‘Jules Verne’

Martian Shergottite
Poikilitic (formerly "lherzolitic" shergottite)
(intermediate, permafic, pyroxene-oikocrystic)

standby for nwa 1950 photo
Found January and March 2001
no coordinates recorded

A meteorite comprising two stones, with weights of 414 g and 398 g, was found in the Atlas Mountains in Morocco by a French team under the organization of Bruno Fectay and Carine Bidaut. The meteorite was classified in collaboration between three French institutions—École Normale Supérieure de Lyon (Gillet), Université d'Angers (Barrat), and Institut Français de Recherche pour l'Exploitation de la Mer (Bohn). An additional paired stone weighing 32 g was purchased by a collector in 2012, and it has been designated NWA 7721 (A. Irving and S. Kuehner, UWS). Northwest Africa 1950 is one of a small number of poikilitic shergottites found to date, and the first one found outside of Antarctica. The olivine-gabbroic NWA 2646 is considered to be the second hot desert poikilitic shergottite find. In addition, poikilitic xenocrysts with similar compositions to this group are present in the shergottite EETA79001 lithology A, while trace element abundances, isotopic signatures, and crystallization and CRE ages suggest that the basaltic shergottite NWA 480/1460 might be genetically related to the poikilitic shergottites.

Perhaps key to the classification of NWA 1950 is the martian meteorites RBT 04262 (along with its pairing RBT 04261) and NWA 4468. Both were originally classified as olivine-phyric shergottites, but suggestions have since been made (Mikouchi, 2009) that they might better be considered anomalous members of the poikilitic shergottite group. They are also compositionally very similar to the poikilitic shergottite NWA 2646. RBT 04262 does has a similar crystallization history to that of the other poikilitic shergottites; i.e., formation of pyroxene oikocrysts and their accumulation to form compact poikilitic areas, followed by the crystallization of intercumulus melt to form the non-poikilitic areas. However, it is mineralogically unique in having certain features in common with a basalt, such as a high abundance of plagioclase (13.3%, as maskelynite) and chemical zoning in grains within the non-poikilitic areas. These features are consistent with rapid cooling (~0.03–0.09°C/hour) near the surface rather than the slow cooling expected in a pluton. RBT 04262 also contains a greater proportion of the non-poikilitic evolved melt component consistent with a basalt. Although olivine in RBT 04262, NWA 4468, and NWA 2646 is more Fe-rich than in other poikilitic shergottites, it is mineralogically more similar to them than it is to other shergottite groups.

These features led Mikouchi et al. (2008) to infer a crystallization for RBT 04262 within a stratagraphic layer closer to the surface than that of the other poikilitic shergottites, but still originating from a common magmatic source region. Based on Ca-zoning, any variation in its crystallization depth could not be resolved to a greater extent than 4–5 m. It was also conjectured that basaltic shergottites with similar young crystallization ages may have formed from this same evolving melt in a stratagraphic layer above that of the poikilitic shergottites.

This martian group has been historically included as a subgroup within the shergottite class, and its members were commonly described as "lherzolitic" shergottites (or shergottitic peridotites) in conformity with the term basaltic shergottites. In actuality, this martian meteorite group does not contain the minimum abundances of olivine or orthopyroxene as those established for terrestrial lherzolites. However, since there was no known petrologic relationship existing between the basaltic and "lherzolitic" shergottite subgroups, and these groups are resolved from each other on an O-isotope plot, the use of the term "lherzolitic" shergottite was proposed by Eugster and Polnau (1997) to represent this unique group of martian meteorites. The discovery of RBT 04262, NWA 2646, and NWA 4468 required further revisions in martian meteorite classification terminology.

The British Geolocical Survey has established a hierarchical classification scheme for terrestrial igneous rocks. The group of igneous rocks that are ultramafic, coarse-grained, crystalline, and have a mafic content >90% are further classified by their content of mafic minerals. Peridotites are distinguished from pyroxenites (at Level 7 of the hierarchy) by containing more than 40% olivine. The peridotites are then divided (at Level 8 of the hierarchy) into the dunite, pyroxene-peridotite, pyroxene–hornblende-peridotite, and hornblende-peridotite groups. The pyroxene-peridotite group is further divided (at Level 9 of the hierarchy) into the harzburgite, lherzolite, and wehrlite groups.

