NORTHWEST AFRICA 2737
‘Diderot’

Several rock fragments having a combined weight of 611 g, which constitute a single stone, were found in the Moroccan Sahara by meteorite hunters under the organization of Bruno Fectay and Carine Bidaut. These black fragments were not recognized as meteoritic for several years, but a sample was eventually submitted for analysis. The importance of this meteorite was soon evident, and multiple analyses were conducted through a collaboration of research groups in France and elsewhere:

1. Laboratoire des Sciences de la Terre, (Centre National de la Recherche Scientifique—Unité Mixte de Recherche [CNRS–UMR]), École Normale Supérieure de Lyon: P. Beck, Ph. Gillet, B. Van de Moortele, B. Reynard
2. Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer: J.A. Barrat, J. Cotten
3. Institut français de recherche pour l'exploitation de la mer (IFREMER), Centre de Brest (Centre national de la recherche scientifique—Unité mixte de recherche [CNRS-UMR]): M. Bohn
4. Planetary and Space Science Research Institute, Open University, United Kingdom: I.A. Franchi, R.C. Greenwood
5. Johnson Space Center, Houston, Texas
6. University of Tokyo, Japan

With the assigned name of NWA 2737, this olivine–chromite cumulate is a dunite that exhibits many mineralogical, chemical, and petrographical similarities to the only known martian chassignite, which fell in Chassigny, France in 1815. Northwest Africa 2737 is composed of 89.6 vol% shock-blackened, cumulus forsteritic olivine, 3.1 vol% chromite, 1.6 vol% sanidine, 1.0 vol% pyroxene (augite, pigeonite, and orthopyroxene), and 0.2 vol% phosphate. Carbonates are present in both chassignites, with at least some of them showing evidence for a martian origin—shock fractures post-date the carbonate formation (Beck et al., 2006). Both chassignites have similar REE patterns, carried mostly in their apatite component. A martian origin is attested to by the abundance ratio of Fe/Mn in olivine, bulk Na/Al ratios, and by the O-isotopic ratio.

Northwest Africa 2737 is one of the least terrestrially altered martian finds as shown by its trace element composition and noble gas signatures. However, in contrast to Chassigny and the nakhlites, NWA 2737 contains trapped noble gases more similar to those in shergottites, as evidenced by the lack of martian mantle Xe, the lack of fission-derived Xe from plutonium, and the enrichment of martian atmospheric Xe (Marty et al., 2006).

While feldspars in Chassigny are composed of mostly plagioclase with lesser amounts of labradorite and sanidine, the feldspar in NWA 2737 only occurs as K-rich sanidine or Na-rich analbite, the cause of which may have been a low Al concentration in the parental melt resulting in the delayed nucleation of plagioclase with a consequent buildup of Na in the melt (Beck et al., 2006; Papike et al., 2009). The olivine and chromite grains in both chassignites contain melt inclusions with a unique mineralogy, including the occurrence of hydrous kaersutite amphiboles, and those in NWA 2737 include an alkali feldspar-rich glass. Six distinct melt inclusion types in NWA 2737 were described by He et al. (2010) based on their diverse mineral assemblages. The minerals include olivine, orthopyroxene, pigeonite, augite, kaersutite, chlorapatite, chromite, troilite, and K-rich glass within a wide range of inclusion sizes (~5–300 µm). It is thought that some melt inclusions may contain trapped parental magma at various stages of fractionation.

Other differences between the two chassignites have been identified (Mikouchi, 2005): interstitial chromite has a larger grain size in NWA 2737, plagioclase is absent from NWA 2737, and all phases of NWA 2737 have a more magnesian composition than Chassigny, perhaps reflecting a higher temperature and pressure during fractional crystallization from a less evolved parent melt (Nekvasil et al., 2005). It was determined experimentally that the parental magma of the chassignites may have resembled a terrestrial, silica-saturated hawaiite magma with a higher than terrestrial Mg# and an aluminum content of ~12 wt% (Filiberto, 2008). The chassignites likely crystallized after ~30% crystallization of mafic phases was attained.

While the overall shock effects in Chassigny are only moderate (S4), portions are present that must have experienced much higher shock levels (S5–S6) consistent with the presence of planar deformation features and localized melting. By contrast, the olivine in NWA 2737 exhibits exceptional darkening, which is thought to reflect both the disordered lattice state of an incomplete transformation to olivine high pressure polymorphs, and the presence of nanophase FeNi-metal particles within the olivine (Reynard et al., 2006). These nanoparticles are thought to have formed through subsolidus reduction of olivine during the high temperature phase (~1300°C) of a shock event corresponding to a minimum shock stage of S5. A partial cause of the darkening (brown color) in olivines as described by Treiman et al. (2006) derives from the conversion of Fe2+ to Fe3+ in an oxidizing environment resulting from the shock heating loss of H+ previously dissolved in a hydrous magma. Only a low ppm abundance of Fe3+ would be required to cause this darkening. However, continued investigation utilizing spectral reflectance techniques, including Mössbauer spectroscopy, indicates that the darkening is primarily the result of shock disseminated nanophase metallic iron particles (Pieters et al., 2007). Still other deformed olivine grains have been recrystallized to a visually colorless phase.

Cooling rate estimates for both chassignites have been set at 28–30°C/year, consistent with a formation within a thick lava flow or a shallow dike. The Sm–Nd data for both chassignites plot on a similar isochron—1.416 (±0.057) b.y. for NWA 2737, and 1.380 (±0.030) b.y. for Chassigny. Both chassignites, as well as the nakhlites, also have similar CRE ages of 10–11 m.y., based on 3He and 21Ne. Therefore, it may be argued that both chassignites, as well as the nakhlites, were ejected during a common impact event from the same igneous region on Mars. Based on Ar–Ar data, it was proposed by Bogard and Garrison (2008) that NWA 2737 may have experienced an intense impact event resulting in its burial ~1–20 m deep under a warm ejecta blanket, while a subsequent, less-intense impact event was responsible for its ejection from Mars 10–11 m.y. ago.

Differences have been noted in the gas retention ages of these two chassignites. After correction for terrestrial contamination, a more accurate determination of the K–Ar crystallization age for NWA 2737 was made, and this revised K–Ar age of 376 (±168) m.y. is significantly younger than that of Chassigny and the nakhlites, and more like that of shergottites (Marty et al., 2005; 2006). This discordant age has been attributed to the shock metamorphism of NWA 2737. The specimen of NWA 2737 shown above is a 0.48 g cut end fragment.