Lunar Mare Basalt
unbrecciated, low-Ti
standby for northwest africa 032 photo
Found October 1999
near 30° 22' N., 5° 3' W.

A small 300 g stone was found by an association of European collectors at a location west of the Kem Kem plateau in Morocco. A second stone weighing 156 g and designated NWA 479 was recovered a year later. It is both chemically and petrographically indistinguishable from NWA 032, and the two stones are obviously paired. Northwest Africa 032 is a previously-unsampled, unbrecciated, interior mare basalt that was not exposed to solar wind implanted gases.

Over the course of several ANSMET seasons, six paired basaltic lunar meteorites with a combined weight of ~1.93 kg were recovered from the Antarctic LaPaz Icefield (LAP). Through chemical, mineral, and petrographic comparisons, including CSD measurements, as well as crystallization and CRE age determinations, the LAP lunaites have been shown to be almost certainly source crater paired with NWA 032/479. However, some differences do exist between the LAP and the NWA samples. There is a disparity in their mineral compositions, considered to be the result of greater fractionation in the LAP samples following lava extrusion. In addition, the LAP samples have larger grain sizes than those in the NWA samples, indicative of crystallization in a more slowly-cooled location within a common parental magma. Another find from Northwest Africa, NWA 4734, has chemical characteristics which overlap those of NWA 032/479 and the LAP 02205 pairing group (e.g., these meteorites have 3% TiO2; Korotev, 2007), and all of these meteorites are likely source crater paired but derive from distinct parental source lava flows in a common volcanic complex (Elardo et al., 2012, 2014).

The mineralogy of NWA 032 consists of phenocrysts of olivine, pyroxene, and chromite in a very fine-grained matrix of radiating pyroxene and feldspar crystals. The olivine phenocrysts (the larger light-colored crystals seen in the photo above) make up ~8 vol% of the meteorite and have measurments of up to 1.3 mm across. The pyroxene phenocrysts constitute ~5 vol% of the rock and constitute a larger range of grain sizes, some appearing as small as ~0.2 mm. The fine- to medium-grained matrix is comprised of pyroxene, primarily in the form of augite and pigeonite. Accessory phases include ilmenite, troilite, and trace metal. A low abundance of late-stage mesostasis (~0.3 vol%) is present. High-silica glass is found in the abundant shock veins that permeate the stone (~6 vol%).

Crystallization of the olivine and pyroxene phenocrysts that would eventually compose the NWA and LAP samples occurred over a temperature range of ~200°C and over a time interval from a few days up to a month under slow-cooling conditions (<2°C/hour), probably within a shallow dyke or sill (Day and Taylor, 2007). Initially, the olivine and chromite phenocrysts crystallized slowly under low pressure conditions, producing the more typical steep zoning profiles reflected in decreasing Mg# from core to rim, with a surrounding FeO-rich rim. Burger et al. (2009) ascertained that the pyroxene phenocrysts show evidence of an additional stage of crystallization from a melt with a fluctuating Ca composition, alternating between pigeonite and augite. Oscillatory zoning occurs in both olivine and pyroxene; in olivine this banding is thought to be caused by rapid crystal growth accompanied by slower P diffusion, known as "solute trapping", while in pyroxene the development of a coarser oscillatory banding profile is considered to be the result of convection cycling associated with variable temperature regimes within the magma chamber (Elardo and Shearer, 2013). Some phenocrysts contain cores with a primitive composition consistent with that predicted for the parental source magma (Fagan et al., 2002).

It has been inferred that this low viscosity magma was then emplaced upon the surface, perhaps 10–20 m thick, where the LAP samples experienced uniformly slow cooling at rates of ~0.2°C/hour within the middle of the flow and crystallized within ~40 days. At the same time, the slowly-cooled phenocrysts that would become the NWA samples were incorporated within the less insulated magma layers, perhaps the upper margin, where they experienced rapid cooling at rates of 20–60°C/hour. This rock solidified rapidly at up to ~60°C/hour, producing the characteristic plumose matrix textures in <10 hours (Fagan et al., 2002; Zeigler et al., 2005; Anand et al., 2006; Joy et al., 2006; Day and Taylor, 2007).

This petrogenetic model was expounded upon by Day and Taylor (2007) to account for other lunar meteorites and to explore the possibility that they also formed within the same differentiated stratigraphic unit as the NWA and LAP samples. Based on chemical compositions, mineralogies, textures, cooling rates, and crystallization and CRE ages, it was initially considered that the lunar pairing group of NWA 773 might represent the more rapidly cooled cumulate-rich base of this magma unit, with the more rapidly cooled basaltic component, as represented by NWA 3160, deriving from the lowermost layer adjacent to local pre-existing rock. However, the finding that NWA 773 was formed from a KREEPy reservoir in contrast to the non-KREEPy source for the NWA 032/LAP/NWA 4734 mare basalt suite rules out such a relationship (Elardo et al., 2014).

