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

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Found 2012
no coordinates recorded

A single partially fusion-crusted stone weighting 2,130 g was found near Smara, Morocco and later purchased by meteorite dealers. A sample was submitted for analysis and classification to the University of Washington in Seattle (A. Irving and S. Kuehner) and the University of Alberta (G. Chen) and it was determined to be a martian poikilitic, enriched, permafic, shergottite. Additional fragments from this same martian meteorite fall, now estimated to have a combined weight of ~5 kg, were sold at the 2013 Tucson Gem and Mineral Show by Moroccan dealers and have been verified as pairings (e.g., NWA 7387, 7755, 7937, 8161).

Northwest Africa 7397 exhibits a unique appearance in hand sample, but has a texture and composition typical of poikilitic shergottites. As with other poikilitic shergottites, it consists of two textural regions: one having a poikilitic texture comprised of large pyroxene oikocrysts (up to 1.5 cm) enclosing olivine (< ~500µm) and chromite (< ~150µm) chadacrysts, and the other having a nonpoikilitic texture comprised of olivine, pyroxene, and interstitial maskelynite, along with minor merrillite, Ti-chromite, pyrrhotite and ilmenite. Olivines in the nonpoikilitic region contain sparse K-Na-Al-Si-rich melt inclusions associated with high-pressure phases such as stishovite and ringwoodite, which attest to shock pressures of ~25 GPa and temperatures of 2300–2500°C (Yoshida et al., 2016). This meteorite has many features consistent with a strong shock stage (plagioclase conversion to maskelynite, melt veins, and melt pockets) and low terrestrial weathering and contamination (carbonate-filled fractures, La-Ce enrichments; Howarth et al., 2014).

Poikilitic shergottites as a group share a similar crystallization history, i.e., formation of pyroxene oikocrysts and their accumulation to form compact poikilitic regions, followed by the crystallization of intercumulus melt to form the nonpoikilitic regions. The nonpoikilitic region was subsequently formed at shallower depths under progressively higher oxidizing conditions and lower temperatures (Howarth et al., 2014; Zemeny et al., 2017). Given that olivine in NWA 7397 is more ferroan than in other poikilitic shergottites, it was located at a shallower depth (Yoshida et al., 2016). The pyroxene oikocrysts of the poikilitic region in NWA 7397 are calculated to have formed at a depth corresponding to ~10 kbar. Rahib et al. (2017) conducted a crystal size distribution analyses for a selection of poikilitic shergottites. From the results (based on growth rates and magma residence times) they inferred that NWA 7397 and NWA 4468 likely crystallized at shallower depths than the other enriched samples if they all derive from a common magmatic unit. Because enriched and depleted basaltic (extrusive) shergottites have similar young crystallization ages as enriched and depleted poikilitic (intrusive) shergottites, it is possible that they formed from the same respective evolving parental source reservoir. Further evidence of a genetic link between these basaltic and poikilitic (and olivine-phyric) shergottites could be revealed through future calculations of their ejection ages.

Rahib et al. (2019) conducted an in-depth study of a comprehensive sampling of shergottites representing different subgroups: poikilitic, gabbroic, basaltic, and olivine-phyric. Based on expansive information including mineralogical, isotopic, and elemental data, crystallization and CRE ages, redox and temperature conditions, and crystal size distribution, spatial distribution pattern, and phosphorus zonation pattern analyses, a model for the formation and emplacement of shergottites was proposed. First, minerals with a poikilitic texture (pyroxene oikocrysts enclosing olivine and chromite chadacrysts) accumulated in ponded magma near the base of the crust. This magmatism involved multiple volcanic systems on Mars, including both geochemically enriched and intermediate source regions. It was demonstrated by Miura and Liang (2020) that poikilitic textures only occurred when the magma remained at an intermediate temperature of ~1200°C within the staging chamber for at least a few days prior to ascent. Thereafter, magma ascended towards the surface while crystallization of sequentially more evolved interstitial melts proceeded; textures of these late-stage phases are manifest as non-poikilitic. This stage occurred under more oxidizing conditions, likely due to auto-oxidation, degassing, and/or assimilation of oxidized crust (Combs et al., 2019).

Potassium-rich metasomatic fluids were incorporated during magma ascent as identified in NWA 7397, NWA 10169, and LAR 06319. The crystallized rock was ultimately emplaced as both shallow intrusive sills (poikilitic and gabbroic shergottites) and extrusive lavas (basaltic and olivine-phyric shergottites). Based on this study, Rahib et al. (2019) inferred that basaltic, olivine-phyric, and poikilitic shergottites are petrogenetically linked to a few common source regions on Mars, and that samples from each of these volcanic complexes were ejected in at least two distinct impact events as demonstrated by CRE age data (see schematic illustrations below).

