NORTHWEST AFRICA 2999

Twelve individual fragments constituting a single meteorite, which have a combined total weight of 392 g, were visually distinguished by Greg Hupé from an assortment of meteorites he had purchased some months earlier in Morocco. Samples from different stones were sent for analysis to Northern Arizona University (T. Bunch and J. Wittke) and the University of Washington in Seattle (A. Irving and S. Kuehner). A preliminary analysis found similarities to known angrites, and a sample was sent to the Carnegie Institution, Washington D.C. (D. Rumble, III) for O-isotopic analysis. By this method it was verified that these meteorites were in fact a new sampling of the angrite parent body. Because of the importance and uniqueness of this find, a sample from each of the twelve fragments was submitted for analysis. Forty-eight additional paired stones have been given a separate designation as NWA 3164 (928 g), while other pairings have been designated as NWA 4662 and NWA 4931.

In contrast to most other quench-textured angrites, NWA 2999 has a polygonal-granular texture more consistent with a slowly-cooled plutonic origin similar to Angra dos Reis and LEW 86010. Evidence in support of a plutonic origin for these three angrites can be found in the homogenous pyroxene compositions compared to the wider compositional range of the other angrites (Kuehner et al., 2006). However, evidence also exists for an extended residence within a regolith. Large (up to 6 mm) anorthite, spinel, and olivine rock fragments are present within the fine-grained groundmass.

In contrast to other angrites, NWA 2999 contains ~8 vol% of coarse FeNi-metal having chondritic abundance patterns, which has been considered to be exogenous and unrelated to any known chondritic or iron chemical groups (Humayun et al., 2007). Consistent with this finding, increased levels of other siderophile elements, such as Co, Ir, and Au, support the presence of a significant meteoritic component. However, it is unknown if this exogenous FeNi-metal source can also explain the increased Mg content and the reduced concentration of refractory elements (Ca, Al, and Ti) observed in this angrite. Such a chondritic impactor would also have necessarily carried an O-isotopic composition close to that of the TFL. Therefore, in order to account for all of the anomalous elemental abundances, an alternate scenario was proposed. Gellissen et al. (2007), followed by Irving and Kuehner (2007), now propose that a large impact onto the angrite parent body, perhaps by an evolved iron object, resulted in the mixture of diverse lithologies from within the same target parent body, which was then followed by deep burial and subsequent thermal metamorphism and annealing.

This angrite meteorite preserves some unique metamorphic features which may suggest that a decompression phase occurred, followed by rapid cooling, events initiated during an extensive multi-km-depth thrust faulting event on a large parent body, postulated by some to be Mercury (Irving et al., 2005). Such features include the clinopyroxene–spinel symplectites that occur between plagioclase and olivine clasts (reflecting a decompression phase), and the plagioclase coronas surrounding portions of spinel grains (reflecting a rapid cooling phase). An alternate explanation for these features has been proposed by Ruzicka and Hutson (2006). They argue that under low pressure oxidizing conditions, at various degrees of melt formation, both plagioclase coronas and clinopyroxene–spinel symplectites can be produced as cooling proceeds. Improved models of these symplectite and corona textures by Irving and Kuehner (2007) has led to their conclusion that these features are more likely the result of the percolation of a S-bearing fluid during a metasomatic phase.

Features which support a very rapid melting and cooling event on the angrite PB have been identified in the angrite NWA 4590. Glass present along mineral grain boundaries attests to a late mobilization of primary phases consistent with a decompression event (Kuehner and Irving, 2007). It has been postulated that the angrite meteorites may represent the impact-related dissemination of a more FeO-rich outer layer during the early history of Mercury, thereby explaining the chemical and mineralogical differences observed on Mercury compared to the angrites; e.g., the higher FeO-abundance of angrites compared to that on the present surface of Mercury, and the reversed Fe/Mn values for both olivine and pyroxene as compared to those of other planetary bodies.

Nevertheless, even accepting that collisional-stripping of a hypothetical FeO-rich basaltic (angritic) crust on Mercury occurred, Hutson et al. (2007) find it implausible that Mercury initially differentiated under oxidizing conditions to form the angritic crust, and then subsequently differentiated under reducing conditions to form the surface that we observe today. They have also determined that other mineralogical features identified in angrites, which on one hand may be attributed to rapid decompresion on a planetary-sized body such as Mercury, may just as well be consistent with the typical cooling processes that occurred during crystallization of a melt.

In contrast to some other angrites, neither kirschsteinite nor orthopyroxene has been found in NWA 2999, and vesicles are absent. Based on Hf–W systematics, NWA 2999 formed ~5 m.y. later than Sahara 99555 and D'Orbigny (Markowski et al., 2007). A precise crystalization age based on the Mn–Cr system indicates an age of 4,557.9 (±1.1) m.y., indistinguishable from that of AdoR and LEW 86010. As deduced by Shukolyukov and Lugmair (2008), two age clusters represent all of the angrites studied thus far, which reflect an early period of magmatic activity.

More recent episodes of impact, disruption, and dissemination of the crust is attested to by the wide range of CRE ages determined for the angrites—0.6–71 m.y. for ten angrites measured; this range is consistent with a large parent body enduring multiple impacts over a very long period of time (Nakashima et al., 2008). A CRE age of 73.4 (±6.6) m.y. was calculated for NWA 2999 (69.6 ±11.2 m.y. for the paired NWA 4931). Taken together, all of the anomalous characteristics observed within the NWA 2999 pairing group could be attributed to contamination through exotic impact event(s), or, as speculated by some, it could be that the NWA 2999 pairing group may even represent a unique parent body with similar O-isotopic values to those measured for the angrite PB. The specimen of NWA 2999 shown above is a 0.228 g partial slice.

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