NORTHWEST AFRICA 2968


Diogenite
Dunite
(ungrouped achondrite in MetBull 91)

standby for northwest africa 2968 photo
Purchased November 2005
no coordinates recorded

Through the untiring efforts of nomads searching the vast Sahara Desert region of Algeria, a remarkable new meteorite type has been recovered. Only 268 g of small broken fragments of a coarse-grained, dark brown meteorite was found, and these fragments were subsequently sold in Erfoud, Morocco to meteorite dealer B. Reed. The small size range of these fragments (17 mm to at least 25 mm wide) is due to fracturing along compression and shear zones. Numerous pieces of this meteorite were submitted for analysis and classification (T. Bunch and J. Wittke, NAU; A. Irving, UWS; D. Rumble III, CIW).

Northwest Africa 2968 is a cumulate, olivine-rich (>95 vol%), dunitic rock, containing minor amounts of orthopyroxene and FeNi-metal, along with troilite and pyrrhotite which primarily fill fractures. Olivines exhibit shock features including domain offsets, mosaicism, and undulatory extinction. The FeO/MnO ratios and O-isotopic compositions of NWA 2968 are consistent with an origin from the howardite/eucrite/diogenite/ (HED) parent body, widely accepted to be the asteroid 4 Vesta. The averged Δ17O value of –0.23 (±0.02) plots within the HED group (Scott et al., 2009); see a linearized O-isotope plot (Miller, 2002). However, in contrast to the olivine and orthopyroxene in known howardites or diogenites, these minerals are considerably more highly magnesian in NWA 2968 (92.5 and 93, respectively), and likely crystallized from a less evolved parental melt. The composition and mineralogy of NWA 2968 is consistent with a mantle or lower crustal origin on a differentiated body, in a formation region analogous to that of the chassignites on Mars.

standby for greenwood diagram
Photo shown courtesy of Greenwood et al., 2015
For an explanation of the diagram components see the open access article in GCA, vol. 169, p. 130 (Nov 2015)

It is noteworthy that continuing studies of the Antarctic MIL 03443, a cumulate, monomict, brecciated dunite, which was previously classified as a mesosiderite clast, have developed strong evidence (e.g., FeO/MnO and Δ17O values, occurrence of olivine melt inclusions and pyrrhotite, Ni and Co abundances) for an origin on the HED parent body, and a relationship to diogenites specifically (Mittlefehldt, 2008; Beck et al., 2011). MIL 03443 has been shown to represent a fractional cumulate rather than a mantle restite (Beck et al., 2011). In a similar way, O-isotopic and trace element data for the unique 1.1 g olivine-rich (dunitic/harzburgitic?) achondrite QUE 93148 have led to the suggestion that it might be derived from the deep mantle of the HED parent body (Goodrich and Righter, 2000; C. Floss, 2003). However, due to its lower Co and Ni abundances than what would otherwise be expected for an olivine-rich mantle lithology or magma ocean cumulate, QUE 93148 could have actually originated on a distinct planetary body such as that of the main-group pallasites (Shearer et al., 2008; Shearer et al., 2010). Two other possible HED-related dunites, NWA 5784 and NWA 5968, will require further study to accurately access their classification. Notably, Beck et al. (2012) identified the first olivine-rich melt material, present in the howardites that constitute the PCA 02009 pairing group. This olivine-rich material was likely derived from harzburgitic and dunitic lithologies exposed on the surface of Vesta.

For further analysis of this dunitic meteorite and its potential pairing relationships, see the NWA 3329 page. An alternative classification system for the diogenites and dunites based on mineralogical and petrographical features has been proposed by Beck and McSween (2010), and modified by Wittke et al. (2011). The photo shown above is a 5.8 g fragment of NWA 2968.