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

We have used full-resolution (250 m/pixel) Clementine images to map the com positions of ejecta from four multiring basins on the Moon: Orientale, Humo rum, Nectaris, and Crisium. All basins have relatively feldspathic ejects, with iron contents ranging from 2 to 6 wt % FeO. Some basins show remarkabl y homogeneous ejecta blankets (Orientale), while others display distinct co mpositional zoning within exterior sectors of the basin (Crisium); in some cases this zoning correlates with morphologically observed ejecta facies (N ectaris). 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 l east several tens of kilometers, the likely sampling depth of basin-forming impacts. The inner rings of all four basins display massifs and small crat er floors of nearly pure anorthosite (FeO 1 wt %), virtually the only such occurrences of this rock type on the Moon. Because basin rings are structur ally 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 s tratigraphic level in the Moon and at these depths, the primordial, pristin e anorthositic crust of the Moon is largely preserved. Below this zone of a northosite is the more mafic region of the lower crust, probably having "hi ghland basalt" composition (FeO similar to 10 wt %, Al2O3 similar to 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.