(Ungrouped achondrite in MetBull 91)
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 averaged Δ17O value of 0.23 (±0.02) plots within the field of 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.
Diagram credit: Greenwood et al., 2015
For an explanation of the diagram components see the open accessarticle in GCA, vol. 169, p. 130 (2015)
'Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection'
Diagram credit: Greenwood et al., 2017
For an explanation of the diagram components see the open accessarticle in Chemie der Erde - Geochemistry, vol. 77, p. 25 (2017)
'Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies'
It is still unresolved whether such dunitic lithologies represent higher-level cumulates or if they are instead mantle material (Greenwood et al., 2015). Mandler and Elkins-Tanton (2013) proposed a formation scenario for such dunites that involves a two-stage crystallization process: first, an equilibrium crystallization process from the late-stage liquid after 6070% solidification of the global magma ocean; second, a fractional crystallization process within an ascended, high-level (crustal) pluton composed of the former extracted residual melt, ultimately resulting in the formation of a thin lower-crustal dunite layer along with more shallow olivine diogenite, diogenite, and cumulate eucrite lithologies. On the other hand, if the dunitic clasts are actually derived from mantle material, a scenario is required to explain how such material was incorporated into the regolith. However, it was argued by Barrat and Yamaguchi (2014) that magma chamber processes are unable to explain the chemical diversity of the diogenites (e.g., the range of heavy-REE ratios in diogenitic orthopyroxenes), and that neither assimilation of wallrock nor incorporation of a trapped melt component can account for this diversity. They contend that the diversity is more likely the result of variability in the respective initial parental melt compositions.
It is noteworthy that through continuing studies of MIL 03443, which is a cumulate, monomict, brecciated dunite previously classified as a mesosiderite clast, strong evidence has been developed for an origin on the HED parent body, and a relationship to diogenites specifically (Mittlefehldt, 2008; Beck et al., 2011). This evidence includes FeO/MnO and Δ17O values (see plot from Greenwood et al., 2015), the occurrence of olivine melt inclusions, and the abundances of pyrrhotite, Ni and Co. 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 assess their classification.
Notably, Beck et al. (2012) identified the first olivine-rich melt material present in the howardites of the PCA 02009 pairing group. This olivine-rich material was likely derived from harzburgitic and dunitic lithologies exposed on the surface of Vesta. Further investigation employing the Antarctic DOM 10 howardite pairing group was conducted by Hahn et al. (2018). They sought to identify Mg-rich harzburgitic (distinguished from diogenitic) silicates (Mg# >80 and >85 for olivine and pyroxene, respectively) that represent HED mantle material. From results of a comprehensive geochemical analysis, they contend that these Mg-rich fragments are not related to cumulate diogenites, but instead are more consistent with a mantle residue that was affected by a late infiltration of metasomatic melt. In addition, they determined that QUE 93148 also likely represents a mantle residue from the HED parent body. The larger degree of partial melting (~3555%) required to produce the observed Mg-rich lithologies, considered to be mantle residua, is attributed by Hahn et al. (2018) to a hybrid magma ocean model that combines aspects of the magma ocean model of Mandler and Elkins-Tanton (2013) and the shallow magma ocean model of Neumann et al. (2014) (see diagrams B and D below).
click on diagrams for a magnified view
Diagram credit: Hahn et al., MAPS, vol. 53, #3, p. 541 (2018)
'Mg-rich harzburgites from Vesta: Mantle residua or cumulates from planetary differentiation?'
For further information about this dunitic meteorite and its potential pairing relationships see the NWA 3329 page. An alternative classification system for the diogenites 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.