Martian Meteorites

Shergottites

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

NWA 2046

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 Fa_15.7 to rims of Fa_47.9, and subhedral to euhedral, prismatic orthopyroxene phenocrysts (up to 2.1 mm long) have cores as magnesian as Fs_17.7Wo_2.5, surrounded by irregularly zoned mantles with both pigeonite and augite, and pigeonite rims as ferroan as Fs_39.0Wo_12.5.  The groundmass consists mainly of zoned pigeonite (Fs₃₀Wo_6.5 to Fs₄₀Wo₁₃) intergrown with maskelynite (zoned from cores of Ab_25.5Or_0.1 to rims of Ab_36.5Or_1.1) and small grains of relatively ferroan olivine (Fa_47.6-58.1).  Accessory minerals include Ti-chromite (Al/(Al+Cr) = 13.8-28.3), chromite, ilmenite, Cr-ulvöspinel, 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.

NWA 2046

NWA 2046. False-color backscattered electron image showing strong compositional zoning in subhedral olivine phenocrysts (green to red to yellow), orthopyroxene microphenocrysts (blue), and groundmass pigeonite (green), maskelynite (black), chromite and ilmenite (both white). Photo © Anthony Irving and Scott Kuehner, 2004

NWA 2046. False-color backscattered electron image  showing prismatic orthopyroxene phenocryst and microphenocrysts (blue) mantled by pigeonite (green), with groundmass pigeonite (red and yellow), maskelynite (black), chromite and ulvöspinel (both white). Photo © Anthony Irving and Scott Kuehner, 2004.

NWA 2046. Pyroxene and olivine compositions

References

A. J. Irving, T. E. Bunch, S. M. Kuehner, & J. H. Wittke, 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.

Another web page: http://www2.jpl.nasa.gov/snc/nwa2046.html

Northwest Africa 2373

    Northwest Africa

    Purchased 2004 August

   Martian (basaltic shergottite)

NWA 2373. Long dimension approximately 3 cm. © John and Dawn Birdsell, 2005

An 18.1 g stone was purchased in Erfoud Morocco. Yellow brown with desert ablation/varnish surface only, no fusion crust. Description and classification (T. Bunch and J. Wittke, NAU): olivine phenocrysts (<1.25 mm in longest dimension) set in a fine-grained groundmass (<0.1 mm) of pigeonite, maskelynite, chromite, Ti-magnetite, augite, chlorapatite and sulfide.  Olivine has cores of Fa_30.7 (FeO/MnO = 51.9 g/g) and rims of Fa_50.3 (FeO/MnO = 53.3 g/g) and contains micron to submicron inclusions of chromite, glass, and barite. One large olivine grain, which may be a xenocryst, is different from the rest with a core of Fa_29.1 (FeO/MnO = 43 g/g) and more abundant tiny inclusions. The wide (0.2 to 0.4 mm), inclusion-free mantle on this grain is Fa_32.3 (FeO/MnO = 39.2 g/g). Pigeonite is zoned from Fs_29.4Wo_8.4 to Fs_36.4 Wo_14.1 (FeO/MnO = 28 g/g); augite is Fs_21.8 Wo_29.1. Maskelynite is slightly zoned with a compositional average of An_52.1Or_2.1. Chromite, Cr/(Cr+Al) = 87, fe# = 90.4. Overall texture, mineral content and mineral compositions strongly suggest that this stone is paired with NWA 1068.  Specimens: type specimen, 3.7 g and one thin section, NAU; main mass, Birdsell.

Another web page: http://www2.jpl.nasa.gov/snc/nwa2373.html

Northwest Africa 2626

    Algeria

    Purchased 2004 November

    Martian (basaltic shergottite)

NWA 2626. Long dimension approximately 3 cm. © Michael Farmer, 2005

NWA 2626. Cut slices of the type specimen showing dark olivine phenocrysts, glass veinlets and thin weathering rind. Photo © Ted Bunch, 2005

A 31.07 g completely crusted stone, found at an unknown site in Algeria, was purchased from a Moroccan dealer in 2004 November by M. Farmer (Farmer).  The ellipsoidal stone measured 28.9 mm by 16 mm by ~15 mm, and has a 1 to 2 mm thick weathering rind; the dark greenish interior is very fresh and unweathered, but contains thin, glass-rich veinlets and pockets, presumably produced by shock.  Classification and mineralogy (J. Wittke and T. Bunch, NAU; A. Irving and S. Kuehner, UWS): olivine-orthopyroxene-phyric basaltic rock composed of euhedral to subhedral olivine phenocrysts and preferentially-oriented, prismatic low-Ca pyroxene phenocrysts in a groundmass of zoned pigeonite (Fs_26.4Wo_12.4 to Fs_34.1Wo_12.4), maskelynite (An_66.0-71.0Or_0.4), olivine (Fa_43.6-47.3), Ti-chromite (Cr/(Cr+Al) = 0.72-0.79; 9.2-19.8 wt. % TiO₂), chromite (Cr/(Cr+Al) = 0.71-0.86; 0.7-2.0 wt. % TiO₂), merrillite, ilmenite, ulvöspinel and pyrrhotite.  Olivine phenocrysts are zoned from Fa_16.7 cores to Fa_43.3 rims, and pyroxene phenocrysts have irregular cores of orthopyroxene (as magnesian as Fs_17.9Wo_2.4, with ~0.03 wt. % TiO₂ and ~0.4 wt. % Al₂O₃) mantled by pigeonite (Fs_25.4Wo_4.4 to Fs_37.0Wo_12.7) with minor subcalcic augite (Fs_21.7-24.0Wo_30.8-31.1).  Although textures and mineral compositions are similar to those in olivine-orthopyroxene-phyric shergottites NWA 1195 and NWA 2046 (Irving et al., 2004), none of these specimens seem to be terrestrially paired.  Both olivine and pyroxene core compositions in NWA 2626 are less magnesian than those in NWA 2046, yet zoned to less ferroan rim and groundmass compositions (Irving et al., 2005), and NWA 1195 is a complete stone from a different, well-documented location near Safsaf, Morocco (Russell et al., 2002).  Specimens: type specimen, 6.4 g, and one polished thin section, NAU; one polished thin section, UWS; main mass, Anonymous.

