[meteorite-list] 30th Martian Meteorite Found in Algeria: NWA 2046

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:10 2004
Message-ID: <200401281723.JAA01200_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/snc/nwa2046.html

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
landing of NASA's Spirit rover 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.

                                  [Image]

                    Image 1: Complete NWA 2046 stone.
                    Photo © Michael Farmer and Jim
                    Strope.
                                  [Image]

                    Image 2: Cross section of NWA 2046
                    main mass. Photo © Michael Farmer
                    and Jim Strope.
                                  [Image]

                    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.
                                  [Image]

                    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.
                                  [Image]

                    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.
                                  [Image]

                    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.
Received on Wed 28 Jan 2004 12:23:59 PM PST