NORTHWEST AFRICA 5789

Three fragments constituting a single fresh stone meteorite and partially covered in a shiny, bubbly, black fusion crust were found in the Morroccan Sahara. The combined weight of the three meteorite fragments was determined to be 49 g. A 1.8 g sample of the meteorite was provided to G Hupé who sent a sample both to the University of Washington (A. Irving and S. Kuehner) for petrographic analysis, and to the Carnegie Institute (D. Rumble, III) for O-isotope analysis. It was ascertained that NWA 5789 is a rare primitive martian meteorite. Thereafter, the remaining quantity of NWA 5789 was purchased by Chladni's Heirs (S. Ralew and M. Altmann).

Northwest Africa 5789 is a slightly friable meteorite with a mafic/ultramafic composition. A terrestrial analog for this meteorite is limburgite, a rock type found in a volcanic quarry in Limburg, Germany. Limburgite is a rapidly cooled, alkaline, sometimes vesicular basaltic lava, primarily composed of olivine and augite. This rock type is often associated with hydrated minerals such as kaersutite. The magma from which NWA 5789 originated was determined to be depleted in incompatible elements (Irving et al., 2010).

Northwest Africa 5789 is almost identical in bulk composition, petrography, and mineral chemistry, and similar in texture to the martian meteorite Yamato 980459, an olivine-websterite, which has crystallized from the most primitive martian melt known (Gross et al., 2010). However, there are textural differences between the two meteorites related to differences in their cooling rate. In Y-980459, it was shown by Greshake et al. (2004) that ascent of the magma to the surface resulted in rapid quenching which suppressed the formation of plagioclase and produced a glassy mesostasis. In contrast, NWA 5789 experienced slower cooling as it ascended from higher to lower pressures, and the mesostasis formed crystalline plagioclase with radiating sprays of clinopyroxene, spinel, ilmenite, FeS, and silica laths. Another difference between the two meteorites that is more indicative of separate parent source regions on Mars is the highly siderophile element content and Os isotopes; the HSE in Y-980459 is higher, while the 187Os/188Os ratio is lower. Furthermore, Hoffmann et al. (2010) determined that significant differences exist between their respective magnetic signatures.

Like Y-980459 (Fo84–86), NWA 5789 is thought to represent a Mg-rich, primitive mantle melt in which the olivine megacryst cores are in Mg# equilibrium with the melt of the bulk rock composition (Fo84.4 compared to the equilibrium value of 85.5). Olivines in both meteorites are virtually identical, present as mm-sized yellow-green megacrysts containing the highest known Mg# values among martian meteorites. In addition, smaller phenocrysts of olivine and pyroxene in equilibrium are scattered throughout a fine-grained groundmass composed of pigeonite, chromite, pyrrhotite and mesostasis. It was suggested that the two meteorites may originate from the same or similar source magmas, with NWA 5789 crystallizing more slowly in a thicker section of the melt.

Based on pyroxene crystallization in NWA 5789, it is shown that the cores formed at high pressure conditions, consistent with the crust or upper mantle, while the mesostasis formed at lower pressures, on or near the surface (Gross et al., 2011). This difference in pressure amounts to ~10 kbar or a difference in depth of 85 km, indicating an extended crystallization history that was completed when the magma erupted onto the surface. Temperatures of the magma at depth for both Y-980459 and NWA 5789 were calculated to have been 1300–1400°C at an oxygen fugacity near the iron–wüstite buffer. Experiments (PMELTS) on the composition of both Y-980459 and NWA 5789 reflect a primary source melt at a pressure of 1.2 (±0.1) GPa and a temperature of 1540 (±20) °C, consistent with a depth of ~100 km. Both Y-980459 and NWA 5789 have a similar CRE age of 1.0 (±0.2) m.y. which is indistinguishable from that of at least 7 other depleted olivine-phyric shergottite falls, evidently representing a common ejection event on Mars (Nishiizumi et al., 2011). Both of these meteorites also had a preatmospheric diameter of <10 cm.

A new systematic classification method was invoked for the shergottites by Irving et al. (2010). They utilized a bulk chemical diagram comparing the CaO content with the Mg#, and they established three separate categories encompassing all possible values: mafic, permafic, and ultramafic (see 41st LPSC, #1547). In addition, they combined the resulting designation with the existing terminology pertaining to trace element abundances and isotopic systematics: enriched, intermediate, and depleted. Finally, these designators are combined with the established meteorite group name: mafic or diabasic shergottite (formerly "basaltic" shergottite), olivine-phyric shergottite, poikilitic shergottite (formerly "lherzolitic" shergottite), nakhlite, chassignite, and ALH 84001. Under this scheme the primitive martian magmatic rock NWA 5789 is classified as a depleted permafic olivine-phyric shergottite.

The martian rover "Spirit" has identified alkaline volcanic rocks at Gusev crater which are geochemically consistent with limburgite, and which may characterize highlands terrane on Mars (Kochemasov, 2006). The specimen of NWA 5789 shown above is a sub-gram fragment. The mesmerizing photos below, kindly provided by Chladni's Heirs, show different views of this unique martian meteorite.