NWA 8159

Martian Shergottite
augite-rich ∗
(depleted, mafic)

standby for nwa 8159 photo
Found 2013
no coordinates recorded At the 2013 Denver Gem and Mineral Show, meteorite dealer Blaine Reed was asked by a fellow meteorite dealer (S. Tutorow) to sort through a box of at least a hundred ‘likely terrestrial’ rocks remaining in a bulk shipment sent from a Moroccan dealer (B. Tahiri). 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—augite-rich basalt. For his part in recovering this rare meteorite, Mr. Reed was kindly given a 2.2 g section.

Based on the initial submitted samples as well as subsequent analyses (Herd et al., 2017), the modal composition of NWA 8159 was determined to be augite (~48–50%), plagioclase (both crystalline and amorphous present in approximately equal proportions: ~37–40%), olivine (~5%), magnetite (~5%), maghemite (~0.5%), and orthopyroxene (~1–2%), 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 (~15–23 GPa; Herd et al., 2017) 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 preventing the formation of post-shock back-transformation phases; such rapid quenching also prevented isotopic resetting. Other features of NWA 8159 are also indicative of rapid cooling, including its fine-grained texture, the intergrown assemblages of augite, plagioclase, and magnetite crystals, and the presence of metastable pyroxene phases.

In their investigation of the shock history of NWA 8159, Sharp et al. (2019) determined that the meteorite experienced a comparatively long shock pulse duration of 100 ms. They described an impact scenario, consistent with all of the measured shock parameters of NWA 8159, in which the ejected meteoroid originated some distance from the point of impact of an object ~1 km in diameter. Other known shergottites that may have been ejected during the same event were located closer to the point of impact and experienced higher shock pressures (>20 GPa) and shorter shock pulse durations (~10–20 ms).

It was initially considered by some that NWA 8159 could possibly represent an extruded evolved intercumulus melt that was displaced from below, and 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 2.37 (±0.25) b.y. (Herd et al., 2017), which is significantly older than the ~1.3 b.y. Sm–Nd age of the nakhlites (Simon 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). Based on the results of cosmogenic nuclide studies, Herd et al. (2017) calculated the pre-atmospheric size of NWA 8159 to be 0.6–1.2 m in diameter.

Coincidentally, the crystallization age of NWA 8159 is consistent with the 2.403 (±0.140) b.y. Sm–Nd age determined for the depleted, mafic, olivine–plagioclase-phyric shergottite NWA 7635 (Righter et al., 2014). Both NWA 8159 and NWA 7635 also share similar REE abundances (unique HREE) and pyroxene compositions; in fact, Herd et al. (2017) suggest that a classification for NWA 7635 as an augite-rich shergottite would also be appropriate. Moreover, they determined that NWA 8159 and NWA 7635 have very similar ejection ages (~1.2 m.y. and ~1.0 m.y., respectively), and speculate that both meteorites may be launch paired. Notably, at least 16 additional depleted shergottites have a similar ejection age (Irving et al., 2017; #2068), and despite some having much younger crystallization ages (as young as 348 m.y.), all of these depleted shergottites might be associated with a common ~2 b.y.-old volcanic site (Lapen et al., 2017; diagram).

The terrestrial ages calculated for NWA 8159 and NWA 7635 are 230 (±60) t.y. and 2.3 (±1.3) t.y., respectively, but this does not exclude a common Mars ejection event. Nevertheless, Kruijer et al. (2017; diagram) determined that these two meteorites have disparate ε182W values of 1.13 (±0.10) and 1.80 (±0.13), respectively. This corresponds to a later formation of the source for NWA 8159 by more than 5 m.y., possibly reflecting a re-melting event in the mantle, and therefore a common source-crater pairing is doubtful.

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mouseover the time machine set to 230 t.y. before present

For additional information on the crystallization and ejection age of NWA 7635 and similar shergottites, read the PSRD article by G. Jeffrey Taylor: ‘Two Billion Years of Magmatism in One Place on Mars‘, May 2017.

Further evidence supporting a unique martian source region for NWA 8159 was presented by Kayzar et al. (2015) through a coupled Nd–W diagram, and by Herd et al. (2015) through a coupled O–Cr diagram; these clearly demonstrate that NWA 8159 is resolved from other martian mantle reservoirs (see below). standby for nd-w diagram
Diagram credit: Kayzar et al., 46th LPSC, #2357 (2015)

standby for o-cr diagram
Diagram credit: Herd et al., GCA, vol. 218, p. 20 (2017)
‘The Northwest Africa 8159 martian meteorite: Expanding the martian sample suite to the early Amazonian’
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.

Utilizing 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 typical 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). However, although NWA 8159 crystallized from an evolved melt, it is the conclusion of Herd et al. (2017), based in part on the bulk rock Sm–Nd and W–Nd values, that NWA 8159 represents a near primary mantle melt derived from a unique, more highly incompatible element-depleted martian mantle reservoir.

Based on petrographic, mineralogical, chemical, and isotopic similarities, it has been suggested 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. The photo below shows the main mass sectioned in half. standby for mars photo
Photo credit: Herd et al., GCA, vol. 218, p. 5 (2017)
‘The Northwest Africa 8159 martian meteorite: Expanding the martian sample suite to the early Amazonian’

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

standby for mars photo

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