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 AlTi-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 FeTi-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 MnCr 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).
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.
This very early period of Solar System history corresponds to a time when the short-lived isotopes 26Al and 60Fe were still extant and could have initiated parent body melting. In a high-precision Cr-isotopic analysis utilizing four volcanic and three plutonic angrites, Zhu et al. (2019) determined that all of the angrites have homogeneous ε54Cr values, and made the assumption that the angrite parent body (APB) experienced a global magma ocean stage. They calculated an absolute MnCr age anchored to D'Orbigny (U-corrected) of 4.5632 (±0.0003) b.y., which they contend corresponds to the magma ocean crystallization and crust formation on the parent body (see diagram below). Because the absolute MnCr age for angrites is slightly younger than that calculated for Vesta (4.5648 [±0.0006] b.y.), which indicates a delayed mantlecrust differentiation stage for the APB, they reasoned that the APB was probably larger than Vesta.
Diagram credit: Zhu et al., The Astrophysical Journal Letters, vol. 877, #1, article L13 p. 5 (2019, open accesslink)
'Timing and Origin of the Angrite Parent Body Inferred from Cr Isotopes'
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.
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), the proto-Earth impactor that produced the Moon. An analysis of the intrinsic nucleosynthetic Mo isotope anomalies present in a comprehensive sampling of meteorite groups, Budde et al. (2019) investigated the dichotomy that exists between meteorites that derive from both the non-carbonaceous (NC) and the carbonaceous (CC) reservoirs. Results from this study enabled them to place constraints on whether the Moon-forming impactor originated from the NC or the CC region of the protoplanetary disk. Based on this study they ascertained that Theia was most likely a carbonaceous body which originated from the CC region, but that it was possibly composed of a mixture of CC and NC material; however, the impactor is inconsistent with an origin from the NC region (see the NWA 032 page for further details about the Moon-forming event). In a new study of the Fe/Mn ratio in olivine grains for a number of angrites, Papike et al. (2017) determined that angrites plot along a trend line between the Earth and Moon, which indicates the possible location of the angrite parent body (see diagram below).
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 extension 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
Diagram credit: Nielsen et al., EPSL, vol. 505, p. 137 (2019)
'Nucleosynthetic vanadium isotope heterogeneity of the early solar system recorded in chondritic meteorites'
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 Eureka could represent a captured fragment of the disrupted angrite parent body now in a stable orbit around Mars. In a separate study based on heliocentric distance calculations involving noble gas data, Obase et al. (2020) propose that the R chondrite PRE 95410 may be another potential representative of asteroid Eureka. They note that both the spectral similarity between the Rumuruti meteorite and asteroid Eureka and the orbital stability of the latter are consistent with such a possibility.
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 accesslink)
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.