Two friable stones with a combined weight of 145 g were found by nomads and sold to A. Habibi in Erfoud, Morocco. A portion of the meteorite was submitted for analysis and classification by Mr. Habibi to the Université Pierre & Marie Curie, Paris VI (Jambon, Boudouma and Badia) and the Universite de Bretagne Occidentale (Barrat). Oxygen isotopic values determined by the Open University (Greenwood and Franchi) are consistent with a martian origin. Northwest Africa 5790 was determined to be a unique unpaired martian nakhlite.
Northwest Africa 5790 is a loosely consolidated composition of mm-sized, cumulus, olive-green, zoned augite crystals (51.2 vol%) along with zoned olivine phenocrysts (9.1 vol%), and rare Ti-magnetite (<1 vol%), all crystallized within an interstitial mesostasis (39.7 vol%) containing oxide phases, pyroxene, olivine, feldspar, tridymite, merrilite, Cl-apatite, and glass, plus a component of Cl-amphibole trapped in melt inclusions. Excluding the contribution of the larger mesostasis component to the modal composition of NWA 5790, the augite:olivine ratio is the same as that for Nakhla. However, the texture is similar to that of NWA 817 and MIL 03346.
Petrographic evidence indicates that NWA 5790 likely formed near the top of the cumulate pile, given the scenario that all nakhlites formed within a common magma structure. For instance, the mesostasis in NWA 5790 is present in the highest proportion of any nakhlite (39.7%), while NWA 817, previously considered to have formed at the top of the pile, contains the next highest proportion of mesostasis (~20%); lesser abundances are present in other nakhlites in descending order from the envisioned top (e.g., 10% in Nakhla and Y-000593). It is notable that NWA 5790 contains the highest Th, U and REE concentrations reported thus far in a nakhlite, exhibiting a strong LREE enrichment. Just as the enrichment of Fe and LREE in the alteration phase of NWA 817 has been attributed to the infiltration of fluids through country rock, incorporating a dissolved plagioclase component, perhaps the same process was active in NWA 5790. By the same token, since NWA 998 has the lowest REE abundance of any nakhlite, it may be attributed to its low retention of intercumulus magma reflected in its relatively small volume of mesostasis (~2%). Further studies may reveal that the location of NWA 5790 at the top of the nakhlite cumulate pile may have resulted in a lack of both water and any accompanying biogenic activity like that tentatively identified in Nakhla, which formed at a greater depth.
The fact that NWA 5790 is the sole member of the nakhlite group that has retained complex primary zoning features on augite grains is indicative that it cooled the fastest of the other nakhlites and formed at the top of the cumulate pile. Further evidence for rapid cooling at this topmost location can be seen in the Cl contamination previously identified in MIL 03346, in the high proportion of evolved mesostasis, and in the lack of equilibrium among the mineral phases. Based on texture and mineralogy (mesostasis [REE] abundance, plagioclase size, olivine composition, intercumulus porosity, and pyroxene composition), the Fe–Mg and Ca zoning profiles in olivine and pyroxene (reflecting cooling rates), and the crystal size distribution analysis, a comparative burial depth within the cumulate pile (lava flow or magma chamber) for each of the nakhlites has been established (Mikouchi et al., 2003, 2005, 2006; Lentz and McSween, 2003; Macrì et al., 2004; Imae et al., 2005; Day et al., 2006; Treiman and Irving, 2008; Jambon et al., 2010):
NWA 5790—top of cumulate pile
MIL 03346—near top; 1-2 m
NWA 817—near top; ~1-2 m
Y-000593/749/802—7 m
Nakhla—middle; ~10 m
Governador Valadares—middle; ~10 m
Lafayette—near bottom; >30 m
NWA 998—bottom of cumulate pile; >30 m
More details about the nakhlite group, including the possible presence of martian bacterial micro-fossils, can be found on the Nakhla page. The intact masses of NWA 5790 are shown below, courtesy of Aziz Habibi.
