NORTHWEST AFRICA 2646


Martian Shergottite
Poikilitic (formerly "lherzolitic" shergottite)
(intermediate, permafic, plagioclase–olivine–clinopyroxenite
or olivine–melagabbro/meladiabase)

standby for nwa 2646 photo standby for nwa 2646 photo
click on photos for a magnified view

Purchased December 2004
no coordinates recorded

A single stone reported to weigh 30.67 g was found in Algeria or Morocco and acquired by Moroccan dealer A. Habibi. A 9.3 g portion of the small meteorite was subsequently purchased by N. Oakes. A sample of the stone was submitted for analysis and classification (T. Bunch and J. Wittke, NAU; A. Irving and S. Kuehner, UWS), and it was determined that NWA 2646 is a unique type of rock from Mars—a plagioclase–olivine-clinopyroxenite (also olivine-gabbro/diabase or poikilitic shergottite).

This meteorite is related to the small martian poikilitic shergottite group, the members of which are generally considered to be plutonic, ultramafic, olivine–clinopyroxenites according to IUGS nomenclature. As is the case for the martian meteorites RBT 04262/261 and NWA 4468, NWA 2646 is a basaltic rock containing less olivine and more plagioclase (in the form of maskelynite) than those amounts required to conform to the IUGS definition for a terrestrial lherzolite. Each of these martian meteorites share some mineralogical characteristics with the poikilitic shergottite group (Walton et al., 2012), and they might have a genetic relationship. Northwest Africa 2646 in particular is related to the martian poikilitic shergottite group through geochemical properties such as trace element abundances and isotopic compositions. Moreover, its Rb–Sr age of 167 (±6) m.y. is within the range of ages of the poikilitic shergottite group, a number of olivine-free enriched basaltic shergottites, and at least one olivine-phyric shergottite (NWA 1068) (Shih et al., 2009).

Interestingly, the unique characteristics observed in NWA 2646, which are inconsistent with those of other established martian meteorite groupings, has led the editor of the Meteoritical Bulletin Database to recommend the use of the less-than-descriptive classification term "Martian". It has been revealed that NWA 2646 bears some close similarities to the Antarctic martian meteorites RBT 04262 and NWA 4468 (see formation information on the NWA 1950 page).

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 lherzolite-like shergottites, Mikouchi (2009) addressed the need for changing the commonly used term of "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 name with which to encompass all of the various martian "lherzolitic" shergottites that exist in the worldwide collections. This would include both intermediate and enriched "lherzolitic" shergottites as reflected by a geochemical classification scheme, as well as any depleted members that may be recovered in the future.

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).

POIKILITIC SHERGOTTITE COMPOSITIONS
BULK MAJOR ELEMENTS vs. TRACE ELEMENTS
(Mg/[Mg + Fe] vs. CaO)
    MAFIC PERMAFIC ULTRAMAFIC
HREE/LREE
    (La/Yb)
ENRICHED   RBT 04261/2
GRV 020090
NWA 4468
NWA 7397
NWA 6342
INTERMEDIATE   NWA 1950
NWA 2646
ALH 77005
Y-793605
LEW 88516
Y-984028/000027/47/97
Y-1075
GRV 99027
NWA 4797
DEPLETED      

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

A modal analysis of NWA 2646 revealed that it has a heterogeneous composition, consisting of the pyroxenes pigeonite (40.7 vol%) and augite (24.3 vol%) together with olivine (21.6 vol%) and maskelynite (11.4 vol%). Minor phases include chromite, ilmenite, merillite, and pyrrhotite. It contains a higher abundance of plagioclase in the form of maskelynite than other members of the martian poikilitic shergottite group with the exception of RBT 04262 and NWA 4468. Possible pre-terrestrial secondary hydrous alteration products are present, including calcite and chlorides.

The CRE age for NWA 2646 based on 10Be, an isotope that is produced by local irradiation of the proto-Sun, has been calculated to be 2.5–3.1 m.y. This is similar to several other martian meteorites including NWA 1068 and NWA 1460 (Nishiizumi and Caffee, 2006), and is also similar to the group of poikilitic shergottites (Walton and Herd, 2007). However, 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 dated at 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. before present—see his chart here. Although NWA 2646 was not included in his study group, the CRE age shown above is most consistent with the impact-ejection event 2.77 m.y. before present time. However, Weiler et al. (2016) conducted noble gas analyses on several meteorites including NWA 2646, with consideration of the shielding parameter (i.e., meteoroid size and sample depth) that affects the calculation of the CRE age. Employing their preferred model for determination of CRE age based on the mean of calculated 3He, 21Ne, and 38Ar production rates, they derived an age of 3.9 m.y. before present; this CRE age is more consistent with the impact event at 4.05 m.y. before present. 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 2646 and at least 14 other intermediate poikilitic shergottites were ejected in a common impact event unique from the others.

The specimen of NWA 2646 shown above is a 0.42 g fragment with a portion of the natural outer surface. The photos below show two views of the main mass, along with a thin section in cross-polarized light.

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Photos courtesy of Nelson Oakes