NORTHWEST AFRICA 4801


Angrite
Plutonic/Metamorphic
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Found May 2007
coordinates not recorded

Four fragments comprising a single 252 g individual stone meteorite was found in Algeria and purchased by G. Hupé in Erfoud, Morocco. This is a friable meteorite in which the fusion crust has been eroded away by prolonged terrestrial weathering processes, leaving only oxidation products on its surface. An analysis was conducted at the University of Washington in Seattle (A. Irving and S. Kuehner), and it was ascertained that NWA 4801 is a plutonic igneous cumulate angrite. .

This angrite has a metamorphosed texture being that it is a coarse-grained rock (0.1–1.2 mm) with 180° triple junctions (Irving and Kuehner, 2007). It is composed of a variety of multi-colored grains, primarily Al–Ti-rich clinopyroxene, and it contains a high abundance of white crystals and aggregates of pure anorthite. Other grains are composed of Cr-pleonaste, Ca-rich olivine, pleonaste, and merrillite. Minor troilite is present in association with FeNi-metal and small oxide grains (Riches et al., 2016). While kirschsteinite is present in most angrites, it has not been observed in this one. Notably, NWA 4801 has a greater abundance of merrillite than in most other angrites.

The crystallization age of NWA 4801 based on Pb isotopes is barely resolvable from that of the youngest angrite, Angra dos Reis (Amelin and Irving, 2007). This young age is also very close to that of the plutonic angrites LEW 86010 and NWA 4590; i.e., NWA 4801 is ~1.2 m.y. younger than LEW 86010. The crystallization/isotope closure age for NWA 4801 based on Mn–Cr systematics is 4.5643 (±0.0005) b.y. When anchored to the absolute Pb–Pb chronometer, which has now been accurately determined for NWA 4801 to be 4.558 (±0.013) b.y., these ages provide the best agreement between these two chronometers yet obtained for angrites (Shukolyukov et al., 2009). The Sm–Nd-based age is concordant with the Pb–Pb-based age and identical within errors to the angrite NWA 4590 (Sanborn et al., 2011). A Lu–Hf isochron was determined for NWA 4801 by Bouvier et al. (2015) to be 4.563 (±0.05) b.y. The time of the last mantle fractionation as determined by Mn–Cr and tied to the new NWA 4801 Pb–Pb anchor is consistent with the crystallization age of the oldest known angrites at 4.5646 (±0.0005) b.y.

With the steadily increasing number of unique angrite samples available to study, new models of their formation are emerging. In an abstract from the Workshop on Chronology of Meteorites 2007, A. Irving and S. Kuehner (UWS) conceive of a rapid progression of events on the angrite parent body following its accretion within ~2 m.y. after CAI formation. Immediately thereafter, the onset of internal heating by 26Al decay, along with significant impact heating (John T. Wasson, 2016), resulted in differentiation of the mantle and formation of a small core (core mass fraction of 0.08; Shirai et al., 2009). Subsequent to this, plutonic and volcanic magmatism occurred, along with metasomatism, metamorphism, and impact-generated regolith formation, all occurring within ~4–11 m.y. after CAIs.

In order to better constrain the properties of the differentiated angrite parent body core, van Westrenen et al. (2016) conducted a study modeling siderophile element depletions along with their metal–silicate partitioning behavior for the hypothesized angrite parental melt composition. A CV chondrite mantle composition was used for their calculations, along with a temperature and pressure (0.1 GPa) appropriate for a solidifying melt on a small planetesimal. Their results indicate that the observed siderophile element depletions of angrites are consistent with a core mass fraction of 0.12–0.29 composed of Fe and Ni in a ratio of ~80:20 (with a low S content), and that it was formed under redox conditions (oxygen fugacity) of ΔIW–1.5 (±0.45).

