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NWA 10463

Angrite
Sub-volcanic/Metamorphic
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click on photos for a magnified view
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 lithology. 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). This angrite exhibits textural features, including chemically zoned olivines (Fa41.6 cores to Fa59.1 rims), thin exsolution lamellae in olivine, and chemically zoned spinels (Al-rich cores to Cr-rich rims; Santos et al., 2017), which are indicative of relatively fast cooling at depth. In many ways these features are 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 which is manifest in 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 during the past 4+ b.y. This storage location might lie within the main asteroid belt, or alternatively, the material could remain associated with the original collisionally-stripped parent body postulated by some to be the planet Mercury (see schematic diagram below). The disparity that exists in FeO content between the angrite group of meteorites (up to 25 wt%) and 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)
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. A noteworthy study involving nucleosynthetic vanadium isotopes was conducted by Nielsen et al. (2019). They ascertained that the V isotope composition of Earth is significantly heavier than that of any other meteorite group they investigated, attesting to the fact that Earth accreted primarily from a unique non-carbonaceous reservoir not otherwise represented in our meteorite collections.
Vanadium vs. Chromium Isotope Diagram
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Diagram credit: Nielsen et al., EPSL, vol. 505, p. 137 (2019)
‘Nucleosynthetic vanadium isotope heterogeneity of the early solar system recorded in chondritic meteorites’
(https://doi.org/10.1016/j.epsl.2018.10.029)

Portions of the angrite asteroid must be in a stable orbit (planetary or asteroid belt) from which spallation has continued to occur over the past ~56 m.y. as indicated by the broad range in angrite CRE ages. Notably, Rivkin et al. (2007) have determined that the largest known co-orbiting ‘Trojan’ asteroid of Mars, the 1.3 km-diameter 5261 Eureka located at a trailing Lagrangian point, is a potentially good spectral analog to the angrites (as measured by Burbine et al., 2006) (see diagrams below). They suggest that 5261 Eureka could represent a captured fragment of the disrupted angrite parent body now in a stable orbit around Mars.
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Diagrams credit: Rivkin et al., Icarus, vol. 192, #2, (2007)
‘Composition of the L5 Mars Trojans: Neighbors, not siblings’
(https://doi.org/10.1016/j.icarus.2007.06.026; open access link)

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–Ž.
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NWA 4590

Angrite
Sub-volcanic/Metamorphic
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Found June 2006
30° 19.025′ N., 4° 56.573′ W. Numerous fragments with a combined weight of 212.8 g and constituting a single, very fresh, friable, fusion-crusted stone were found 21 km south-southwest of Tamassint oasis and 18 km south of Agoult, Morocco near the Morocco–Algeria border. The fragments were purchased by G. Hupé and an analysis was conducted at the University of Washington in Seattle (A. Irving and S. Kuehner). Northwest Africa 4590 has been determined to be a previously unsampled angrite type. This classification was verified by an O-isotopic analysis completed at Carnegie Institute in Washington D.C. (D. Rumble III).

Northwest Africa 4590 exhibits textural and mineralogical features that are unique from most other angrites, and it is considered to be neither plutonic/metamorphic nor basaltic/quenched (McKibbin et al., 2015). Instead, it is more similar to the LEW 86010 angrite, with mineralogical features (e.g., chemically zoned pyroxenes, exsolution lamellae in olivine) indicative of relatively fast cooling over a period of a few thousand years at a depth of ~240 m; a lithology formed in such an environment is more appropriately termed sub-volcanic rather than plutonic. By comparison, it was ascertained that LEW 86010 is also a sub-volcanic angrite, cooling over a period of ~10–250 years at a depth of <100 m (McKay et al., 1998).

Northwest Africa 4590 is a coarse-grained, cumulate rock composed of zoned, purple-brown, Al–Ti-rich clinopyroxene (33%), white intercumulus anorthite (28%), yellow-green Ca-rich olivine (14%), kirschsteinite (5%), and black ulvöspinel (18%), along with minor merrillite and Cl-bearing Ca silico-phosphate (now resolved to be silico-apatite; Mikouchi et al., 2011). In addition, troilite is present typically associated with FeNi-metal and small oxide grains (Riches et al., 2016). Glass having a compositional range between anorthite and ulvöspinel is present at the grain boundaries, and it incorporates re-precipitated primary minerals. This glass is thought to have formed by a rapid melting and cooling process, perhaps during a late impact event, or possibly through a decompression event such as that occurring from the collisional stripping of the lithosphere on a large planet. This event might be recorded in NWA 4590 and two other angrites as a disturbance of the Mn–Cr and U–Pb chronometers at ~4.5572 b.y. (McKibbin et al., 2015). Timescales for Angrites
standby for angrite ages diagram The first-time discovery of the rare mineral rhönite in an angrite has been made in a sample of NWA 4590 at the University of Washington, Seattle (Kuehner and Irving, 2007). The rhönite mineral is associated with ferric iron in NWA 4590, which, when taken together with other Fe-metal–oxide associations present in some angrites, is indicative of an oxidizing environment during their formation, possibly accompanied by metasomatic processes. Rhönite forms where low silica is present in Ca- and Ti-rich melts (Jambon and Boudouma, 2011). It is commonly associated with terrestrial alkalic rocks.

