NWA 2975 Mars Shergottite Meteorite

Official Classification by Dr. Ted Bunch of Northern Arizona University and Dr. Tony Irving of the University of Washington, below:

Northwest Africa 2975

Algeria
Find: 2005
Achondrite (Martian, basaltic shergottite)

History: A minimally weathered fully encrusted whole stone of 70.1 g was purchased in Erfoud, Morocco, by M. Farmer in November 2005.

Petrography: (T. Bunch and J. Wittke, NAU; A. Irving, UWS) A medium-grained (<3.1 mm greatest dimension) basaltic shergottite that consists of ~57.3 vol% augite and pigeonite pyroxenes, and 38.3 vol% plagioclase (present as shock-formed maskelynite and glasses) with minor opaques (2.7 vol%) and phosphates (1.7 vol%) arranged in a weakly foliated subophitic to granular texture. Accessory phases include ulvöspinel, ilmenite, chlorapatite, merrillite, pyrrhotite, Si-Al-Na-K-rich glasses, and baddeleyite. Vesicular black glass veins (<3 mm in width) and pockets (up to 6 mm) are prominent.

Geochemistry: Pigeonite (Fs_35.2–57.6Wo_12.6–16.5; FeO/MnO = 28–38) and augite (Fs_27.2–41.5Wo_30.8–35.2) show mottled compositional zoning. Fe-rich margins of pigeonite contain very thin (0.2–0.5 µm) orthopyroxene exsolution lamellae. Maskelynite is compositionally homogeneous An₅₅Or_1.8 in contrast to mesostasis maskelynite (An_48–60Or_9.2) and glasses. Melt inclusions (15–60 µm, longest dimension) in ulvöspinel have rims of Fe-rich pigeonite (Fs_73.5Wo_5.8), merrillite, and pyrrhotite; cores are Si-Al-K-Na-rich glass. Bulk composition: Calculated average bulk composition based on analyzed phases and their modes of three melt inclusions is: SiO₂ = 70.2, Al₂O₃ = 8.5, TiO₂ = 0.8, FeO = 2.1, MnO = 0.2, MgO = 2.35, CaO = 3.5, Na₂O = 0.95, K₂O = 2. 1, and P₂O₅ = 2.8 (all wt%) with trace amounts of NiO, CoO, and S. One melt inclusion contains only fayalite (Fa₈₄) mantled around a core of glass (K₂O = 6.4, SiO₂ = 78; both wt%).

Classification: Achondrite (Martian basaltic shergottite); moderate to high shock, minimal weathering.

Specimens: A 20.2 g type specimen and one thin section are on deposit at NAU. An anonymous owner holds the main mass.

Ted's quotes in recent emails to us:

"Simply gorgeous" it would be a shame to cut it.

Melt pockets of bubbly dark glass; pyroxene phenocrysts < 2 mm; plagioclase (maskelynite) ~one mm; sparse “spinel”; Very fresh with a weathering grade of only 0/1; shock 4-6.

This is a 2 pyroxene basaltic shergottite that lacks most of the strong compositional zoning of other basaltic shergottites. In addition, it appears to be blessed with a higher abundance of K-rich glasses and plagioclase (maskeynite) relative to most others.

The following PDF documents require an Acrobat Reader (it is free):

NORTHWEST AFRICA 2975: AN EVOLVED BASALTIC SHERGOTTITE WITH VESICULAR GLASS POCKETS AND TRAPPED MELT INCLUSIONS. J.H. Wittke, T. E. Bunch, A. J. Irving, M. Farmer, J. Strope, Dept. of Gelogy, Northern Arizona University, Flagstaff, AZ 86011, Dept. of Earth & Space Sciences, University of Washington, Seattle, WA 98195. Abstract submitted for presentation as poster at the LPSC in Houston in March, 2006.

The Mars Meteorite Compendium - Compiled by Charles Meyer for Astromaterials Research and Exploration Science (ARES)