The basaltic shergottite Northwest Africa 856 (NWA 856): Petrology and chemistry

Abstract

We report on the discovery of a new shergottite from South Morocco. This single stone weighing 320 g is referenced as Northwest Africa 856 (NWA 856) with Djel Ibone as a synonymous name. It is a fresh, fine-grained basaltic rock consisting mainly of 2 pyroxenes (total ~68 vol%: 45% pigeonite En61-16 Wo9-22 Fs26-68, 23% augite En46-26 Wo34-29 Fs21-43) and plagioclase converted to maskelynite (about 23 %, Ab43-57 Or1-5 An54-36). Accessory minerals include merrillite, Cl-apatite, pyrrhotite, ilmenite, ulvöspinel, silica (stishovite and glass), amorphous K-feldspar and baddeleyite. Amorphous mixtures of maskelynite and silica occur most commonly as median layers inside maskelynite laths. In addition, melt pockets (about 2%) were recognized with relics of maskelynite, pyroxene and both dense silica glass and stishovite occurring as both grains and sub-micrometer needles. The compositions of the melt pockets are consistent with mixtures of maskelynite and pyroxenes with an average of about 50 % maskelynite. The meteorite is highly fractured at all scales. The bulk composition of NWA 856 has been measured for 44 elements. It is an Al-poor ferroan basaltic rock which strongly resembles Shergotty and Zagami in its major and trace element composition. The nearly flat REE pattern (La/Lu)n= 0.9, is similar to that of Shergotty or Zagami and differs significantly from NWA 480, another Moroccan shergottite recently described. According to the U, Ba and Sr abundances, NWA 856 is not significantly weathered. The oxygen isotopes (delta-18O = +5.03 ppm, delta-17O = +3.09 ppm, and Delta-17O = +0.47 ppm) are in agreement with the Martian origin of this meteorite. On the basis of grain size, pyroxene zoning and composition, abundance of silica inclusions associated with maskelynite, trace element abundances, REE pattern and oxygen isotopes, pairing with NWA 480 is excluded. The similarity with Shergotty and Zagami is striking. The only significant differences are a larger grain size, a greater abundance of silica and melt pockets, a slightly more restricted range of pyroxene compositions and the absence of significant mesostasis.