Northwest Africa 4797

Morocco

Find: 2001

Achondrite (Martian, shergottite)

History: Found in 2001 near Missour, Morocco, and acquired by Morocco Import of Erfoud, Morocco.

Physical characteristics: A single, dense, broken stone (15.0 g) with fusion crust on one side and visible regions of pale gray; vesicular glass interstitial to coarse silicate grains (Fig. 1). Several thin, dark-colored shock veins crosscut the specimen.

Petrography: (A. Irving and S. Kuehner, UWS; T. Bunch and J. Wittke, NAU) This very fresh specimen is predominantly a peridotite composed mostly of olivine chadacrysts (up to 1.5 mm) and zoned clinopyroxene oikocrysts (subcalcic augite and pigeonite, up to 9 mm across) with interstitial regions (apparently originally intercumulus) now consisting of vesicular glass containing sparse, very small, birefringent plagioclase microlites. Accessory phases are Ti-chromite, Mg-ilmenite (some with associated tiny baddeleyite grains), merrillite, and pyrrhotite. A shock injection vein (~1 mm wide) contains reacted xenocrysts and polycrystalline fragments of the peridotite material enclosed in a vesicular aggregate of fine grained (5-10 microns) interlocking olivine, augite and glass, and both margins of the vein exhibit even finer grained (<2 microns) selvages against the peridotite wallrock. This entire specimen has experienced very high shock pressures.

Geochemistry: Peridotite lithology: olivine (Fa_31.3-32.7, FeO/ MnO = 49.3-53.0), augite (Fs_14.6-16.0Wo_35.3-29.9, FeO/MnO = 23.8-26.2), interstitial pigeonite (Fs_25.2-26.3Wo_14.8-7.9, FeO/ MnO = 27.8-31.8), plagioclase microlites in interstitial glass (An_56.8Or_0.8). Shock vein: olivine (Fa_23.7-28.7, FeO/ MnO = 39.5-48.0, P₂O₅ = 0.28-0.46 wt%), augite (Fs_14.3-16.3Wo_41.4-36.4, FeO/MnO = 24.7-26.7), glass (average of 6 analyses, in wt%: SiO₂ = 58.7, TiO₂ = 1.4, Al₂O₃ = 14.3, FeO = 6.5, MnO = 0.22, MgO = 2.1, CaO = 11.8, Na₂O = 2.7, K₂O = 0.12, P₂O₅ = 1.8, SO₃ = 0.17, Cl = 0.05). Bulk composition: (R. Korotev, WUSL; INAA data) FeO = 19.6, Na₂O = 0.40 (both wt%), Ni = 330, La = 0.69, Sm = 0.52, Eu = 0.18, Tb = 0.19, Yb = 0.57, Lu = 0.080, Hf = 0.5 (all ppm). The chondrite-normalized rare earth element pattern is subparallel to that for Zagami, but at about half the absolute abundances, and there is a small negative Eu anomaly.

Classification: Achondrite (Martian, shergottite); extensive shock.

Specimens: A total of 3.3 g of sample, one polished thin section and one polished mount are on deposit at UWS. Morocco Import holds the main mass.

Submitted by: A. Irving, UWS.