NORTHWEST AFRICA 7397

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

standby for nwa 7397 photo
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. Like 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). Since 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. Since enriched and depleted basaltic (extrusive) shergottites have similar young crystallization ages as enriched and depleted poikilitic (intrusive) shergottites, it is considered they might have formed from the same respective evolving parental source reservoirs; 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.

While most all poikilitic shergottites studied thus far are depleted in REEs, Northwest Africa 7397 and three others are enriched in REEs (RBT 04261/2, GRV 020090, and NWA 4468). 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 suface, crystallizing the nonpoikilitic lithology. The differences in formation temperature and oxygen fugacity 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 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).

POIKILITIC SHERGOTTITE COMPOSITIONS
BULK MAJOR ELEMENTS vs. TRACE ELEMENTS
(Mg/[Mg + Fe] vs. CaO)
    MAFIC PERMAFIC ULTRAMAFIC
HREE/LREE
    (La/Yb)
ENRICHED   NWA 4468
NWA 7397
NWA 10618
RBT 04261/2
 
INTERMEDIATE   NWA 1950
NWA 2646
NWA 10169
NWA 11065
NWA 11214
ALH 77005
GRV 99027
LEW 88516
Y-793605
Y-984028
NWA 4797
NWA 6342
NWA 10697
NWA 11261
NWA 10961
DEPLETED      

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

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 7397 was launched from Mars during the 2.12 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 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.

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.

standby for nwa 7397 photo
standby for nwa 7397 photo
Photos courtesy of Darryl Pitt—The Backplate Collection