NORTHWEST AFRICA 4590
‘Tamassint’

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 has a texture unique from other angrites, which is neither metamorphic nor basaltic/quenched. It is a coarse-grained, plutonic, 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 troilite, merrillite, Cl-bearing Ca-silicophosphate, and kamacite. In addition, glass having a compositional range between anorthite and ulvöspinel is present at the grain boundaries, and incorporates re-precipitated primary minerals. This glass is thought to have formed by a rapid melting and cooling process, possibly through a decompression event such as that occurring from the collisional stripping of the lithosphere on a large planet.

The first-time discovery of the 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.

Northwest Africa 4590 has a Pb–Pb-based crystallization age of 4,558.86 m.y. A Hf–W 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 ages are identical within error margins to that of the plutonic angrite LEW 86010, and are very close to two other plutonic angrites, Angra dos Reis 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. However, in a comparison between NWA 4590 and NWA 4801, they concluded that despite their many similarities, the significantly different REE abundances observed in NWA 4801 suggests that it crystallized from a distinct source magma. 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 angrites—0.6–71 m.y. for the ten angrites measured—suggests that the APB was/is a large body which has experienced multiple episodes of impact and dissemination of the crust over a very long period of time.

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 4,568.6 (±0.7) m.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. The specimen of NWA 4590 shown above is a 0.66 g fragment showing its coarse-grained composition and clear glass component.

---