Schematic of the Emplacement of Enriched and Intermediate Shergottites
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Diagram credit: Rahib et al., GCA, vol. 266, p. 489 (2019, open access link)
'Mantle source to near-surface emplacement of enriched and intermediate poikilitic shergottites in Mars'

Schematic of the Emplacement of Enriched Shergottites
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Diagram credit: Combs et al., GCA, vol. 266, p. 458 (2019, open access link)
'Petrology of the enriched poikilitic shergottite Northwest Africa 10169: Insight into the martian interior'

While most all poikilitic shergottites studied thus far are depleted in REEs, Northwest Africa 7397 and three others are enriched in REEs (NWA 4468, NWA 10618, and RBT 04261/2). Northwest Africa 7397 shows many chemical and mineralogical similarities with these enriched poikilitic shergottites and shows differences with depleted members (He and Xiao, 2014). A two-stage formation model has been proposed by Howarth et al. (2014) for these enriched samples. The first stage is similar to that of all poikilitic shergottites—crystallization of a parental source melt and crystal accumulation at the bottom of a magma chamber to form a poikilitic lithology. However, in these few enriched shergottites, the second stage occurred later at a shallower depth. It is proposed that a succeeding flow of magma entrained former olivine and pyroxene cumulate crystals from the initial magma chamber and rose toward the surface, crystallizing the nonpoikilitic lithology. The differences in formation temperature and oxygen fugacity (oxygen partial pressure) between the poikilitic and nonpoikilitic lithologies calculated for NWA 7397 and the other enriched members are consistent with this scenario (He and Xiao, 2014). Based on their textural similarities, both enriched and depleted poikilitic shergottites are believed to have formed through similar processes, but to have derived from isotopically distinct LREE-enriched and -depleted parental source reservoirs. Howarth et al. (2014) conclude that the alternative scenario of enrichment through assimilation of a crustal component by a depleted source magma is not consistent with their research.

In an effort to rectify the discrepancies 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)
NWA 7397
NWA 10169
NWA 10618
NWA 10808
RBT 04261/2
NWA 2646
NWA 11065
NWA 12241/837
NWA 13250/76
ALH 77005
GRV 99027
LEW 88516
NWA 4797
NWA 6342
NWA 10697
NWA 10961
NWA 11261
DEPLETED Asuka 12325*
(Mg# 0.42, CaO 4.1 wt%)

After Irving et al. (2010, see diagram), Walton et al. (2012), and Dr. Anthony Irving's List of Martian Meteorites
*Asuka 12325 data from Debaille et al., 2019, 2022; Takenouchi et al., 2019, 2020

Cosmic ray exposure ages have now been determined for many martian meteorites, and Mahajan (2015 #1166) 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 7397 was launched from Mars during the 2.12 m.y.-old impact event. A noble gas study was conducted by Wieler et al. (2016) for 18 martian meteorites including NWA 7397, which they determined has an average CRE age (3He, 21Ne, 38Ar) of ~4.3 m.y. 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 7397 and several other enriched poikilitic shergottites (e.g., NWA 4468, NWA 10618, RBT 04261/62) were ejected 3–4 m.y. ago in a common impact event unique from the others.

The Sm–Nd isochron age for NWA 7397 was calculated by Ferdous and Brandon (2020) to be 182 (±28) m.y. In addition, they found this crystallization age to be similar to that of other enriched shergottites (150–230 m.y., see diagram below). Moreover, they recognized that enriched poikilitic NWA 7397 has a similar CRE age to some intermediate poikilitic shergottites (e.g., LEW 88516, Y-000097, Y-793605).

Sm–Nd Crystallization Age vs. Ejection Age
purple = enriched; green = intermediate; yellow = depleted
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Diagram credit: Ferdous and Brandon, 51st LPSC, #1064 (2020)

More detailed information about the petrogenetic and classification history of the poikilitic shergottites can be found on the NWA 1950 page. The specimen of NWA 7397 shown above is a 0.5 g partial slice with a small amount of fusion crust. The top photo below shows the cut face of the main mass, while the bottom photo is a close-up view of the poikilitic texture of NWA 7397, shown courtesy of Darryl Pitt.

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Photos courtesy of Darryl Pitt—The Backplate Collection

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