NORTHWEST AFRICA 1459


Diogenite
Harzburgite
(>40 vol% olivine)

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click on photo for a magnified view

Purchased 2002
no coordinates recorded

A single 49 g stone with weathered fusion crust was found near Lake Iriqui, Morocco, and was subsequently purchased through a Moroccan dealer in March 2002 by A. and G. Hupé. Analysis of this meteorite was conducted at the University of Washington in Seattle (A. Irving and S. Kuehner). Oxygen isotope data indicate that NWA 1459 originated on the HED parent body, and it was classified as a rare olivine-rich diogenite.

This meteorite has a coarse-grained texture and a dark yellow-green color, and it exhibits minor staining caused by terrestrial weathering products such as iron oxides/hydroxides. It contains an unusual abundance of chromite grains, the presence of which is associated with high temperatures. Some of the chromite grains protrude across the crusted surface, selectively exposed and polished by atmospheric ablation processes. Minor constituents that are present include anorthitic plagioclase, clinopyroxene, troilite, and Ni-free (0.06%) Fe-metal.

At the time of its classification, the olivine diogenites included only a few like members, some of which had been previously found in Antarctica. The olivine-rich GRA 98108 has an orthopyroxene content of 70% (Mg# = 69–79) and an olivine content of 30% (Mg# = 73), along with minor plagioclase, chromite, FeNi-metal, and troilite (Righter, 2001). Another unusually olivine-rich diogenite is the unbrecciated ALHA77256. In addition, the genomict breccia EETA79002 contains a large amount of olivine (Mg# = ~76) in at least one lithology, mainly associated with highly magnesian orthopyroxene grains (Mittlefehldt, 2000). As with the other olivine diogenite samples, this highly magnesian orthopyroxene lithology is thought to have originated from a harzburgitic (olivine + orthopyroxene) pluton. However, olivine in this diogenite is not in equilibrium with some other orthopyroxene lithologies that are present, suggesting that this is a genomict breccia consisting of a mixture of harzburgite and more common Fe-rich orthopyroxene (orthopyroxenite) clasts (Beck and McSween Jr., 2009).

Northwest Africa 1459 is classified as a harzburgitic peridotite rock, with a higher olivine (~40 vol%) and Fe content than that found in all previously analyzed olivine-rich diogenites. In 2003, a fifth member of this olivine-rich diogenite group was found, NWA 1877 and pairings, which contains ~45 vol% olivine with no plagioclase, suggesting that it formed at a greater depth than did NWA 1459. This meteorite, along with NWA 1459, GRA 98108, ALHA 77256, and EETA 79002, provided the requisite number required to form a new group, which was proposed to be called the olivine diogenite group—a term generally associated with those diogenites containing at least 5 vol% olivine (A. Irving et al., 2005). For those diogenites with between 1 vol% and 5 vol% olivine the term "olivine-bearing" is most commonly used. Even more recently, the classification of several new olivine diogenites was published; namely, NWA 5405 (~10 vol% olivine), NWA 5312 (24 vol% olivine), MIL 07001 (>40 vol% olivine), NWA 4223 (50 vol% olivine), and NWA 5480 (57 vol% olivine). 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.

It is noteworthy that O-isotope 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 may have actually originated on a distinct planetary body such as that of the main-group pallasites (Shearer et al., 2008; Shearer et al., 2010).

Northwest Africa 1459 may be different from the other olivine-rich diogenites in being a recrystallized cumulate (Irving et al., 2003). Some have suggested a formation of olivine diogenites from basalt depleted source regions—melts from which eucrites had previously been derived. However, due to the minor and trace element overlaps observed among the diogenite types, they may in fact form a continuum of orthopyroxene accumulation within a fractionated magma ocean or layered intrusion emplaced into the crust. They may represent distinct magma sources rather than originating in a separate mantle setting (Shearer et al., 2007, 2010). It was argued that only the largest impact event could excavate olivine-rich mantle material on Vesta, and this is exemplified by the central uplift within the Southern Lowlands crater where even core material may have been exposed (Delaney, 2009). This tremendous impact event is estimated to have displaced 10–20% of the mass of the asteroid. Notably, Beck et al. (2012) identified the first olivine-rich melt material 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.

Importantly, the dunitic, Vesta-related meteorite NWA 2968 (>95 vol% olivine) may be the deepest sampled lithology known from the HED parent asteroid, widely thought to be 4 Vesta and its derivitive Vestoids. That said, 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 break-up 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.

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 above shows the cut side of a 0.42 g partial end section of NWA 1459. The photo of the main mass shown below is courtesy of A. and G. Hupé.

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NORTHWEST AFRICA 1459
Photo courtesy of A. and G. Hupé