NORTHWEST AFRICA 8159

Martian Augite Basalt
(depleted, mafic)
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Found 2013
no coordinates recorded

At the 2013 Denver Gem and Mineral Show, meteorite dealer Blaine Reed was asked by a fellow meteorite dealer to sort through a box of at least a hundred "likely terrestrial" rocks remaining in a bulk shipment sent from Morocco. During his examination he noticed a small 149.4 g stone with visible shock veins and thought it was probably a eucrite; however, an XRF probe on a cut face indicated that the meteorite was possibly martian. The owner subsequently brought this stone to the University of New Mexico (C. Agee) for analysis and classification, and NWA 8159 was determined to be a new martian type-specimen—augite basalt. For his part in recovering this rare meteorite, Mr. Reed was kindly given a 2.2 g section.

Based on the submitted samples, the modal composition of NWA 8159 was determined to be augite (~50%), plagioclase/maskelynite (present in approximately equal proportions: ~40%), olivine (5%), magnetite/maghemite (~3%), and orthopyroxene (~2%), along with minor amounts of ilmenite, merrillite, Cl-apatite, and Cr-spinel. Sharp et al. (2015) reported the presence of shock-melt veins of unique texture associated with high-pressure mineral phases including majoritic garnet, stishovite, coesite, tissintite (a Ca-analog of jadeite), and ahrensite (an Fe-analog of ringwoodite), the two latter phases having been discovered in the Tissint shergottite (Ma et al., 2014). These high-pressure mineral phases are indicative of a weak to moderate shock stage and a high temperature (~2000°C). In their analysis of a shock melt vein in NWA 8159, Hu and Sharp (2016) observed high-pressure phases along with maskelynite and partially amorphized plagioclase associated with shear along a shock melt vein; these phases were probably produced by solid-state transformation during a localized shock event without significant melting. In a nano-scale mineralogical study of shock veins in NWA 8159, Sharp and Walton (2016) determined that the high-pressure minerals were formed under weak shock pressures of ~16 GPa and were rapidly quenched to temperatures below 1200 K within seconds, which prevented the formation of post-shock back-transformation phases; such rapid quenching also prevented isotopic resetting.

It was initially considered by some that NWA 8159 could possibly represent an extruded, evolved intercumulus melt that was displaced from below, which contained a low abundance of xenocrystic olivines spalled from the walls of the nakhlite magma chamber (Herd et al., 2014). Notably, NWA 8159 has a similar δ37Cl value as the nakhlites, 1.5‰ vs. 1.8‰, respectively (Sharp et al., 2016). However, the Sm–Nd-based crystallization age for NWA 8159 was determined to be 2.3 (±0.5) b.y., which is significantly older than the ~1.3 b.y. Sm–Nd age of the nakhlites (Simon et al., 2014). Coincidentally, the age of NWA 8159 is consistent with the 2.33 (±0.13) b.y. Sm–Nd age determined for the depleted, mafic, olivine–plagioclase-phyric shergottite NWA 7635 (Righter et al., 2014). Based on Ar–Ar chronometry, a minimum age of 2.15 (±0.10) b.y. was obtained for a shock melt vein in NWA 8159; however, the timing of melt vein formation has not yet been established (Cassata, 2016). Further evidence supporting a unique martian source region for NWA 8159 was presented by Kayzar et al. (2015) through Hf–W isotope systematics. They demonstrated that NWA 8159 is clearly resolved from other martian mantle reservoirs on a ε142Nd vs. ε182W diagram (see below).

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Diagram credit: Kayzar et al., 46th LPSC, #2357 (2015)

An O-isotopic analysis was conducted at the University of New Mexico (K. Ziegler), and the values plot on the martian fractionation line. The CRE age for NWA 8159 was calculated for both shock melt glass and bulk rock, which yielded an age of ~1.5 and ~1.1 b.y., respectively (Cassata, 2016). In addition, they identified a martian atmospheric Xe component, but the timing of its incorporation is so far indeterminate.

Based on Nd–Nd isochron age models, an age of 4.504 (±0.005) b.y. has been calculated for the parental source region of the martian shergottites. On the same basis, a younger age of 4.451(+0.020/–0.013) b.y. was calculated by Kayzar et al. (2015) for the parental source region of NWA 8159. However, to account for the much higher ε182W value obtained for NWA 8159 (~ +1.3) compared to that of the shergottites (~ +0.2 to +0.9) , the initial content of the 182Hf parent isotope would necessarily have been significantly higher given its relatively short half-life of 8.9 m.y. (i.e., 182Hf would have been virtually extinct by the time the source for NWA 8159 was formed). Therefore, it was concluded that NWA 8159 experienced a more complex fractionation history than typical basaltic shergottites, possibly forming at much greater depths within the martian mantle, and the Nd–Nd age calculated for its source region might not be accurate.

Studies by Agee et al. (2014) show that NWA 8159 contains an unusually pure form of magnetite compared to that of other martian meteorites, and they recognized that this magnetite represents the most oxidized martian material analyzed thus far. In addition, they discovered that the typical correlation between LREE pattern and oxidation state that is present in other martian meteorites is not observed in this meteorite, providing further evidence for a unique petrogenetic history. The parental source composition for NWA 8159 was LREE depleted in a similar manner to the depleted, high-Al basaltic martian meteorite QUE 94201, which is considered to be a product of extensive silicate fractionation of a Y-980459-like primary mantle melt (Kayzar et al., 2015; Treiman and Filiberto, 2014).

Based on petrographic, mineralogical, chemical, and isotopic similarities, it has been argued that the olivine-phyric basaltic shergottite NWA 10416 might be genetically related to NWA 8159. Of particular note, both meteorites contain shock features and veins associated with a similar suite of high-pressure minerals and feldspathic glass phases. At the same time, a significant abundance of primary crystalline feldspar still remains in NWA 8159 with much less persisting in NWA 10416, while in other martian basalts complete transformation to maskelynite has occurred (Walton et al., 2016). In addition, Vaci et al. (2016) employed multiple high-precision techniques and recognized that both meteorites likely experienced a low degree of parent body aqueous alteration, which is evident in the replacement of olivine cores by micro-scale hydrated minerals including a Mg-bearing form of a ferric iron-bearing fayalite known as laihunite.

Following the classification of NWA 8159, the 2.2 g block held by Mr. Reed was cut into two thin slices, from which smaller specimens were broken for distribution. The specimen of NWA 8159 shown above is a 0.25 g interior partial slice.

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

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