Shock pressures greater than 25 GPa are revealed by the melt pockets and veins, maskelynized feldspar, and mosaisicm in olivine. High-pressure polymorphs of olivine are present in impact melts, including ringwoodite and wadsleyite. Northwest Africa 032 has a low-Ti composition often associated with an earlier-formed basaltic unit. It represents a unique crystalline mare basalt source containing high olivine phenocryst abundances. A relatively short terrestrial age of ~5 t.y. has produced only very minor terrestrial weathering (W0), evidenced by a thin calcite covering on the exterior with some calcite veining, but with most of the interior remaining virtually free of alteration. Typical for many hot-desert meteorite finds, the trace element Ba is enriched in NWA 032 compared to Apollo basalts.

Subsequent to the finding of NWA 032, a 154 g mare basalt meteorite, Dhofar 287A, was found in the desert of Oman. These two mare basalt lithologies exhibit some close similarities as well as some important differences (Anand et al., 2003). They are both low-Ti mare basalts similar to Apollo 12 and 15 basalts. However, Dhofar 287A contains a higher abundance of olivine phenocrysts (~20 vol% vs. ~8 vol%) in a larger size range (>2 mm vs. 0.4 mm), and which are more highly zoned compared to those in NWA 032. It also contains a larger vol% of late-stage mesostasis (~3 vs. 0.3), some of which is surrounded by highly Fe-enriched pyroxferroite. Moreover, Dhofar 287A has a coarser-grained texture than NWA 032 (50–100 µm vs. <20 µm, respectively), inferring a faster cooling rate for the latter. Compared to NWA 032, Dhofar 287A has a REE pattern consistent with the assimilation of a KREEP component, although a limited association of NWA 032 with a KREEP component (similar to the La Paz basalt) has been proposed by Barrat et al. (2005) to explain certain anomalous trace element ratios. As a further distinction between the two, NWA 032 has a much greater abundance of impact melt veins. There is ample evidence to indicate that these two mare lithologies were derived from distinct source regions, and have experienced different petrogenetic histories.

The International Union of Geological Sciences—Subcommission on the Systematics of Igneous Rocks, having established a Working Party on the classification of lunar rocks, has adopted a Classification System for Lunar Rocks. The terms "highland" and "mare", which were originally established as geographical terms based on the surface morphology of the Moon and its resulting albedo, are now utilized on a geochemical basis composing two subdivisions of the broader igneous group.

Mare basalts cover ~17% of the lunar surface but account for only ~1% of the total volume of the crust. They are largely the result of eruptions within basins located asymmetrically on the nearside of the Moon, and predominantly in the western region including the Oceanus Procellarum and Mare Imbrium basins. It has been argued that this asymmetry is the result of a complex sequence of events that began within the first 50 m.y. of solar system history. In the upper levels of the fully enveloping magma ocean, which extended to a depth of at least 500 km, crystallization proceeded until reaching a point of 82–94% completion. At this time the cumulate precursor to the low-Ti basalts was formed. Finally, a low-degree partial melt from this precursor material underwent fractionation to form the low-Ti mare basalts. The low-Ti NWA 032 is similar to basalts collected by Apollo 12 and Apollo 15 with respect to Ti content, but not with respect to major element, REE, and LREE compositions (e.g., REE abundances are enriched compared to that of Apollo basalts). The parent melt is thought to have had a bulk composition similar to that of the Apollo 15 low-Ti yellow picritic glasses, representing the residue that remained after ~20% olivine had crystallized at low pressure (Zeigler et al., 2005). Despite the finding of elevated incompatible element abundances in the bulk rock, isotopic compositions indicate that the parental source of NWA 032 was highly depleted in incompatible elements; this discrepancy is not yet completely understood.

The more evolved material that was the precursor of the high-Ti basalts was not produced until after 95% crystallization had occurred. Consequently, trapped residual melt occurs only in high-Ti basalts. The most widely accepted formation scenario suggests that following the crystallization of the lunar magma ocean, a gravitationally unstable, Ti-rich, cumulate ilmenite layer was crystallized beneath the crust. Having a greater density than the peridotite mantle below it, this ilmenite began to founder and sink to the deep interior, bringing with it the heat-producing elements U and Th. Situated above a small metallic core, this ilmenite layer experienced thermal expansion. Coupled with a lack of convection in the overlying peridotite mantle preventing the removal of heat, an eventual instability occurred leading to the migration of this molten plume to one hemisphere of the Moon and to the formation of a stable, cumulus stratigraphy. The melting, and eventual convective mixing, of this Ti-rich ilmenite with the peridotite mantle, resulted in the olivine-bearing, Ti-rich basalt that was eventually carried to the surface through volcanism. An alternative scenario argues that a negatively-buoyant, high-Ti cumulate could not have formed, but instead, the Ti-rich ilmenite cumulate layer became gravitationally unstable to the point of sinking due to assimilation of 10–20% olivine prior to crystallization.

