The magma ocean

The geochemical evidence is decisive that at least half and probably the whole Moon was molten at or shortly following its accretion. This stupendous mass of molten rock is referred to as the ''magma ocean'' and a very energetic mode of origin of the Moon, such as delivered by the giant impact hypothesis, is required to account for it.

The concept of the magma ocean has proven robust. Several decisive pieces of evidence require that the Moon was mostly melted at or shortly following accretion. The first is the presence of the 50-60 km thick anorthositic crust. This volume of feldspar with 36% Al2O3 requires the concentration in the crust of around 50% of the Al (and Eu) in the Moon.

The second is the high concentration of many incompatible elements at the lunar surface. The near-surface concentrations of aluminium, europium and of the many incompatible elements in KREEP constitute a significant proportion of the entire lunar budget for these elements. Their abundances are not related to element volatility but are due to crystal-liquid fractionation and so indicative of an internal origin. This near-surface concentration occurred shortly after the formation of the Moon so requiring a catastrophic event rather than the slow uniformitarian processes that resulted in the similar concentrations of incompatible elements in the continental crust of the Earth [22]. (Fig. 3.3)

The third point is that the mare basalts that are derived from the deep lunar interior are all depleted in europium. As discussed later, early crystallization of the plagio-clase that floated to form the highland crust, depleted the mantle, the later source of the mare basalts, in europium [23]. Finally all these events happened within a short timeframe so that the Moon formed a thick crust closely following its formation. The Earth in contrast has taken four Gyr to build a much smaller crust relative to planetary volume.

The crystallization of such a large body is difficult to constrain from our limited terrestrial experience. A reasonable scenario is that olivine and orthopyroxene were the first minerals to crystallize. These dense phases sank to form a zone of deep cumulate minerals. The aluminium and calcium content of the magma increased as a result of the removal of the phases dominated by iron and magnesium so that plagioclase crystallized. This phase was less dense than the bone-dry melt and floated, forming ''rockbergs'' that eventually coalesced to form the lunar highland crust, apparently around 4460 Myr [24]. (Fig. 3.4)

The first-order variation in thickness from nearside to farside is probably a relic of primordial convection currents in the magma ocean. Excavation by large basin impacts has subsequently imposed additional substantial variations in crustal thickness. The magma ocean was enriched in calcium and aluminium over typical terrestrial values, a conclusion from the Apollo samples that was reinforced by the Galileo, Clementine and Lunar Prospector data. The implication is that the Moon was enriched in these and other refractory elements compared to our estimates of the terrestrial mantle (Chapters 2, 8).

As crystallization of the magma ocean proceeded, those elements unable to enter the major mineral phases olivine, ortho- or clinopyroxene, plagioclase, and ilmenite were concentrated in a near-surface residual melt zone (KREEP), that eventually constituted about 2% of the volume of the magma ocean. Most of the incompatible trace elements such as K, REE (less Eu), P, Th, U, Ba, Zr, Hf, and Nb, were concentrated in the residual melt. Many of these elements are concentrated in near-surface rocks by factors of a thousand relative to bulk-Moon or primitive nebular values. This again constitutes evidence for large-scale lunar melting as these elements have to be sequestered from a large volume of the Moon on a short timescale [12]. The uniformity of elemental ratios in KREEP and the concordant model ages around 4360 Myr also argue for a single-stage process and so for a magma ocean rather than a disconnected series of igneous events.

Mare basalt

Mare basalt

Fig. 3.4 Two alternatives for the internal structure of the Moon. On the left, only half the Moon was melted and differentiated and the deep interior has primitive lunar composition. Some partial melting has occurred due to the presence of K, U and Th. On the right, the Moon was totally melted and differentiated, forming a small metallic core.

Fig. 3.4 Two alternatives for the internal structure of the Moon. On the left, only half the Moon was melted and differentiated and the deep interior has primitive lunar composition. Some partial melting has occurred due to the presence of K, U and Th. On the right, the Moon was totally melted and differentiated, forming a small metallic core.

Possibly a small iron core that incorporated the siderophile elements formed in the center of the Moon. The crystallization of the magma ocean was probably asymmetric, as shown by the variations in crustal thickness and the apparent concentration of the residual KREEP melt under the lunar nearside. Crystallization of the main mineral phases (olivine, pyroxenes, ilmenite) in the lunar mantle was complete, in most interpretations of the isotopic data, by about 4400 Myr and the final KREEP residuum was solid by about 4360 Myr [25].

The crystallization sequence portrayed here was far from peaceful. During all this time, the outer portions of the Moon were subjected to a continuing bombardment, which broke up and mixed the various components of the highland crust. Thus although the lunar crust is often thought to be formed simply from the accumulation of feldspar crystals floating on the magma ocean, the crust is much more complex as discussed in Chapter 2. During its formation, a massive bombardment continued, producing impact melts, mixing and pulverizing the crust and mixing in the KREEP component.

The solidification of the magma ocean was followed by the formation of the enigmatic Mg-suite. As discussed in Chapter 2, the origin of this suite is unclear as its unrelated members combine extremely primitive Mg/(Mg + Fe) ratios with high concentrations of incompatible trace elements (KREEP) typical of highly evolved magmas. An origin by mixing (from impacts?) is indicated by the REE patterns, that parallel those of the other highland rocks. This observation raises serious problems for the popular notion of serial magmatism.

Probably some local overturning of the deeper cumulate pile may have occurred, but such events did not homogenize the interior. As we have seen, this region eventually produced a wide variety of mare basalt compositions.

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