NWA 2626. Thin section in plane-polarized light (approximately 12 mm across). showing olivine phenocrysts (yellow-brown) and smaller, prismatic orthopyroxene phenocrysts (pale brown) in a finer grained groundmass of mainly pigeonite (pale brown) and maskelynite (white). Note the prominent dark brown, shock-produced glassy veinlets. Photo © Ted Bunch, 2005

NWA 2626. Portion of the thin section above in partially cross-polarized light (approximately 10 mm across). Several orthopyroxene phenocrysts are visible as prismatic grains with prominent twinning. © Ted Bunch, 2005.

NWA 2626. False-color backscattered electron image showing strong compositional zoning in subhedral olivine phenocrysts (green to red to yellow), orthopyroxene microphenocrysts (blue), and groundmass pigeonite (green), maskelynite (black), chromite and ilmenite (both white). Photo © Anthony Irving and Scott Kuehner, 2005

NWA 2626. False-color backscattered electron image  showing prismatic orthopyroxene phenocryst and microphenocrysts (blue) mantled by pigeonite (green), with groundmass pigeonite (red and yellow), maskelynite (black), chromite and ulvöspinel (both white). Photo © Anthony Irving and Scott Kuehner, 2005.

References

A. J. Irving, T. E. Bunch, S. M. Kuehner & J. H. Wittke (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.

A.J. Irving, T. E. Bunch, J. H. Wittke & S. M. Kuehner (2005) Olivine-orthopyroxene-phyric shergottites NWA 2626 and DaG 476: The Tharsis connection.  Abstr. 36th Lunar Planet. Sci. Conf., #1229.

Northwest Africa 2646

    Algeria or Morocco

    Found 2004 December

    Martian (plagioclase-olivine clinopyroxenite)

History: A 9.3 g, broken stone (allegedly part of a larger specimen found at an unknown site in Algeria or Morocco) was purchased from a Moroccan dealer in December 2004 for N. Oakes. Petrography and Geochemistry: (T. Bunch and J. Wittke, NAU; A. Irving and S. Kuehner, UWS) The specimen is relatively coarse-grained and has an overall gray color, but grain surfaces have a thin whitish coating. It is modally heterogeneous and is composed of ~40.7 % pigeonite, ~24.3 % augite, ~21.6 % olivine, and ~11.4 % maskelynite, with 2 % chromite, ilmenite, merrillite, and pyrrhotite. Euhedral to subhedral chromite (Cr/(Cr+Al) = 0.869, Mg/(Mg+Fe) = 0.12) and augite (Fs_19–23Wo_26–36; FeO/MnO = 22–27; Cr₂O₃ = 0.6, Al₂O₃ = 2.2 [both wt. %]) appear to have crystallized first, followed by subhedral to anhedral olivine (Fa_38–44; FeO/MnO = 35–56). These are poikilitically enclosed in large oikocrysts of pigeonite (Fs_24.4Wo_5.7 zoned to Fs_34.4Wo_12.1; FeO/MnO = 26–32); augite and olivine chadacrysts tend to be clustered. Laths of plagioclase (now maskelynite, An_58.4–60.7Or_0.9), some in clusters, occur interstitially to pigeonite grains. All maskelynite grains are rimmed by zones up to 40 μm in width consisting of a mixture of fine grained calcite, hydrous Al-rich silicate (possibly kaolinite), and very minor calcium chloride, which appear to be replacing the primary feldspar. This specimen has affinities to the six recognized “lherzolitic shergottites,” but differs from them in having a higher abundance of plagioclase and more ferroan mafic mineral compositions. Classification: Achondrite (Martian, plagioclase-olivine clinopyroxenite); minimal weathering. Specimens: A 3.4 g type specimen and one polished thin section are on deposit at NAU. One polished mount is on deposit at UWS. N. Oakes holds the main mass.

NWA 2646. Backscattered electron image showing overall texture; minerals in order of decreasing gray: maskelynite, pigeonite, augite and olivine, chromite and ilmenite (both white). Scale bar at right is 1 mm. Photo © J. H. Wittke 2006

NWA 2646. Typical ophitic to granular texture that shows pigeonite, augite, olivine, maskelynite (plagioclase) and chromite. Image width is 6 mm. Photo © T E Bunch 2006

NWA 2646. Large, compositionally zoned pigeonite grain that poikilitically encloses augite and olivine. Image width is 6 mm. Photo © T E Bunch 2006