NORTHWEST AFRICA 10463

Angrite
Sub-volcanic/Metamorphic

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Found 2015
no coordinates recorded

A 202.5 g friable stone lacking fusion crust but with adhering desert soil was found in Morocco and ultimately sold to D. Pitt. Analysis and classification was conducted at the University of New Mexico (Agee et al.), and NWA 10463 was determined to be a previously unsampled angrite type. A number of smaller paired stones and tiny fragments were recovered during further searches, including a 14 g stone classified as NWA 10646.

Northwest Africa 10463 is a coarse-grained polycrystalline aggregate composed of Al–Ti-rich clinopyroxene (~28 vol%), both high-Fe and high-Ca olivine (~26 vol%), and plagioclase (~37 vol%), along with minor phases including oxides (3 vol% spinel and Fe–Ti-oxides), troilite (3 vol%), and silico-phosphate (<1 vol%), the latter resolved by Mikouchi et al. (2011) to be silico-apatite in other angrites (Agee et al., 2015; Santos et al., 2016). The meteorite exhibits textural features (e.g., chemically zoned olivines (Fa41.6 cores to Fa59.1 rims), thin exsolution lamellae in olivine) indicative of relatively fast cooling at depth, in many ways similar to the angrites NWA 4590 and LEW 86010 which have been termed sub-volcanic/metamorphic (McKay et al., 1998).

The chemistry of NWA 10463 indicates that post-crystallization metamorphic processing (i.e., re-equilibration) occurred; this is attested by several petrographic features including the fractionation and redistribution of divalent and trivalent 53Cr from olivine into other phases (Papike et al., 2016). This metamorphism has affected the Mn–Cr chronometer, reducing its usefulness in dating the crystallization of angrites. Santos et al. (2016) found that olivine in NWA 10463 contains Ca that spans a larger range than in other angrites, and they suggest that the meteorite could have experienced a unique petrogenetic history (see diagram below).

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Olivine composition system for fayalite (Fe2SiO4)–forsterite (Mg2SiO4)–larnite (Ca2SiO4)
Diagram credit: Santos et al., 47th LPSC, #2590 (2016)

In-depth studies of the diverse angrite samples collected thus far are bringing to light a scenario in which a large planetary body accreted and crystallized over an extended period of time, perhaps as long as 7 m.y., beginning only a couple of m.y. after the formation of the earliest nebular condensates. The refractory bulk composition of this body, along with features such as a high abundance of trapped solar noble gases, attests to an origin in close proximity to the Sun. The oldest angritic material is recognized in the form of early crustal vesicular rocks such as Sahara 99555, D'Orbigny, and NWA 1296. Younger angritic material, in the form of impact-mixed extrusive and intrusive magmatic rocks together with regolith material, is represented by A-881371, LEW 87051, and NWA 1670. The youngest angritic rocks known, represented by the meteorites Angra dos Reis, LEW 86010, NWA 2999, NWA 4590, and NWA 4801, are composed of annealed regolith and late intrusive plutonic lithologies.

It was proposed by A. Irving and S. Kuehner (2007) that one or more severe collisional impacts onto the angrite parent body resulted in the stripping of a significant fraction of its crust and upper mantle, accompanied by dissemination of large sections of this material into a stable orbit for the past 4+ b.y. This location may lie within the main asteroid belt, or may possibly remain associated with the original collisionally-stripped parent body, postulated by some to be the planet Mercury (see schematic diagram below). The disparity in FeO content that exists between the angrite group of meteorites (up to 25 wt%) and that which is observed on the surface of Mercury (~5 wt%) may reflect the existence of a redox gradient in which the lower mantle region, now the present surface of Mercury, has a more magnesian composition.

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Diagram credit: A. Irving and S. Kuehner, Workshop on Chronology of Meteorites, #4050 (2007)

While this angrite could be a piece of "Maia", mother of Hermes (Mercury), an alternate hypothesis speculates that it might represent a piece of "Theia", mother of Selene (the Moon goddess). In a new study of the Fe/Mn ratio in olivine grains for a number of angrites, Papike et al. (2017) determined that these meteorites plot along a trend line between the Earth and Moon, which indicates the possible location of the angrite parent body (see diagram below).

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Diagram credit: Papike et al., 48th LPSC, #2688 (2017)

The specimen of NWA 10463 shown above is a small 1.7 g individual exhibiting a coarse-grained granular texture. The photo below shows this 1.7 g specimen along with a small group of individuals weighing 3.3, 6, 11.7, 14, and 15 g, shown courtesy of Habib-naji Naji‎.

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