In an effort to resolve the discrepencies that exist between the official IUGS definition of lherzolites and the application of that term to the varied group of "lherzolitic" shergottites, Mikouchi (2009) addressed the need for changing the name of the "lherzolitic" shergottites to one that is more consistent and more broadly applicable. Since a texturally-based nomenclature is already employed for some shergottite subgroups, e.g., olivine-phyric, it was suggested that the term "pyroxene-oikocrystic" shergottites would be an appropriate designation to comprise all of the various martian "lherzolitic" shergottites that exist in the worldwide collections. This would include intermediate, enriched, and postulated depleted "lherzolitic" shergottites, as reflected by a geochemical classification scheme.

More recently, in an effort to rectify the discrepencies that exist in martian meteorite nomenclature, the textural term "poikilitic" was proposed by Walton et al. (2012) to apply to those meteorites previously referred to as "lherzolitic" shergottites, which is to be used along with additional descriptive terms for bulk major element compositions (based on a plot of Mg/[Mg + Fe] vs. CaO, where this ratio increases along the sequence from mafic to permafic to ultramafic) and trace element content (based on the enrichment of HREE over LREE, increasing along the sequence from depleted to intermediate to enriched).

(Mg/[Mg + Fe] vs. CaO)
ENRICHED   RBT 04261/2
GRV 020090
NWA 4468
NWA 7397
NWA 6342
NWA 2646
ALH 77005
LEW 88516
GRV 99027
NWA 4797

After Irving et al. (2010), Walton et al. (2012), and Dr. Anthony Irving's List of Martian Meteorites

Poikilitic shergottites are defined as cumulate, plutonic rocks (e.g., ALH 77005 formed at a depth of ~18 km; Szymanski et al., 2004) which are derived from primary magmas containing >90% mafic minerals. These minerals are composed of >40% olivine (45.3 vol% in NWA 1950), ~>10% low-Ca pyroxene, and ~>10% high-Ca pyroxene (34.5 vol% pyroxenes in NWA 1950). The olivine in poikilitic shergottites is chemically similar to the olivine in the martian dunite Chassigny, but pre-terrestrial Fe redox processes gives olivine in poikilitic shergottites a distinctive brown color.

In contrast to the dark olivine present in one of the chassignites, which contains both magnetite and FeNi-metal nano-particles, it was demonstrated by Kurihara et al. (2009, 2010) that the dark olivine in martian poikilitic shergottites (and in certain other SNCs; Hoffmann et al., 2009) reflects the presence of 200–500 nm-wide parallel bands containing Ni-free hematite nano-particles measuring ~20 nm in size. These are thought to have formed by recrystallization following shock melting during impact ejection, or possibly during shear stresses. Further studies of NWA 1950 by Mikouchi et al. (2013) verified the presence of abundant 5~100 nm-sized rounded particles, but they also identified rod-shaped Fe-metal particles larger than 100 nm. The nano-particles in NWA 1950 contain a core of Fe-metal that formed by reduction of olivine under high temperature annealing (non-melting) conditions, and some are rimmed by magnetite that formed as temperatures decreased under more oxidizing conditions (Takenouchi et al., 2014). Takenouchi et al. (2015) suggested that the Fe nanoparticles were formed in association with high-pressure polymorphs such as wadsleyite during a high-pressure, high-temperature shock event. Ultimately, the wadsleyite was back-transformed to olivine during a period of high post-shock temperatures, and this olivine now appears brown due to the presence of the Fe nanoparticles. Subsequent studies were conducted by Takenouchi and Mikouchi (2016) of several shergottites including NWA 1950 as well as the chassignite NWA 2737, which contain darkened olivine associated with shock-melt phases. They found higher Fe3+ ratios in both brown (or brownish) olivine compared to adjacent colorless olivine located within the same grain, attesting to the formation of Fe nanoparticles by olivine oxidation and reduction during transformation to high-pressure polymorphs such as ringwoodite and wadsleyite.

Poikilitic shergottites also contain a significant amount of feldspathic glass in the form of maskelynite (11.0 vol% in NWA 1950), along with accessory chromite (5.7 vol% in NWA 1950). Other mineral phases include ilmenite, phosphates (merrillite in NWA 1950), sulfides (pyrrhotite in NWA 1950), and an interstitial K-rich glass. Trapped martian atmospheric gases have been identified in maskelynite and melt pockets in some poikilitic shergottites, and this gas is thought to have produced the vesicles present in these specimens through exsolution during decompresion. Oxygen isotopic ratios for NWA 1950 are identical to those of other martian poikilitic shergottites.

