Martian Shergottite
olivine–orthopyroxene-phyric ∗
(depleted, permafic, reduced)

standby for nwa 2046 photo
Purchased September 2003
no coordinates recorded

A 63 g partially fusion-crusted stone was found in the Sahara Desert near Lakhbi, Algeria and subsequently purchased by an American dealer in Rissani, Morocco. Analysis and classification was completed in a collaboration between Northern Arizona University (J. Wittke and T. Bunch) and the University of Washington in Seattle (A. Irving and S. Kuehner).

The shergottite NWA 2046 contains abundant zoned olivine and orthopyroxene crystals exhibiting a preferred alignment indicative of magmatic flow and/or crystal accumulation. Although it is similar in many respects to the primitive shergottites NWA 1195 and DaG 476/489, both previously classified as olivine-phyric shergottites, NWA 2046 has olivine megacrysts that are more strongly zoned and have more magnesian cores (Fo84.3) with more ferroan rims. The highly magnesian nature of the olivine megacryst cores is consistent with an origin from a less evolved magma source region, which was at the same time depleted in incompatible elements (LREE). The crystal faces of the megacrysts in NWA 2046 are better defined than those in NWA 1195, and are more consistent with a phenocrystic rather than xenocrystic origin.

A new shergottite subgroup name was proposed to distinguish these unique meteorites—olivine–orthopyroxene-phyric (Irving et al., 2004). Northwest Africa 2046 is a very primitive martian meteorite with similarities to the olivine-phyric shergottite Y-980459 (containing olivine cores with a more magnesian composition of Fo86, and lacking plagioclase [Mikouchi et al., 2003]). These zoning features of NWA 2046 suggest that it was derived from a very primitive source magma and experienced rapid cooling.

The formation of NWA 2046 and some other depleted shergottites has been successfully modeled by a fractional crystallization process based on a parental magma source having a composition similar to that of the most primitive, magnesian shergottite, Y-980459 (Symes et al., 2008). It was demonstrated that 43% fractionation can cause the melt to progress from a primitive Y-980459-like composition to that of the most evolved composition represented by QUE 94201. This fractionation hypothesis is in contrast to the most commonly discussed hypothesis cited to explain the compositional variation among the shergottite group, which calls for the assimilation of differing proportions of evolved crustal material by a common parental magma.

Other shergottites having close to very similar REE depletions, Sr–Nd isotopic compositions, and old crystallization ages (e.g., Y-980459, SaU 005/008, Dhofar 019, QUE 94201, DaG 476, NWA 1195, and NWA 480/1460) might share some aspects of their petrogenetic history. For example, the Sm–Nd isotopic data for NWA 1195 reflects a crystallization age of 347 (±13) m.y., very similar to that of the basaltic shergottite QUE 94201; however, these two shergottites are not isotopically related (Symes et al., 2005, 2008).

A new classification scheme for the shergottites (and ‘lherzolites’) based on REE patterns and trace element systematics such as Sr–Nd has been proposed by Symes et al. (2008) and expanded upon by Bunch et al. (2008) and Shih et al. (2009). The new classification scheme shown is based on the Mg# of a sample indicating the extent of evolution of the parent magma, along with the initial εNd which serves as a measure of the enrichment or depletion of the source composition.

Using spectral data obtained from the Viking and Pathfinder Mars landers, it was proposed that the olivine–orthopyroxene-phyric meteorites are compositionally similar to certain rocks analyzed on the martian surface—the high-silica andesites. These silica-rich rocks are analogous to terrestrial boninites (SiO2 >53%, MgO >8%, and TiO2 <0.5%), and both of these rock types may have experienced a similar petrogenesis. They suggested a possible process for the formation of olivine-phyric shergottite magmas: partial melting and dehydration of hydrous harzburgitic peridotite as a result of the heat from rising mantle plumes.

The groundmass of NWA 2046 has a coarse texture that consists of well-developed, Mg-rich, orthopyroxene megacrysts, some of which measure ~2mm in length. A petrofabric analysis revealed a preferred orientation among the orthopyroxene and olivine megacrysts, presumably reflecting their entrainment within a magma flow, perhaps in a shallow dike, although a cumulate origin is possible.

Estimates of the crustal thickness of Mars give a range of ~30–100 km, with an average thickness of ~50 km; this is equivalent to ~3% of the planetary mass. The crustal composition of Mars is enriched in moderately volatile large-ion lithophiles (e.g. K, Rb, and Cs), refractory large-ion lithophiles (e.g. Th and U), and in large rare earth elements (e.g. La), values which are complementary to a large-ion lithophile depletion in the mantle (McLennan, 2003). This is evidenced by the LREE/REE/Nd-isotopic compositions of basaltic shergottites, the values of which are consistent with the derivation of basaltic shergottites from a highly depleted mantle source that experienced mixing and assimilation of an LREE-enriched crustal component. An alternative model proposed by Symes et al. (2008) suggests that fractional crystallization is responsible for the compositional variability among the shergottites. In an effort by another investigator to account for the chondritic elemental proportions that are assumed to have been present on Mars initially, in particular, to explain the low La/Th ratios found in shergottites, the existence of an additional unsampled, primitve mantle reservoir was proposed. Since certain nakhlites contain a relatively high La content, they may in fact sample this reservoir.

A CRE age for NWA 2046 was determined to be 1.1 (±0.2) m.y., which is indistinguishable from at least 8 other depleted olivine-phyric shergottite falls (DaG 476 and pairings, NWA 1195, 2626, 4925, 5789, SaU 005, Y-980459, and Tissint), evidently representing a common ejection event on Mars (Nishiizumi et al., 2011). Variations in flight trajectories has resulted in a wide range of terrestrial ages. For instance, that of NWA 2046 is calculated to be 90 (±50) t.y., whereas that of NWA 1195 is 240 (±60) t.y. and Tissint is a recent fall. Cosmic ray exposure ages have now been determined for many martian meteorites, and Mahajan (2015) compiled a chart based on the reported CRE ages for 53 of them. He concluded that together these 53 meteorites represent 10 distinct impact events which occurred 0.92 m.y., 2.12 m.y., 2.77 m.y., 4.05 m.y., 7.3 m.y., 9.6 m.y., 11.07 m.y., 12.27 m.y., 15 m.y., and 16.73 m.y.—see his chart here. It was argued that NWA 2046 was launched from Mars during the 0.92 m.y.-old impact event. In a subsequent review based on multiple criteria, Irving et al. (2017 [#2068]) made a new determination of the number of separate launch events associated with the known (101) martian meteorites. They speculate that the number could be as few as twenty, and suggest that NWA 2046 and at least 18 other depleted (predominantly olivine-phyric) shergottites were ejected 1.1 m.y. ago in a common impact event unique from the others.

The photo of NWA 2046 shown above is a 0.44 g partial slice. The top photo below shows the complete mass as found, while the middle photo shows a complete slice that exhibits abundant olivine megacrysts embedded within a groundmass consisting of pyroxene and maskelynite, along with other minor constituents. The bottom photo shows a thin section viewed in both plane light and polarized light.

∗ Recent geochemical research on the martian basalts has led to new petrogenetic models and classification schemes.  read more >>

standby for nwa 2046 photo

standby for nwa 2046 photo

standby for nwa 2046 photo
Photos courtesy of Mike Farmer—Mike's Meteorites and Tektites