NORTHWEST AFRICA 4485


Lunar Mafic, Th-rich, Impact-Melt Breccia
(KREEP-melt regolith breccia)
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Found Summer of 2006
no coordinates recorded

Two paired stones weighing 64.3 g (NWA 4472) and 188 g (NWA 4485) were found in the Algerian desert and subsequently purchased by separate collectors (G. Hupé and S. Ralew, respectively). A portion of each was submitted for analysis to the University of Washington in Seattle (S. Kuehner and A. Irving) and Washington University in St. Louis (R. Korotev), and the meteorite was classified as a unique KREEP-rich, basaltic melt breccia.

This is a polymict breccia composed of clasts from diverse lunar locations including mare basalt, High-Mg Suite (HMS), High-Alkali Suite (HAS), ferroan anorthosite (FAN), and impact-melt lithologies dispersed in the matrix. The KREEP-bearing assemblages are composed of granophyric textured clasts consisting of intergrowths of silica and K-feldspar, together with the high-temperature mineral zirconium oxide, or baddeleyite, and the Fe(Zr,Y)Ti-silicate known as tranquillityite, a mineral first recognized as a late-stage fractional crystallization product in Apollo 11 and 12 basalts. Other mineral fragments identified in NWA 4485 include olivine, pyroxene, plagioclase, ilmenite, chromite, K-feldspar, apatite, merrillite, silica, Fe-metal, and FeS, most reflecting derivation from a KREEP-rich precursor magma (Arai et al., 2009) referred to as urKREEP. The investigators observed that some of these KREEP basalt clasts exhibit chemical zoning and thick exsolution lamellae, attesting to slower cooling conditions at a deeper location compared to the Apollo mare basalts. The matrix also contains a variety of glasses, some containing vesicles and others taking the form of spherules enriched in P and K. In their study of apatite grains in NWA 4472, the pairing to NWA 4485, Tartèse et al. (2014) found that they contain moderate amounts of water, in the range of 2,000–6,000 ppm. Associated isotopic studies on the apatite demonstrated elevated δ37Cl values compared to terrestrial values, which suggests this meteorite has retained its original lunar isotopic signature. Moreover, they recognized that the δD values are consistent with lunar rocks associated with HMS, HAS, and KREEP-rich basalts.

The bulk composition of NWA 4485 reflects a high REE abundance with a strong negative Eu anomaly, with overall incompatible element abundances in the range of the only known KREEP-rich lunaites—the impact-melt breccias Sayh al Uhaymir 169 and Dhofar 1442. SaU 169 contains ~4.0 b.y. old clasts containing very high-K KREEP which best reflect the composition of primordial urKREEP (Lin et al. 2010). Basaltic clasts in NWA 4472/4485 sample low-Ti to very low-Ti source regions and exhibit a range of metamorphic textures. Some of these are fayalite-rich, quenched-textured glass thought to be derived from impact melting of mare basalt lithologies. Also present are a variety of feldspathic impact-melt, fragmental, and granulitic breccias, as well as metal clasts and glass spherules, all consistent with lithification within the lunar regolith.

The composition of NWA 4472/4485 is similar to that of the Th-rich, mafic, LKFM (low-K Fra Mauro) impact-melt breccias recognized from the Apollo collection; specifically, group-C melt breccias of Apollo 15, group-1S melt breccias of Apollo 16, and the aphanitic and poikilitic impact-melt breccias of Apollo 17 (Korotev 2000). The four constituents of the LKFM material—KREEP norite, forsteritic dunite, feldspathic upper crust, and FeNi-metal—are thought to be the likely products of a basin-sized impact into the ancient ‘Great Lunar Hot Spot’, which created the Imbrium basin within the Th-rich Procellarum KREEP Terrane (PKT) (Korotev, 1999). The impactor is thought to have been an iron meteorite that mixed upper mantle dunite with KREEP-contaminated Mg-rich magma, and incorporated clasts of ferroan anorthositic upper crust. The LKFM composition is unique to the PKT, and with its high FeO (noritic) composition and incompatible element abundances (>10 ppm Sm; 5.9–7.9 ppm Th), NWA 4472/4485 is likely derived from this nearside region (Joy et al., 2008). Other possible source locations are northwest of Sinus Iridium within the Jura mountains, northwest of Sinus Roris at Herschel crater, regions of the Mons Alpes formation in western Mare Imbrium, regions of the Apennine mountains near Mons Caucasus and Mons Bradley near Apollo 15, and regions near the craters Ptolemaeus and Lalande, the latter suggested to be the source location of SaU 169.

Calzada-Diaz et al. (2015) compared compositional and age data from a large number of lunar meteorites with elemental remote sensing data obtained by the Lunar Prospector gamma ray spectrometer, primarily for Fe, Ti, and Th, to better constrain the meteorite's source regions. For the KREEP-rich basaltic melt breccia NWA 4485/4472, plausible ejection sites were identified near the John Herschel crater and in Mons Caucasus, having a composition consistent with ejecta from the Imbrium basin (see image below).


Image credit: A. Calzada-Diaz et al.
Meteoritics & Planetary Science, vol. 50, p. 220 (2015)

Establishment of a thorough chronological history of this lunar rock following the initial basin-forming impact, and including the time spent on the lunar surface, in Moon–Earth transit, and in terrestrial residence has begun (Joy et al., 2009). Cosmogenic Ar–Ar data are indicative of a ~300 m.y. near-surface residence as part of the ancient lunar regolith. The Pb–Pb and U–Pb ages were calculated from the phosphates fluorapatite and merrillite in matrix and basalt clasts, as well as from zircon grains in the KREEP basalt component (Joy et al., 2011). The ages found within this regolith breccia reflect a diversity of lithic fragments with a wide range of crystallization ages (~4.35–3.94 b.y.); the younger ages in this range are consistent with those of Apollo KREEPy mare basalts possibly dating the formation of Mare Imbrium, while the oldest ages were derived from a matrix apatite grain and might reflect the crystallization of the HAS lithology. The lower Ar–Ar apparent age of 1.7–2.2 b.y. obtained for trapped solar wind Ar is thought to reflect a recent impact-resetting event which could represent the consolidation of the NWA 4472/4485 meteorite components.

A more thorough treatment of the chemical classification of lunar meteorites can be found on the WUSL—Lunar Meteorites website, including information on the other (unpaired) KREEP-rich meteorites SaU 169 and Dhofar 961/960/925/SaU 449, the KREEPy clast bearing meteorites Calcalong Creek and Y-983885, and the KREEP basalt meteorites comprising the NWA 733 pairing group, the LAP pairing group, and Dhofar 287a.

NWA 4472/4485 contains Sr and Ba indicative of terrestrial weathering. Both portions of the meteorite also contain high Br concentrations, suggesting that they were contaminated by seawater. The photo of NWA 4485 shown above is a 0.32 g partial slice, while that pictured below is the uncut mass as found (both photos courtesy of Chladni's-Heirs.com).

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Photo courtesy of Chladni's-Heirs.com