Northwest Africa 4590 is a member of the younger, slowly-cooled angrites that crystallized after 60Fe was extinct. Amelin et al. (2011) calculated a Pb–Pb-based crystallization age of 4.55886 (±0.0003) b.y. (pyroxene isochron) and 4.557381 (±0.00023) b.y. (silico-apatite isochron), while Amelin and Sapah (2012) report 4.55728 (±0.00016) b.y. (merrillite isochron). When taken together, these isochrons establish a relatively rapid cooling rate of a minimim of 450–550 K/m.y., and probably 700–850 K/m.y. for NWA 4590 (though less than the probable cooling rate of >1000 K/m.y. for Angra dos Reis). These cooling rates are up to 100× faster than the cooling rate of the H chondrite parent body.

Furthermore, the Sm–Nd-based crystallization age for NWA 4590 is concordant with the Pb–Pb-based age (Sanborn et al., 2011), while a Hf–W isochron for NWA 4590 calculated by Kleine et al. (2008) resulted in a slightly older age of 4.5591 (±0.0006) b.y. These Pb–Pb ages are also identical within error margins to that of the plutonic angrite LEW 86010, and are very close to two other plutonic angrites, Angra dos Reis (pyroxene: 4.55651 [±0.00011] b.y.; merrillite: 4.55657 [±0.00071] b.y.) and NWA 4801. Still, these ages are relatively young compared to most other angrites, some having ages as old as 4.564 m.y. From radiometric age data it can be inferred that basalt extrusion on the angrite parent body occurred over an extended period of time, between ~4 m.y. and ~10 m.y. after CAI formation (Nyquist et al, 2009).

It was shown by Sanborn and Wadhwa (2009) that both NWA 4590 and LEW 86010 were derived from parental source melts having almost identical compositions, and that they also experienced the same thermal histories involving fast cooling. Still, cooling rate estimates indicate that NWA 4590 cooled ~10 times slower than LEW 86010, corresponding to a greater depth of ~240 m compared to <100 m calculated for LEW 86010 (McKibbin et al., 2015 and references therein).

In a comparison between NWA 4590 and NWA 4801, Sanborn and Wadhwa (2009) concluded that despite their many similarities, the significantly different REE abundances in NWA 4801 suggests that it crystallized from a distinct magma source. In a similar manner, their CRE ages reflect different ejection events, calculated to be 26.4 (±1.2) m.y. for NWA 4590 and 31.6 (±1.5) m.y. for NWA 4801 (Nakashima et al., 2008). A more precise noble gas analysis conducted by Nakashima et al. (2018) established a CRE age for NWA 4590 and NWA 4801 of 20.0 (±4.0) m.y. and 26.4 (±6.1) m.y., respectively. Multiple episodes of impact, disruption, and dissemination of the crust can be inferred by the wide range of CRE ages determined for the angrites—<0.2–56 m.y. for thirteen angrites measured to date, possibly representing as many ejection events (Nakashima et al., 2008; Wieler et al., 2016; Nakashima et al., 2018). This range is consistent with a single large parent body enduring multiple impacts over a very long period of time, which would suggest that the parent object resides in a stable orbit (planetary or asteroid belt) permitting continuous sampling over at least the past 56 m.y. Alternatively, Nakashima et al. (2018) consider it plausible that there is currently at least two angrite (daughter) objects occupying distinct orbits: one representing the fine-grained (quenched) angrites with the shorter CRE age range of <0.2–22 m.y., and another representing the coarse-grained (plutonic) angrites with the longer CRE age range of 18–56 m.y. (see diagram below). Cosmic-ray Exposure Ages of Angrites
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Diagram credit: Nakashima et al., MAPS, Early View, p. 14 (2018)
Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures’
(http://dx.doi.org/10.1111/maps.13039)
Utilizing the unique characteristics of the angrites, an absolute timescale was determined based on the short-lived H–W and long-lived Pb–Pb chronometers (Kleine et al, 2008). Relative to angrites, a more accurately resolved absolute age of 4.5686 (±0.0007) b.y. was determined for the formation of CAIs. Consequently, with consideration of the Al–Mg-based age previously determined for ordinary chondrules, a revised formation interval was calculated for ordinary and carbonaceous chondrites, which concluded that carbonaceous chondrites are the youngest. The investigators note that this younger age is consistent with a commensurate decrease in radiogenic 26Al resulting in a less thermally metamorphosed nature for the carbonaceous chondrite groups.

A small number of unique angrites are represented in our collections today, which are commonly grouped as either plutonic/metamorphic or basaltic/quenched, 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 4590 shown above is a 0.66 g fragment showing its coarse-grained composition and clear glass component. The photo below is an excellent petrographic thin section micrograph of NWA 4590, shown courtesy of Peter Marmet. standby for nwa 4590 ts photo
Photo courtesy of Peter Marmet