The role of magma buoyancy on the eruption of lunar basalts

Abstract

It has long been recognized that mare basalts on the Moon are preferentially located both on the Earth-facing hemisphere and within large impact basins. A popular model that accounts for this observation assumes that these magmas were denser than the lunar crust, that they accumulated at the crust^mantle interface, and that eruptions occurred only when this magma chamber became overpressurized. In this paper, we re-evaluate this model and argue that it is not consistent with the available data nor with models of dike propagation. As an alternative hypothesis, we propose that magma buoyancy is the predominant factor that determines whether mare basalts erupt at the surface or form crustal intrusions instead. We have computed the densities of mare basaltic magmas and find that some are, in fact, less dense than the Moon's upper anorthositic crust. Based on the widely accepted view that the lunar crust becomes more mafic with depth, we also show that all mare basaltic magmas should be less dense than the lower portion of the crust. Thus, if the upper anorthositic crust was regionally removed by an impact event, then any mare basaltic magma could have risen to the surface there based on buoyancy considerations alone. In support of this model, we note that mare basalts are indeed found wherever geophysical crustal thickness models predict the upper crust to be absent. Furthermore, many of the basalts that erupted within the anorthositic highlands are predicted to be less dense than the underlying crust based on remote sensing data. The high titanium flows within Oceanus Procellarum are somewhat problematical to our model in that an anorthositic crust is predicted to be present beneath them. Using results from recent lunar thermal models, we suggest that these magmas may have overcome their negative buoyancy in the crust by possessing superliquidus temperatures. If magma buoyancy does indeed control whether or not a basaltic eruption will occur, then this implies that the quantity of magma produced beneath the South Pole-Aitken basin was about 10 times less than that of the nearside. ß

Formation of lunar mare domes along crustal fractures: Rheologic conditions, dimensions of feeder dikes, and the role of magma evolution

“…In this model, the plane of the dike is perpendicular to the direction of minimum compressive stress, leading to dikes following the same paths as pre-existing fractures. The seismic profiles obtained at the Apollo 12 and 14 landing sites indicate that impact-induced fractures in the lunar crust are closed at depths of ∼ 20 km (Wieczorek et al, 2001) and thus do not extend as far down as the mantle source zones of most dikes. But according to the model by Rubin (1993b) the dike orientations were guided by the same stress fields that caused the crustal fractures, leading to dikes extending radially with respect to the centres of the major basins.…”

“…Without an excess pressure in the source region, the magma was able to rise above the base of the crust to a distance where the negative buoyancy component from the magma above content at liquidus temperature (Wieczorek et al, 2001). This value is very similar…”

“…tures of about 1200-1400 • C (Wieczorek et al, 2001) and that the magma solidifies at ∼ 800 • C (Spera, 2000). According to Carrigan et al (1992), a preheated crust is a realistic assumption if the region has previously undergone an extended phase of active volcanism.…”

“…We put forward the hypothesis that on the Moon, impact-induced stress fields caused by large impact events, manifesting themselves near the surface as crustal fractures, facilitated the ascent of dikes in an analogous manner. It is the stress field, not the actual presence of the pre-existing fractures, which favoured the formation and ascent of dikes (Rubin, 1993b), since the fractures are closed at depths no larger than ∼ 20 km (Wieczorek et al, 2001). The resulting low compressive stress normal 41…”

40Ar‐39Ar chronology of lunar meteorites Northwest Africa 032 and 773

“…Shock-melt veins comprise only 3.2 vol% of the bulk meteorite and are present as linear features (Fagan et al 2002), so it is reasonable to expect that each sample will contain a different proportion of vein material. Furthermore, using the mass of each sample and assuming each is a sphere with a density of 2.95 g/cm 2 (using the method of Wieczorek et al 2001), a size range of 300-400 µm is estimated. The samples are, therefore, of similar size to the phenocrysts and significantly smaller than the distance between shock veins, which are typically ~1 mm apart.…”