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 MoroccoAlgeria 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 ~10250 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, AlTi-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 MnCr and UPb chronometers at ~4.5572 b.y. (McKibbin et al., 2015).
Timescales for Angrites
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-metaloxide 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 PbPb-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 450550 K/m.y., and probably 700850 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 SmNd-based crystallization age for NWA 4590 is concordant with the PbPb-based age (Sanborn et al., 2011), while a HfW isochron for NWA 4590 calculated by Kleine et al. (2008) resulted in a slightly older age of 4,559.1 (±0.6) m.y. These PbPb 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). The wide range of CRE ages determined for the angrites0.673 m.y. for the eleven angrites measuredsuggests that the APB was/is a large body that has experienced multiple 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.
Utilizing the unique characteristics of the angrites, an absolute timescale was determined based on the short-lived HW and long-lived PbPb 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 AlMg-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.