At 1:30 A.M., fragments of a rare diogenite fell over a radius of 500 meters, 4 km NE of a small village in Tunisia. Local Bedouins immediately collected over 12 kg of mostly minute fragments which were sent to the Muséum National d'Histoire Naturelle in Paris.
Because the larger mass broke up along mineral grain boundaries late in its entry, the unbrecciated orthopyroxene (hypersthene) crystals were not melted and fusion crust is rare (see photo below), rendering this meteorite difficult to recognize. Tatahouine is light olive-green in color, translucent-to-opaque, and is crisscrossed by small black veinlets up to 2 mm wide. Tatahouine has a cumulate texture with exceptionally large crystals, many reaching 2 cm long. Inclusions of silica, troilite, chromite, and minor metal occur throughout. Other diogenites with compositions similar to Tatahouine but with various grain textures have since been found, including NWA 1821 and NWA 3329.
In 1994, Dr. Alain Carion revisited the strewn field and recovered many more specimens, raising the total recovered weight to ~13.5 kg. Interestingly, these recent specimens were found to be contaminated by secondary minerals including carbonates containing rod-shaped objects, these being similar in size and shape to those found in the martian meteorite ALH 84001. Other terrestrial weathering effects include increased concentrations of Ba and Sr, and formation of calcite and iron hydroxide.
Diogenites exhibit a wide range of incompatible element abundances. Tatahouine, NWA 5480, and MET 00424 represent a subgroup of diogenites exhibiting an HREE enrichment and Eu anomalies considered to be inherited from compositionally distinct parental melts, possibly a product of remelting of previously formed magma ocean orthopyroxene cumulates, some of which were contaminated by partial melts from the eucritic crust and a possible phase of fractional crystallization (Barrat et al., 2010). Consistent with this hypothesis is the conclusion by Yamaguchi et al. (2013) from their analyses of NWA 5480 that this olivine diogenite is an impact melt rock formed at the bottom of a large basin such as Rheasilvia. In a another study, Fukuoka et al. (1977) speculated that Tatahouine could be a restite of orthopyroxene cumulates. The REE patterns among the three recognized members of this subgroup are virtually indistinguishable, and they possibly formed from the same parental source magma (Barrat et al., 2010). While it is hypothesized that olivine diogenites like NWA 5480 are cumulates constituting distinct layered magmatic intrusions emplaced into the crust, the unusual features exhibited in NWA 5480 suggest a possible origin as a mantle residue; however, analyses by Yamaguchi et al. (2013) led them to conclude that this diogenite is an impact melt rock formed at the bottom of a large basin such as Rheasilvia which formed ~1 b.y. ago.
Newly revised calculations of cosmic-ray exposure ages using the 10Be/21Ne method have shown that Tatahouine coincides with the major HED peak at 38 m.y. A more detailed scenario for the petrogenesis of the diogenites can be found on the Johnstown page. To see an alternative classification system for the diogenites based on mineralogical and petrographical features, proposed by Beck and McSween (2010) and modified by Wittke et al. (2011), click here. The specimen of Tatahouine shown above is a 2.9 g fragment containing multiple black shock veins. The photos below show a Tatahouine mass found in 2013 on which remnant fusion crust has been preserved, shown courtesy of Sergey Vasiliev.