Lunar Mare Basalt
unbrecciated high-Al basalt
standby for northwest africa 4898 photo
Found 2005/2006
no coordinates recorded

A single fusion-crusted stone weighing 137 g was found in the Sahara Desert. It was sold by the finder to German dealer S. Ralew in 2007, and a sample was submitted to the Museum für Naturkunde (A. Greshake) in Berlin, Germany for analysis and classification under the designation NWA 4898.

This coarse-grained (gabbroic), magmatic rock (Li and Xue, 2022 #6230) is composed primarily of phenocrysts of pyroxene (Ti-rich pigeonite and augite, 50 vol%) and Ca-rich plagioclase (32 vol%, all of which is converted to maskelynite), with lesser amounts of Fe-zoned olivine megacrysts, needle-shaped grains of ilmenite, high-silica glass, and chromite, along with minor FeNi-metal and FeS. The matrix texture reveals radiating, densely branched, polycrystalline sprays (called 'spherulites' because of their large-scale average spherical shape) of clinopyroxene and plagioclase, attesting to rapid quenching of the magma during eruption to the surface. It has been demonstrated that this spherulitic texture forms when lunar basalts cool at a rate of ~20–60°C/hour. Besides the presence of maskelynite, other features that reflect moderate shock levels include the occurrence of planar fractures and mosaicism in silicates, twinning in ilmenite, partial melt veining, and localized melt pockets with embedded FeS droplets.

Northwest Africa 4898 is a low-Ti, low-Fe, high-Al basalt compositionally similar to Apollo 14 mare basalts. However, NWA 4898 is significantly younger than Apollo 14 samples, with a Rb–Sr-based crystallization age of 3,578 (±40) m.y. compared to the calculated age range of 3,950–4,330 m.y. for Apollo 14 samples. Certain elemental ratios also reflect significant differences exist between the NWA 4898 and Apollo 14 basalt mantle sources. Moreover, NWA 4898 is derived from a highly evolved mantle source, and has the highest Sm/Nd ratio known among lunar mare basalts, demonstrating the highest incompatible element depletion of any lunar mantle source studied at the time (perhaps as a result of later-crystallized Fe–Ti–ITE-rich mafic cumulates that became unstable and sank below the NWA 4898 source region).

Spacecraft have found that basalts are not present in all topographic low areas, but instead, reservoirs of basaltic magma are confined at great depth with eruptions being contingent on a combination of factors such as the crustal thickness, the concentration of heat-producing elements (K–U–Th), and the extent of the underlying magma columns. Based on spectral data, it was determined that many different basalt units exist within individual maria, these representing a wide range of crystallization ages from ~4.35 b.y. (components of the lunar breccia Kalahari 009) to as young as ~1.3 b.y. Crater counting methods indicate that some maria could have formed as recently as 1 b.y. ago (G. T. Taylor, 2007). With a crystallization age (Rb–Sr) of 3.578 b.y., NWA 4898 is somewhat older than the mare basalt NWA 032, the latter crystallizing only 2.852 b.y. ago (Rb–Sr) and is considered to be one of the youngest mare basalts in our collections. A look at the ages determined for the lunar meteorites reveals that none of them have a bulk rock age older than ~3.85 b.y., upholding the lunar cataclysm hypothesis.

Further information detailing the formation of mare basalts can be found on the NWA 032 page of this website. In-depth information about lunar meteorites in general, and NWA 4898 in particular, can be found on the lunar meteorite website of the Department of Earth Sciences, Washington University. The specimen of NWA 4898 pictured above is a crusted partial slice weighing 0.207 g. The photo below is a high-resolution close-up of the pyroxene–plagioclase sprays that were formed in the matrix during quenching of this rock, shown courtesy of Stefan Ralew.

standby for northwest africa 4898 pyroxene–plagioclase sprays photo
Photo courtesy of Stefan Ralew