The lunar mare basalts can help constrain the origin and formation history of the Moon and reveal information about the magma ocean phase. The magma ocean is consistent with the theory in which the Moon formed as a result of an impact on the proto-Earth by a differentiated Mars-sized projectile, which was given the name 'Theia' after the mythological Greek Titan who gave birth to the Moon goddess Selene. During this collision, which occurred 30–110 m.y. after the start of the solar system (Yin et al., 2002; Kleine et al., 2005), the metallic core accreted to Earth while the silicate mantle eventually became stabilized in orbit. Alternatively, a modified fission model involving a rapidly rotating proto-Earth and a nuclear explosion is propounded by Westrenen et al. (2012).

Abundances of platinum group elements (PGEs: Ru, Rh, Pd, Ir, Pt) are unique relative to terrestrial and martian basalts. One particular lunar signature is the depletion of Pd due to its partitioning into Fe-metal during the magma ocean phase under conditions of low oxygen fugacity (related to the partial pressure of available oxygen). The correlation between high-PGE abundances and the high-Ti basalts suggests that the PGEs were concentrated in the same residue from which the high-Ti basalts were derived.

A study of Li abundances and isotopic compositions in NWA 479 phenocrysts, conducted by Barrat et al (2005), led to their consideration that the Moon may be enriched in 7Li, possibly the result of volatilization and loss of 6Li during the Moon's impact-generated formation. The strong variability of Li isotopic compositions between phenocryst core and rim in NWA 479 is thought to have been established through diffusion processes.

Northwest Africa 032 has the highest Th:REE ratio of any other mare basalt, with probable source areas considered to be (in decreasing order of probability) Mare Humorum, Mare Fecunditatis, western Mare Serenitatis, Mare Crisium, and far western Oceanus Procellarum. 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 including 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, ranging from ~4.35 b.y. (components of the lunar breccia Kalahari 009) to as young as ~1.3 b.y. Crater counting methods indicate some maria could be as young as 1 b.y. old (G. T. Taylor, 2007).

Northwest Africa 032 has a mid-range crystallization age of 2.779 (±0.014) b.y., making it one of the youngest mare basalts in our collections (identical within error margins as NWA 4734). This K–Ar age has been interpreted by some as reflecting the shock event that produced the impact-melt veins rather than its crystallization age. However, a concordant Rb–Sr age of 2.852 (±0.065) b.y. and Sm–Nd age of 2.692 (±0.160) b.y. was obtained for NWA 032 by Borg et al. (2007), disputing the association of the K–Ar age with a late shock event. K–Ar dating of basaltic lithologies in Kalahari 009 conducted by Fernandes et al. (2007) revealed the youngest age for any lunar basalt measured thus far, indicating an age of <1.70 (±0.04) b.y. However, utilizing more precise U–Pb systematics within phosphate grains, a chronometer which is not prone to shock resetting effects, an age of 4.35 (± 0.15) b.y. was obtained for this meteorite (Terada et al., 2007).

A look at the ages of the various lunar meteorites reveals that none have a bulk rock age older than ~3.85 b.y., upholding the lunar cataclysm hypothesis. This hypothesis suggests that a large number of impacts occurring over a brief time interval (beginning ~3.9 b.y. ago and lasting 200 m.y.) initiated a metamorphic phase over much of the Moon's surface. Noble gas studies indicate that NWA 032 resided within the lunar regolith for 207 (±43) m.y., and was subsequently buried to an appropriate shielding depth. After its ejection from the Moon, the NWA 032 meteoroid spent 42 (±5) t.y. in transit to Earth (Lorenzetti et al., 2005). According to Nishiizumi and Caffee (2010), this relatively short Moon–Earth transit time would be consistent with a launch from a depth of <1–4.7 m. The meteorite then resided for another ~5 t.y. on Earth (Nishiizumi, 2003). This was a small meteoroid, having a pre-atmospheric size of less than 10 cm in diameter. The presence of solar cosmic ray produced 26Al is indicative of a very low ablation rate, consistent with a low entry velocity and/or a low entry angle.

The specimen pictured above is a sub-gram partial slice of Northwest Africa 032. Shown in the top photo below is a high-resolution close-up photo showing the large olivine phenocrysts surrounded by the feathery feldspathic crystals within the matrix of this meteorite.

standby for northwest africa 032 photo
Photo by Walt Radomsky
Courtesy of R. A. Langheinrich Meteorites
standby for northwest africa 032 photo

For more information on the lunar basalt NWA 032, read the PSRD article by G. Jeffrey Taylor—"The Growing Diversity of Lunar Basalts", Sept. 2009, from this link. In addition, extensive information can be found on the lunar meteorite website of the Department of Earth Sciences, Washington University. Additional details about the linkage of NWA 032, NWA 4734, and the LAP pairing group mare basalts can be found on the NWA 4734 page.