NWA 4801

standby for nwa 4801 photo
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 contains a high abundance of pure anorthite in the form of white crystals and aggregates. 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—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 is identical within error to the angrite NWA 4590 (Sanborn et al., 2011). A Lu–Hf isochron for NWA 4801 was determined 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 for study, new models of their formation are now 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 core formation, plutonic and volcanic magmatism, metasomatism, metamorphism, and impact-generated regolith formation occurred 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, attest to an origin in close proximity to the Sun. The oldest angritic material is recognized in the form of early crustal vesicular rocks represented by such meteorites as Sahara 99555, D’Orbigny, and NWA 1296. Younger angritic material, occurring in the form of impact-mixed extrusive and intrusive magmatic rocks combined 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 Irving and 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, with the dissemination of large sections of this material into a stable orbit that has been maintained for the past 4+ b.y. The source of the delivery of angrite material to Earth might lie within the main asteroid belt, or it could 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. standby for angrite schematic diagram
click on image for a magnified view

Diagram credit: A. Irving and S. Kuehner, Workshop on Chronology of Meteorites, #4050 (2007) While angrites could possibly be original fragments from ‘Maia’, mother of Hermes (Mercury), they may instead derive from ‘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 a possible location for the angrite parent body (see diagram below). standby for angrite fe/mn diagram
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 is available for ε100Ru), NWA 5363/5400 and angrites have values for each of these isotopic anomalies that 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 for 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 nebular isotopic reservoir most similar to that of enstatite and ordinary chondrites.

The CRE age calculated for NWA 4801 is 31.6 (±1.5) m.y. (Nakashima et al., 2008). A more precise noble gas analysis conducted by Nakashima et al. (2018) established a CRE age for NWA 4801 of 26.4 (±6.1) m.y. 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
standby for angrite cre age diagram
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’
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 could be attributed to a number of possible causes such as accretion to an orbit in close proximity to the early T-Tauri phase solar field, or perhaps more likely, to a magnetic field generated by an internal core-dynamo mechanism.

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). standby for bahia blanca angritic fragments photo
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 number of unique angrites identified today is quite 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. standby for nwa 4801 ts photo
click on image for a magnified view
Photo courtesy of Peter Marmet

Leave a Reply