Lunar initial 143Nd/144Nd: Differential evolution of the lunar crust and mantle

The current state of understanding of the lunar interior is the sum of nearly four decades of work and a range of exploration programs spanning that same time period. Missions of the 1960s including the Rangers, Surveyors, and Lunar Orbiters, as well as Earth-based telescopic studies, laid the groundwork for the Apollo program and provided a basic understanding of the surface, its stratigraphy, and chronology. Through a combination of remote sensing, surface exploration, and sample return, the Apollo missions provided a general picture of the lunar interior and spawned the concept of the lunar magma ocean. In particular, the discovery of anorthite clasts in the returned samples led to the view that a large portion of the Moon was initially molten, and that crystallization of this magma ocean gave rise to mafic cumulates that make up the mantle, and plagioclase flotation cumulates that make up the crust (Smith et al. 1970; Wood et al. 1970). This model is now generally accepted and is the framework that unifies our knowledge of the structure and composition of the Moon. The intention of this chapter is to review the major advances that have been made over the past decade regarding the constitution of the Moon’s interior. Much of this new knowledge is a direct result of data acquired from the successful Clementine and Lunar Prospector missions, as well as the analysis of new lunar meteorites. As will be seen, results from these studies have led to many fundamental amendments to the magma ocean model.

Much of what we know from sample analyses has been previously summarized elsewhere, and only their most important aspects will be discussed in this chapter. The reader is referred to the relevant chapters in the books Basaltic Volcanism on the Terrestrial Planets (Basaltic Volcanism Study Project 1981), The …

The Constitution and Structure of the Lunar Interior

Theory and observations point to the occurrence of magma ponds or oceans in the early evolution of terrestrial planets and in many early-accreting planetesimals. The apparent ubiquity of melting during giant accretionary impacts suggests that silicate and metallic material may be processed through multiple magma oceans before reaching solidity in a planet. The processes of magma ocean formation and solidification, therefore, strongly influence the earliest compositional differentiation and volatile content of the terrestrial planets, and they form the starting point for cooling to clement, habitable conditions and for the onset of thermally driven mantle convection and plate tectonics. This review focuses on evidence for magma oceans on planetesimals and planets and on research concerning the processes of compositional differentiation in the silicate magma ocean, distribution and degassing of volatiles, and cooling.

Magma oceans in the inner solar system

The model of lunar evolution in which the anorthositic plagioclase-rich oldest crust of the moon is formed over a period of 300 Myr or less by crystallization as it floats on a global ocean of magma tens or hundreds of km thick is examined in a review of petrological and theoretical studies. Consideration is given to the classification of lunar rocks, the evidence for primordial deep global differentiation, constraints on the depth of the molten zone, the effects of pressure on mineral stability relationships, mainly-liquid vs mainly-magmifer ocean models, and the evidence for multiple ancient differentiation episodes. A synthesis of the model of primordial differentiation and its aftereffects is presented, and the generalization of the model to the earth and to Mars, Mercury, Venus, and the asteroids is discussed.

The magma ocean concept and lunar evolution

A chemical model for generating the sources of mare basalts: Combined equilibrium and fractional crystallization of the lunar magmasphere

The mineralogy and petrology of the Apollo 11 rocks are consistent with impact melting
of ilmenite and pyroxene crystals plus liquid derived from fractional crystallization of basaltic magama.
The complementary plagioclase accumulate should exist in the highlands. The ferrobasaltic magma is
derived from a differentiated hot moon of modified chondritic composition. The one-sided distribution
of large irregular seas is explained by tidal attraction of the last fraction of liquid to the
near side of the moon and release of liquid by meteorite impact.
The vesicular ferrobasalt lavas contain ilmenite, clinopyroxene and plagioclase and have textures
similar to some terrestrial basalts. Delayed appearance of plagioclase indicates an unusual source of
the magma. The iron-enrichment of the coarser microgabbros is extreme at the end of crystallization
resulting in a new iron metasilicate, pyroxferroite.
The oxygen fugacity at 1050°C of these rocks indicated by the composition of opaque oxides and
troilite-iron intergrowths is 10^(-14)-10^(-16) compared to 10^(- 11) for terrestrial basalts and about 10^(16) 
for ordinary chondrites. Absence of hydrous minerals indicates loss of volatiles at some stage in
the moon's history.
The breccias and soil have the same mineralogy modified by shock. Plagioclase vitrophyres are
probably melted cumulates and may derive from the highlands. Lithification of the breccia results
principally from welding of debris discharged from a hot gas cloud created by meteorite impact.
Small glass spheres have surface features consistent with a fiery rain of boiling silicate liquid rounded
by surface tension and later impacted by high-velocity micrometeorites.
Melting experiments of synthetic material of mean Apollo 11 rock composition revealed low
liquidus temperature, delayed appearance of plagioclase and narrow crystal-liquid interval. These
support the concept of advanced fractional crystallization of a basaltic liquid under reducing conditions leading to high iron enrichment. Flotation of plagioclase and sin king of ilmenite and pyroxene
should occur in the liquid basalt.
We propose that meteorite impact blasted away a plagioclase-rich crust, melting a mixture of
fractionated basalt liquid and ilmenite plus pyroxene crystals. The new liquid formed the Sea of
Tranquillity and yielded the near-surface lava flows represented by the ferrobasalts and microgabbros.
This new liquid could yield plagioclase only after the extra ilmenite and pyroxene had
crystallized.
We propose that the original basalt liquid was derived by fractional crystallization of a molten
moon of modified chondritic composition yielding a metallic core surrounded by pressure-stable
Mg-rich o li vine and pyroxene. The fractionated liquid became Fe-rich resulting in an inverse density
stratification. Primitive ultra basic crust should occur in the highlands, along with dominant plagio clase-
rich cumulates. The temperature-time relations of the model are discussed qualitatively. Key factors are early
removal of radioactive material from the center by fractional crystallization and possible enhancement
of radiative heat transfer in volatile-free silicates. The moon rocks should be more refractory
and rigid because of the low volatile content.

Petrologic history of the moon inferred from petrography, mineralogy, and petrogenesis of Apollo 11 rocks

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Petrologic history of the moon inferred from petrography, mineralogy, and petrogenesis of Apollo 11 rocks