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NWA 2626



Another Olivine-Orthopyroxene-Phyric Shergottite From Algeria
Dr. Anthony Irving (University of Washington)
Dr. Ted Bunch (Northern Arizona University)
February 16, 2005

A small, crusted meteorite found in Algeria in November 2004 joins NWA 1195, NWA 2046 and DaG 476 as olivine-orthopyroxene-phyric shergottites, which represent the most primitive igneous rocks known from Mars, and may be samples from the young Tharsis volcanoes. This latest 31.07 gram stone, purchased in Morocco by Mike Farmer and Jim Strope, was studied by Drs. Ted Bunch and Tony Irving and their colleagues Drs. James Wittke and Scott Kuehner at Northern Arizona University and the University of Washington in Seattle. Like NWA 2046, this specimen contains relatively large crystals of both olivine and orthopyroxene which exhibit a preferred alignment as a result of magmatic flow processes. The finer grained groundmass of the rock consists of compositionally-zoned clinopyroxene (pigeonite and some augite), maskelynitized plagioclase, smaller olivine grains, titanium-rich chromite, chromite, merrillite, ilmenite, ulvöspinel and pyrrhotite. Although this sample is similar to NWA 1195 and NWA 2046, it differs from them in having much less extensive compositional zoning in pyroxene and olivine. Other distinctive features are irregular patches of late-crystallizing merrillite adjacent to groundmass pigeonite grains and within maskelynite grains, and moat-like cavities around maskelynite grains (which formerly may have contained soluble salts or soft alteration assemblages of Martian origin). NWA 2626 is further distinguished by the presence of cross-cutting veinlets and small pockets of dark glass with quenched crystallites, which are interpreted to have formed by shock-induced melting as this specimen was ejected from Mars.

Speculation that NWA 2626 and most of the other olivine-phyric to basaltic shergottites are samples of volcanic and subvolcanic rocks from the large Tharsis volcanoes is based mainly on the relatively young crystallization ages for shergottites (575-165 million years) coupled with the evidence from Mars Global Surveyor and Mars Express imagery for very young volcanism on Olympus Mons and Arsia Mons. Very few other candidate sites on Mars are as likely to have had the sustained and intermittently replenished magmatic systems capable of fractional crystallization, mixing and flow processes reflected in the textures and compositional range of the shergottites, and furthermore these immense volcanic edifices may have greater potential for efficient ejection of rock fragments by low-angle impacts.


Image 1: Complete NWA 2626 stone. Photo © Michael Farmer.

Image 2: Cut slices of NWA 2626 type specimen showing dark olivine phenocrysts, glass veinlets and thin weathering rind. Photo © Ted Bunch.

Image 3: Thin section image (width 12 mm) in plane-polarized light of NWA 2626 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.

Image 4: Portion of the same thin section shown in Image 3 in partially cross-polarized light. Several orthopyroxene phenocrysts are visible as prismatic grains with prominent twinning. Photo © Ted Bunch.

Image 5: False-color backscattered electron image of NWA 2626 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.

Image 6: False-color backscattered electron image of NWA 2626 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.

Image 7: False-color backscattered electron image showing late stage merrillite (yellow) along the margins of groundmass pigeonite grains and within maskelynite grains in NWA 2626. Photo © Anthony Irving and Scott Kuehner.

Image 8: Backscattered electron image showing moat-like cavities around the margins of maskelynite grains in NWA 2626. Photo © Anthony Irving and Scott Kuehner.

Description Submitted for Publication in the Meteoritical Bulletin

Northwest Africa 2626
     Purchased 2004 November
     Martian meteorite (basaltic shergottite)

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 (A. Irving and S. Kuehner, UWS; J. Wittke and T. Bunch, NAU): 
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 
(Fs26.4Wo12.4 to Fs34.1Wo12.4), maskelynite (An66.0-71.0Or0.4), olivine 
(Fa43.6-47.3), 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 Fa16.7 cores 
to Fa43.3 rims, and pyroxene phenocrysts have irregular cores of orthopyroxene 
(as magnesian as Fs17.9Wo2.4, with ~0.03 wt.% TiO2 and ~0.4 wt.% Al2O3) 
mantled by pigeonite (Fs25.4Wo4.4 to Fs37.0Wo12.7) with minor subcalcic 
augite (Fs21.7-24.0Wo30.8-31.1).  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, 

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 m antle.  Abstr. 35th Lunar Planet. Sci. 
Conf., #1444.

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

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