The composition and origin of the moon

Abstract

Many of the properties of the moon, including its “enrichment” in Ca, Al, Ti, U, Th, Ba, Sr and the REE and “depletion” in Fe, Rb, K, Na and other volatiles can be explained by early condensation processes in the solar nebula. Thermodynamic calculations show that Ca-, Al- and Ti-rich compounds condense first in a cooling nebula. The initial high temperature mineralogy is gehlenite, spinel, perovskite, Ca-Al-rich pyroxenes and anorthite. Inclusions in type III carbonaceous chondrites such as the Allende meteorite are composed primarily of these minerals and are highly enriched in trace refractories such as REE relative to carbonaceous chondrites. These inclusions can yield basalt and anorthosite in the proportions required to eliminate the europium anomaly, leaving a residual spinel-melilite interior. A high Ca-Al deep interior does not imply an unacceptable mean density or moment of inertia for the moon. The inferred high U content of the lunar interior, both from the Allende analogy and the high heat flow, indicates a high temperature interior. The model is consistent with extensive early melting, shallow melting at 3 AE, and with presently high deep internal temperatures. It is predicted that the outer 250 km is rich in plagioclase and FeO. The low iron content of the interior in this model raises the interior temperatures estimated from electrical conductivity by some 800°C. The deep interior may be partially molten at the present time.