A New Shergottite from North Western Sahara
64th Annual Meteoritical Society Meeting (September 2001)
A. Jambon[1], J-A. Barrat[2], P. Gillet[3], C. Gpel[4], M. Javoy[5], J-L.
Joron[6], V. Sautter[7].
[1] Laboratoire MAGIE UMR 7057 Universit P. et M. Curie 4 place Jussieu
75252 Paris
[2] CNRS UMR6112 and Universit d’Angers, France
[3] Laboratoire des Sciences de la Terre, Ecole Normale suprieure de Lyon,
France
[4] Laboratoire de Gochimie et Cosmochimie, Institut de Physique du Globe,
Paris, France
[5] Laboratoire de Gochimie des Isotopes Stables, Institut de Physique du
Globe Paris, France
[6] Laboratoire Pierre Se, CEA-CNRS, Saclay, France
[7] Laboratoire de Minralogie, Musum National d’Histoire Naturelle, Paris,
France.
Introduction: We report on the discovery of a new shergottite from Morocco.
This single stone weighing 320 g is not officially named yet. The location
of its find is unknown but we used "Djel Ibone" as a working name. It is a
fine-grained basaltic rock consisting mainly of pyroxenes (about 70 vol%,
pigeonite En48Wo13 Fs39, augite En36 Wo32 Fs32 ) and plagioclase converted
to maskelynite (about 23 vol%, Ab41-47 Or2 An57-51). Accessory minerals
include merrilite, apatite, pyrrhotite, chromite, Fe-Ti oxides, silica
(stishovite), and baddeleyite. Melt pockets were recognized with stishovite
occuring as both phenocrysts and submicrometric needles. The specimen is
highly fractured at all scales: pyroxene cores are cut by large and medium
sized fractures whereas their rims are affected by numerous small fractures.
A circular fracture often delineates the Mg rich core. Maskelynite is only
affected by a few major fractures. Terrestrial calcite is present mainly as
veins cross-cutting the meteorite, as in many other Saharan finds.
Chemistry: The bulk composition of "Djel Ibone" has been determined using a
combination of three analytical techniques: ICP-AES for major elements,
ICP-MS for trace elements and INAA for additional trace elements. About 50
elements were determined with an accuracy better than 5 % in most cases. The
bulk composition of "Djel Ibone" is for selected elements : TiO2 0.81 wt%,
Al2O3 6.83 wt%, FeO* 17.8 wt%, MnO 0.49 wt%, MgO 9.51 wt%, CaO 10.2 wt%,
Na2O 1.28 wt%, K2O 0.13 wt%, Ni 77 ppm, La 2.16 ppm, Sm 3.88 ppm, Eu 1.5
ppm, Gd 0.58 ppm, Yb 1.64 ppm, Th 0.4 ppm. The REE pattern is similar to
that of Shergotty or Zagami. "Djel Ibone" is only marginaly weathered as
illustrated by the U, Ba and Sr abundances which are sensitive indicators of
surface processes [1]: It displays a normal Th/U ratio and its Ba and Sr
abundances are not outside the trend defined by other unweathered
shergottites. Therefore, key element ratios such as FeO*/MnO (w 30), Na/Al
(w 0.40), K/La (= 500) or Ga/Al (= 4.1 10-4) indicate that "Djel Ibone" is a
new member of the martian meteorite clan[2].
Oxygen isotopes: "Djel Ibone" has a O18 of +5.03 . and a O17 of +3.09 .. The
corresponding O17 (=+0.47 .) is a little high but in agreement with the
martian origin of this meteorite[3].
References:
[1] BarratJ-A. et al. (2001) Meteorit. Planet. Sci. 35, 95-106.
[2] Meyer C. (1998) Mars Meteorite.Compendium 1998, Nasa, Houston.
[3] Clayton and Mayeda. (1996) Geochim. Cosmochim. Acta, 60, 1999-2017.