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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click on image for a magnified view
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)

In connection with their in-depth study of NWA 5363/5400, Burkhardt et al. (2017) published comparative data for nucleosynthetic anomalies among parent bodies for O, Cr, Ca, Ti, Ni, Mo, Ru and Nd. It is interesting to note that with the exception of ε48Ca (no angrite data for ε100Ru), NWA 5363/5400 and angrites have values for each of these isotopic anomalies which are nearly the same or overlap within uncertainties. Results of their studies indicate that while both angrites and NWA 5363/5400 have Δ17O values indistinguishable from Earth, and that other anomaly values for angrites overlap with Earth within uncertainties (ε92Ni, ε92Mo, ε145Nd), the ε54Cr and ε50Ti values of angrites are distinct from Earth. Based on their studies, Burkhardt et al. (2017) concluded that the parent body of NWA 5363/5400, and perhaps by extention that of angrites, originated in a unique nebula isotopic reservoir most similar to enstatite and ordinary chondrites.

Small fine-grained basalt clasts exhibiting textures and mineralogy generally consistent with a quenched angrite-like impactor are preserved in impact melt glass fragments recovered from a significant impact event that occurred ~5.28 m.y. ago near Bahía Blanca, Argentina (Schultz et al., 2006; Harris and Schultz, 2009, 2017; see photo below). This impactor is considered to have been very large, perhaps at least one km³, and its source object could plausibly reside near the Earth–Moon system. Interestingly, analyses of other grains obtained from Bahía Blanca impact melt glass have a geochemistry similar to the Moon (Harris and Schultz, 2017).

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Photo credit (left): Schultz et al., MAPS, vol. 41, #5, p. 755 (2006) (http://dx.doi.org/10.1111/j.1945-5100.2006.tb00990.x)
Diagram credit (right): Harris and Schultz, 40th LPSC, #2453 (2009)

The CRE age calculated for NWA 4801 is 31.6 (±1.5) m.y. (Nakashima et al., 2008). It was concluded that the wide range of CRE ages determined for the angrites—0.6–73 m.y. for eleven angrites measured—indicates that the APB was/is a large body which has experienced recurrent episodes of impact and dissemination of the crust over a very long period of time, and suggests that the parent object resides in a stable orbit (planetary or asteroid belt) permitting continuous sampling over at least the past 73 m.y.

Although NWA 4801 has been remagnetized by hand magnets, a study by Weiss et al. (2008) of remanent magnetism in angrites revealed that a magnetic field with a strength of ~10 µT, ~20% of that of present-day Earth, was imparted to the angrite PB during its earliest phase of crystallization (as observed particularly from the angrite D'Orbigny). This magnetic field may be attributable to a number of possible causes, e.g., accretion to an orbit in close proximity to the early T-Tauri phase solar field, or perhaps more plausible, a magnetic field generated through an internal core-dynamo mechanism.

The number of unique angrites represented in our collections today is limited, and they have been grouped as basaltic/quenched, sub-volcanic/metamorphic, or plutonic/metamorphic, along with a single dunitic sample in NWA 8535 (photo courtesy of Habib Naji). In a recent study based on a comparison of Hf/Sm ratios for a diverse sampling of both angrites and eucrites, Bouvier et al. (2015) inferred that these two meteorite groups reflect the existence of three distinct crustal reservoirs on their respective parent bodies. These three reservoirs reflect similar chemical differentiation processes on both parent bodies: 1) subchondritic Hf/Sm ratios for the Angra dos Reis angrite and the cumulate eucrites (such as Moama); 2) chondritic Hf/Sm ratios for the quenched angrites (such as D'Orbigny and Sahara 99555) and the basaltic eucrites; 3) superchondritic Hf/Sm ratios for the plutonic angrites (NWA 4590 and NWA 4801) and the unusual cumulate eucrite Binda. The unique metamorphic NWA 2999 pairing group was not included in the Bouvier et al. (2015) study. The specimen of NWA 4801 shown above is a 0.98 g partial slice. The photo below is an excellent petrographic thin section micrograph of NWA 4801, shown courtesy of Peter Marmet.

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click on photo for a magnified view
Photo courtesy of Peter Marmet