Three fragments constituting a single fresh stone meteorite and partially covered in a glossy, 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 permafic 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 reservoir from which the parent magma for NWA 5789 was derived had a composition that was depleted in incompatible elements, to which 2% olivine was added (Irving et al., 2010; Treiman and Filiberto, 2014).
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 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 that 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 which is more indicative of separate parental source regions on Mars is the high-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.
As with Y-980459 (Fo8486), NWA 5789 is thought to represent a Mg-rich, primitive mantle melt in which olivine megacryst cores have a Mg# in equilibrium with the melt of the bulk rock composition (Fo84.4 compared to the equilibrium value of Fo85.5) (Gross et al., 2011). 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 might 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 was 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 13001400°C, and were formed in reduced redox conditions (oxygen fugacity near the ironwüstite buffer). Modeling of the composition of both Y-980459 and NWA 5789 by Filiberto and Dasgupta (2012) suggests their parent magmas separated from a primary mantle source region at a pressure of 1.2 (±0.1) GPa and temperature of 1540 (±20) °C, consistent with a relatively shallow depth of ~100 km. This temperature is at the high end of the range calculated for the average mantle temperature during the Noachian period (4.53.6 b.y. ago) of 1450 [±80] °C, and they attribute this to a thermal anomaly.
It was calculated that both NWA 5789 and Y-980459 had a pre-atmospheric diameter of <10 cm, and they exhibit similar CRE ages of 1.0 (±0.2) m.y. This CRE age is indistinguishable from that of at least 7 other depleted olivine-phyric shergottite falls, all of which evidently represent a common ejection event on Mars (Nishiizumi et al., 2011). 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 5789 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 at the time of their study) martian meteorites. They speculate that the number could be as few as twenty, and suggest that NWA 5789 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.
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 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. Next, these designations 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, or orthopyroxenite. 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.
∗ Recent geochemical research on the martian shergottites has led to new petrogenetic models and classification schemes. read more >>