NORTHWEST AFRICA 4734


Lunar Mare Basalt
unbrecciated, low-Ti
standby for northwest africa 4734 photo
Specimen in the collection of Chladni's Heirs

Two partially fusion-crusted fragments, weighing 409 g and 68 g, were purchased from nomads in Rissani, Morocco in 2007 by A. Habibi. Additional fragments of this same meteorite, having a combined weight of 895 g, are in the possession of M. Oumama. A portion of the meteorite was submitted by Mr. Habibi for analysis and classification to UPVI, Paris (A. Jambon, O. Boudouma and D. Badia) and UBO (J-A Barrat) under the designation NWA 4734. Northwest Africa 4734 was determined to be of lunar origin.

Not unexpectantly, separate analyses of this unique and unusual lunar mare basalt have resulted in some differences in the classification nomenclature among the various investigating teams. The initial analysis conducted in France on a portion of NWA 4734 resulted in its classification as a highly shocked lunar monzogabbro (inferring a plutonic crystallization origin rather than a volcanic/basaltic origin), with a measured anorthite content within the monzogabbro field (89%). However, the minimum amount of alkali feldspar (10%) was not observed in NWA 4734, and therefore the "monzo" modifier is not supported (R. Korotev).

Given the very close geochemical, mineralogical, textural, compositional (bulk-rock major, minor, and trace element), and chronological similarities that exist among NWA 4734, NWA 032/479, the LaPaz 02205 lunar mare basalt pairing group, it is considered that these meteorites, recovered on distinct continents, are most likely source-crater paired (Elardo et al. 2014 and references therein). However, differences in isotopic ratios (e.g., Sm–Nd) and REE abundances suggest that they derive from distinct parental source melts. Accordingly, it is plausible that these meteorites derive from a common volcanic complex, with NWA 4734 and LaPaz 02205 crystallizing slowly deep within their distinct lava flows, and NWA 032/479 crystallizing more rapidly within the less insulated outer margins of its flow (Elardo et al., 2013).

Through Pb–Pb dating utilizing zirconolite and baddeleyite, Wang and Hsu (2010) and Wang et al. (2012) determined a crystallization age for NWA 4734 of 3,073 (±15) m.y., and an age for the LAP pairing group of 3,039 (±12) m.y. These ages are consistent with results obtained by other chronometry methods (Sm–Nd: 3,024 ±27 m.y. Rb–Sr: 3,083 ±42 m.y.; Elardo et al., 2013), and support a scenario involving a concurrent formation late in lunar history; i.e., during the final stages of crystallization of the Lunar Magma Ocean (after ~86–95% solidification), as a 1% partial melt phase involving a non-KREEPy source (Elardo et al., 2014). A concurrent ejection of these NWA and LAP basalts then occurred through a significant impact event—the lunar ejection ages calculated for NWA 032 and the LAP pairing group are concordant. Notably, the CRE age calculated for NWA 4734 (569 m.y.) differs from that of both NWA 032 (212–275 m.y.) and the LAP pairing group (44 m.y.), possibly due to their formation in distinct lava flows, or to their relative pre-impact burial depths (Elardo et al., 2014).

Further information can be found on the Lunar Meteorites website of the Department of Earth Sciences, Washington University.