Initially, a single, partially fusion-crusted stone weighing 312 g was found in the Northwest Africa desert and purchased through a Moroccan dealer by A. and G. Hupé. The total weight of this material was recorded at 934 g. This meteorite was designated NWA 1877 and classified at the University of Washington in Seattle (A. Irving and S. Kuehner; see MetBull #88). A second, larger stone, weighing 642.3 g, was subsequently recovered and purchased through a Moroccan dealer by N. Oakes. This mass, designated NWA 2115, was analyzed at Northern Arizona University (T. Bunch and J. Wittke) and determined to be likely paired with NWA 1877 (see MetBull 95). The meteorite is moderately weathered to grade W2 and shows features of being weakly shocked to stage S3, with some portions (NWA 1877; Fagan et al., 2019) showing evidence of brecciation. Additional likely paired fragments have been assigned the names NWA 2286 and NWA 2629 (see MetBull 89).
Northwest Africa 2115 is a yellow-green harzburgitic peridotite composed primarily of orthopyroxene (up to 11 mm) and large phenocrysts of olivine (up to 18.5 mm) within a disrupted, cataclasized matrix. Minor amounts of chromite (mm-sized), troilite, and virtually Ni-free metal are present. It contains highly magnesian silicates similar to those in ALHA77256 and GRA 98108. When found, the NWA 1877/2115/2286/2629 masses constituted the fifth representative of the peridotitic olivine diogenite suite of meteorites, which included NWA 1459, GRA 98108, ALHA77256, and EETA79002. It was proposed that this new group, with members containing at least 5% olivine, be described as olivine diogenites (A. Irving et al., 2005). Since then, other representative olivine diogenites have been recovered, including NWA 5405 (~10 vol% olivine), NWA 5312 (24 vol% olivine), NWA 4223 (50 vol% olivine), NWA 5480 (57 vol% olivine), and MIL 07001. While it is hypothesized that olivine diogenites are cumulates constituting distinct layered magmatic intrusions emplaced into the crust, the unusual features exhibited in NWA 5480 suggest a possible origin as a mantle residue; however, analyses by Yamaguchi et al. (2013) led them to conclude that this diogenite is an impact melt rock formed at the bottom of a large basin such as Rheasilvia which formed ~1 b.y. ago overlapping the Veneneia basin which formed ~2.1 b.y. ago. The Rheasilvia basin is deeper than its predecessor at 2080 km and <25 km, respectively (Brasser et. al., 2019).
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). In a manner consistent with the IUGS nomenclature, the term harzburgitic diogenite has been proposed to describe the olivine diogenites.
A study of the paired meteorite NWA 1877 conducted by Irving et al. (2005, #2188) revealed that it contains ~47 vol% olivine with no plagioclase observed, and that it has a very Cr-rich chromite. Each of these features suggests that NWA 1877 (=NWA 2115) formed at great depth. The higher olivine abundance observed in some of the other olivine diogenites suggests that they formed in strata at even greater depth, with the features of NWA 5480 possibly indicating an origin as a mantle residue. However, due to the minor and trace element overlaps observed among the diogenite types, they each might in fact form a continuum of orthopyroxene accumulation within a fractionated magma ocean or layered intrusion emplaced into the crust. Alternatively, they could represent distinct parental magma sources (Shearer et al., 2007; 2010).
The dunitic meteorite NWA 2968 (>95 vol% olivine) may be the deepest sampled lithology from the HED parent asteroid, widely thought to be 4 Vesta and its derivative Vestoids. Notably, Beck et al. (2012) identified the first olivine-rich melt material in 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. It is envisaged by one investigative team that Vesta and the Vestoids, along with the isotopically similar mesosiderites and IIAB irons, were themselves the products of a breakup of an even larger parent object which they have named Opis (Irving et al., 2009). In Greek mythology, Opis was the wife of Saturn, whose children were Jupiter, Neptune, Pluto, Juno, Ceres, and Vesta.
Further information about this newly recognized group can be found on the NWA 1459 page. The 1.11 g specimen of NWA 2115 shown above is from the 642.3 g main mass, a photo of which is shown below.