Angrite
Plutonic/Metamorphic
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 a coarse-grained rock (0.1–1.2 mm) which exhibits 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, along with minor troilite and kamacite. 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 based on Mn–Cr systematics and anchored to the absolute Pb–Pb chronometer, which has now been accurately determined for NWA 4801 to be 4.558 (±0.00013) b.y., provides the best agreement between these two chronometers yet obtained for angrites (Shukolyukov et al., 2009). 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 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 resulted in differentiation of the mantle and formation of a small core (8 mass%; 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-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 few 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 like 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 is proposed 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. 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.
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—0.6–71 m.y. for ten 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.
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 µteslas, ~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 specimen of NWA 4801 shown above is a 0.98 g partial slice.
