A fragmented stone weighing 252 g was collected in Algeria from a similar location as NWA 2968. The fragments were subsequently purchased in Er-Rachidia, Morocco by collector F. Kuntz. A sample of these fragments was analyzed at the University of Washington at Seattle (A. Irving and S. Kuehner) and NWA 3329 was determined to be a diogenite composed primarily of coarse-grained, dark brown orthopyroxene, together with interstitial plagioclase, silica, phosphate, FeNi-metal, and FeS.
The NWA 3329 fragments were subsequently studied by Barrat et al. (2010). Interestingly, some fragments from the batch were found to be identical to the dunitic diogenite NWA 2968, and petrographic evidence indicates that both lithologies were collected from the same location, with both showing similar degrees of weathering. Importantly, other recovered fragments consist of both lithologies togetherdiogenite and dunitewhich are each identical to their respective type samples. Furthermore, trace element studies conducted on both lithologies were found to be consistent with pairing, and their Δ17O values are indistinguishable as well (Greenwood et al., 2015). These investigators, in accord with an earlier suggestion by Barrat et al. (2010), interpret the evidence to indicate that both the orthopyroxenite and dunite lithologies are fragments from a common fall, likely as components of a mesosiderite. It was also noted by Greenwood et al. (2015) that the REE pattern previously determined for the diogenite NWA 5613 (Barrat et al., 2010) is virtually identical to that for NWA 3329. Further evidence for a mesosideriteHED genetic relationship is revealed by the identical Δ17O values among the diogenites, such as NWA 3329 and NWA 2968, and the olivine-rich (dunite) clasts that have been identified in most all mesosiderites (Greenwood et al., 2015, 2017). It is considered likely that the dunitic clasts in mesosiderites were initially formed as upper-crustal plutonic cumulates, which were subsequently disrupted through impacts and incorporated with other HED lithologies prior to the formation of the mesosiderites.
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'
A comparison of reflectance spectra of seven near-Earth asteroids to those of HED-group meteorites revealed that all of the pyroxene mineralogies were consistent with eucrites and howardites, but not to diogenites. Therefore, they suggest that there are no km-sized or larger objects composed strictly of diogenite material, but instead, diogenites might exist as a single component within a mixture of lithologies on the HED asteroid. 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 image 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?'
Further information regarding the origin of the dunitic clasts in our collections can be found on the Vaca Muerta page. To see an alternative classification system for the diogenites and dunites based on mineralogical and petrographical features, proposed by Beck and McSween (2010) and modified by Wittke et al. (2011), click here. The photo shown above is a 0.52g fragment of NWA 3329. The photo below is an excellent petrographic thin section micrograph of NWA 3329, shown courtesy of Peter Marmet.
click on image for a magnified view
Photo courtesy of Peter Marmet