of magmatic flow and/or crystal accumulation, possibly in a subsurface dike; olivine phenocrysts are weakly oriented and tend to occur in clusters. Textures and mineral compositions are similar to those in olivine-orthopyroxene-phyric shergottite NWA 1195, but the olivine cores in NWA 2046 are more magnesian (Irving et al., 2004). Specimens: type specimen, 12.2 g, and one polished thin section, NAU; one polished thin section, UWS; main mass, Strope.

Irving, A. J., Bunch, T. E., Kuehner, S. M. and Wittke, J. H. (2004) Petrology of primitive olivine-orthopyroxene-phyric shergottites NWA 2046 and NWA 1195: Analogies with terrestrial boninites and implications for partial melting of hydrous Martian mantle. Abstr. 35th Lunar Planet. Sci. Conf., #1444.

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