Lunar and terrestrial crusts: a constrast in origin and evolution

Abstract

Planetary crusts, differing in composition from that of the bulk planet, and containing large concentrations of incompatible elements such as K, U and Th, may arise in two distinctly different ways. The first, exemplified by the Moon, involves flotation of a feldspathic crust during an initial melting episode in which much of the planetary body was molten. The second, probably more common type of crust is formed as a consequence of partial melting, in response to radioactive heating, in the planetary interior and the eruption of basaltic lavas. Examples include the lunar maria and the oceanic crust of the Earth. Subsequent partial melting episodes, involving recycling into the mantle, or lower crustal melting episodes produce more acidic crusts, of which the terrestrial continental crust forms the type example. The upper crust requires at least three successive partial melting events.