Compositional studies of the Orientale, Humorum, Nectaris, and Crisium lunar basins

Abstract

Abstract. We have used full-resolution (250 m/pixel) Clementine images to map the compositions of ejecta from four multiring basins on the Moon: Orientale, Humorurn, Nectaris, and Crisium. All basins have relatively feldspathic ejecta, with iron contents ranging from 2 to 6 wt % Fee. Some basins show remarkably homogeneous ejecta blankets (Orientale), while others display distinct compositional zoning within exterior sectors of the basin (Crisium); in some cases this zoning correlates with morphologically observed ejecta facies (Nectaris). The iron-poor composition of most basin ejecta indicates that the crust of the Moon at these locations is feldspathic down to depths of at least several tens of kilometers, the likely sampling depth of basin-forming impacts. The inner rings of all four basins display massifs and small crater floors of nearly pure anorthosite (Fee < 1 wt %), virtually the only such occurrences of this rock type on the Moon. Because basin rings are structurally emplaced topographic elements derived from depths of at least 10-20 km into the Moon, we postulate that basin inner rings are sampling a unique stratigraphic level in the Moon and at these depths, the primordial, pristine anorthositic crust of the Moon is largely preserved. Below this zone of anorthosite is the more mafic region of the lower crust, probably having "highland basalt" composition (Fee -10 wt %, A12e 3 -18-22 wt %). The petrologic nature of lower crustal levels remains uncertain but must at least in part include members of the mafic highland suite, such as norites and troctolites. IntroductionThe formation of impact basins has had a profound effect in shaping the Moon. These impacts excavated vast amounts of material and redistributed it across the lunar surface. We are using basins as probes to build up a three-dimensional picture of the crust. By studying the variations in composition of basin ejecta deposits, we have the opportunity to look "into" the crust of the Moon to learn about its makeup and origin. In this paper we discuss the results of a study of several nearside impact basins using the Clementine multispectral data set. The Clementine mission has given us complete coverage of the Moon, allowing all exposed basin ejecta to be characterized. Moreover, the newly developed techniques

A new model of lunar crust: asymmetry in crustal composition and evolution

“…Here, we propose a new crust model and provide the rationale behind our model in comparison to previously proposed models. Korotev et al (2003) suggested a possible crustal configuration of a ferroan anorthositic upper layer and a magnesian, slightly more mafic lower layer by linking the FAN and the magnesian feldspathic rocks as the precursor of the magnesian granulites with the remotely sensed crustal configuration (Tompkins and Pieters, 1999;Bussey and Spudis, 2000;Hawke et al, 2003). This model seems to be consistent with the simultaneous occurrence of the magnesian and ferroan granulites within the Apollo samples and the feldspathic lunar meteorites.…”

“…3). Combined with the crustal zonation indicated by remote sensing (Bussey and Spudis, 2000;Hawke et al, 2003), where a more mafic layer should be located beneath a highly feldspathic layer in the crust, it is the magnesian portion that comprises a magnesian anorthositic, upper crust and a magnesian troctolitic, lower crust, and the ferroan portion that is formed by a ferroan anorthositic, upper crust and a ferroan noritic, lower crust (Fig. 3).…”

Compositional heterogeneity of central peaks within the South Pole‐Aitken Basin

“…On average, the crust is~40 km thick, as shown by recent models incorporating Gravity Recovery and Interior Laboratory (GRAIL) gravity and Lunar Orbiter Laser Altimeter (LOLA) topography data [Wieczorek et al, 2013]. Several lines of evidence from lunar sample analyses as well as remote sensing studies suggest that the mafic content of the crust increases with depth [Reid et al, 1977;Ryder and Wood, 1977;Bussey and Spudis, 2000;Tompkins and Pieters, 1999;Cahill et al, 2009;Ohtake et al, 2009]. However, impacts throughout lunar history have fractured, brecciated, mixed, melted, and redistributed crustal materials.…”

The role of magma buoyancy on the eruption of lunar basalts

“…(2) The noritic impact melt breccias sampled at the Apollo stations most likely have a deep crustal origin (e.g., [42]). (3) The ejecta blankets of large basins have been shown to be more ma¢c than average crustal materials (e.g., [43]). (4) The compositions of some central peaks in complex craters are highly noritic or troctolitic (e.g., [44]).…”