Two main phases are typically evident in poikilitic shergottites, reflecting two stages of crystallization: 1) a lighter, postcumulus phase in which large orthopyroxenes poikilitically enclose cumulus olivine and chromite grains, and 2) a darker, non-poikilitic phase consisting of olivine, orthopyroxene, maskelynite, chromite, clinopyroxene, ilmenite, phosphates, and sulfides. This latter phase occurs interstitially to the larger orthopyroxenes and incorporates a significant component of trapped intercumulus melt (Treiman et al., 1994; Mittlefehldt et al., 1997). Olivine and pyroxene in the non-poikilitic areas have experienced a considerable degree of re-equilibration with evolved melts (Mikouchi, 2005).

Variation in the intensity of shock metamorphism is apparent in olivine and pyroxene, which ranges from mosaicism and planar deformation features, shock-induced veining, high-pressure polymorphs of olivine, partial melting and recrystallization, and culminating in localized sub-mm- to mm-sized melt veins and melt pockets which constitute 1.8 vol% of the meteorite. Microporphyritic textures including euhedral and dendritic host rock crystallites, chromite stringers, sulfide globules, and vesiculated maskelynite with flow textures are all associated with the melt pockets (Walton and Herd, 2007). Stishovite has been identified in the maskelynite near melt zones by Raman spectra (Gillet et al., 2005). These shock features, along with the content of trapped 40Ar (Terribilini et al., 1998), are indicative of very strong shock pressures equivalent to ~35–45 GPa (S5), and a post-shock temperature of ~200°C, conditions similar to those experienced by LEW 88516 and Y-793605. Compared to the degree of shock observed in NWA 1950, the poikilitic shergottites GRV 99027 and ALH 77005 experienced significantly higher degrees of shock, up to at least 55 GPa, reflecting post-shock temperatures of 1000°C. These shock levels are manifest in the interconnected melt pockets and other shock melt components constituting up to 29 vol% of the bulk of ALH 77005.

Based on several radiometric chronometers, a young isotopic age of ~160–180 m.y. was determined for the igneous crystallization of all poikilitic shergottites studied to date, as well as for many of the basaltic shergottites. An Ar–Ar age of 382 (±36) m.y. was determined for NWA 1950 by Walton et al. (2008). Yamato 793605 has a much shorter terrestrial age of 35 (±35) t.y. compared to 190 (±70) t.y. for ALH 77005, while its terrestrial age is within the range of error of 21 (±1) t.y. calculated for LEW 88516. An ejection age (CRE age + terrestrial age) of ~4 m.y was ascertained for the three poikilitic shergottites Y-793605, ALH 77005 and LEW 88516, which is about 1 m.y. earlier than the ejection event calculated for many of the basaltic shergottites. The CRE age for NWA 1950, based on various rare gas chronometers, shows a range of between 2.3 (±1.0) m.y. and 5.3 (±3.0) m.y., which is similar to the ranges determined for the other martian poikilitic shergottites. In addition, they all share similar chemical compositions (including REE abundances; Hsu et al., 2004) and petrologies, and therefore it is presumed that they experienced a simultaneous ejection from a common lithological unit on Mars.

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 1950 was launched from Mars during the 4.05 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 1950 and at least 14 other intermediate poikilitic shergottites were ejected in a common impact event unique from the others.

Shock pressure comparisons indicate that NWA 1950 was in a shallower position within the magma unit than ALH 77005—compare shock pressures of 30–44 GPa for NWA 1950 to those of 45–55 GPa for ALH 77005. It is apparent that some of these martian samples existed as separate meteoroids during their journey to Earth. Northwest Africa 1950 has sustained only slight terrestrial weathering effects. The specimen shown above is a 0.44 g partial slice with a small amount of fusion crust. The top photo below shows the poikilitic shergottite ALH 77005, which has an olivine composition and re-equilibration stage very similar to that of NWA 1950. The three photos below that are views of the main mass of NWA 1950.

ALH 77005—NASA photo #S78-37989
Photos courtesy of B. Fectay and C. Bidaut—