Highlands

The lunar highlands are the pale, elevated regions around the giant basins that are filled with dark basalt. Since the giant impacts basins are formed within the highlands, the highlands are necessarily older than the basins.Their composition was unknown until the Apollo missions, particularly Apollo 16, which returned a large quantity of samples from a part of the highlands.Their composition is truly intriguing to planetary scientists. All the rocks retrieved are igneous, that is, they crystallized from a liquid silicate magma. Unlike the dark basalts, a common lava composition on many planets (the Earth,Venus, Mars, and the Moon), the highlands are composed of a number of more exotic rock types.

The palest rocks on the Moon are anorthosites. These igneous rocks are composed of 90 percent or more of plagioclase, as described above in the section on the lunar magma ocean. The rocks are called anorthosites because their plagioclase is of a particular composition called anorthite, the calcium-rich type of plagioclase (CaAl Si O ). Anorthite requires a strange composition of magma to crystallize, and anorthosites on Earth are rare.The magma needs to be highly enriched in aluminum and in calcium to make anorthite, but more problematically, the magma has to be very poor in iron and magnesium to produce a rock with 90 percent or more anorthite. With more iron and magnesium, minerals such as olivine and pyroxene will also crystallize from the magma, reducing the percentage of anorthite. Unfortunately, the production of such a magma is a major problem to petrologists, the branch of earth scientists who specialize in minerals and rock compositions.

Assuming the Moon began with a magma ocean requires that the initial magma ocean have a high iron and magnesium composition: It was likely close to the composition of primordial Earth material, minus a large portion of its iron, which sank to depths to form the core. When this initial magma begins to cool it will produce olivine crystals, which are rich in iron and magnesium themselves. Each batch of minerals that crystallize from the magma ocean and settle to the bottom change the composition of the remaining magma a little bit: If the olivine incorporates a lot of magnesium, then the magma left behind will be somewhat depleted in magnesium.

Generations of petrologists have done experiments with rocks and have developed the thermodynamic rules that govern the order of minerals that crystallize from a magma ocean, along with their compositions. The order of minerals and the evolution of the remaining liquid is now known in some level of detail. After olivine and pyroxene have crystallized from the magma ocean to the point that about 70 percent of the magma ocean is solid, the remaining liquid has become enriched enough in aluminum and calcium that plagioclase begins to crystallize.

This is the moment that scientists wait for to make the anorthosite highlands: Once plagioclase begins to crystallize, it can float and make the highlands. Pyroxene and olivine are also still crystallizing from the magma ocean, preventing the plagioclase from ever attaining 90 percent of the crystallizing minerals. There is no magma composition, in fact, at any point in the evolution of a magma ocean that would simply produce a rock with 90 percent anorthite. This problem makes the idea of a magma ocean all the more compelling: If there is no magma that can crystallize 90 percent plagioclase, then the plagioclase has to simply float to the top of the crystallizing magma ocean, separating itself from the more iron- and magnesium-rich minerals. Marc Norman at the Australian National University, Lars Borg at the Institute for Meteorites at the University of New Mexico, and colleagues at the Johnson Space Center have been determining the formation ages of anorthosites from the Apollo 16 landing site.This site is the only one that resembles the large anorthositic (feldspar-rich) regions of the lunar highlands, so detailed study of the anorthositic rocks from the site can shed light on the formation of the oldest lunar crust.The four anorthosites dated so far have ages determined by the samarium-neodymium radiometric system of 4.29 to 4.54 billion years.

Norman noted that the plagioclase in the rocks has been composi-tionally modified by exchange after crystallization, most likely by a large impact event around 3.9 billion years ago, but that pyroxene is more likely to have remained unchanged. The pyroxene isotopic data alone from all four rocks define a precise age of 4.456 billion years, well within the possible time of magma ocean crystallization. He suggested that this is a robust estimate for the crystallization age of lunar anorthosites.

Borg, however, is convinced by the care taken with the anorthosite dates and states that the young age of 4.29 billion years is a good date describing a crystallization age.Therefore there may be two problems with the plagioclase flotation model. First, the anorthosites with radiometric dates of 4.29 billion years are far younger than any likely magma ocean would have survived. If those dates are correct, then those anorthosites must have been made in an event that came later than the magma ocean. The total time needed for crystallizing the magma ocean is difficult to calculate, but is highly unlikely to be more than 50 million years. Some scientists estimate complete crystallization in as little as a few million years.These anorthosites are younger than the last possible magma ocean liquids by over 100 million years. Second, by the time plagioclase began to crystallize from the magma ocean, the entire remaining ocean was filled with crystals and had formed a sort of stiff crystal mush. Plagioclase is physically unlikely to have been able to float out of such a dense stiff mush. On Earth in

A time line of early events on the Moon shows the progression from early anorthosite crust formation to more iron-rich crustal materials to the eventual eruption of the mare basalts and fire-fountaining volcanic glasses.

such cases plagioclase forms clumps with dense pyroxene and olivine crystals and sinks under the influence of their density. Because of the late ages, shown in the figure here called "Time Line of Early Lunar Events," and because of the physical constraint of a highly crystalline magma ocean, some scientists are suggesting that the anorthosite highlands formed in a second event.

As minerals crystallize from the magma ocean they preferentially take magnesium out of the remaining magma, causing it to be relatively enriched in iron. Eventually the magma is sufficiently depleted in magnesium that the minerals are forced to incorporate more iron, becoming increasingly iron-rich. As the minerals become more iron-rich, they become more dense, since iron is denser than magnesium.

A Time Line for Early Lunar Events

"O Anorthosites

Magneskium suite of crustal rocks _

Peak/end of the Late Heavy Bombardment

Magneskium suite of crustal rocks _

r Alkali suite of crustal rocks

Formation of the Moon 10 to 30 Ma after 4.56

Crystallization of the magma ocean complete 10 to 50 Ma after formation

□ Apollo 14 aluminous basalts C

KREEP basalts

I Very low titanium basalts

□ Apollo u high titanium basalts

Apollo 15 and 12 low titanium basalts 1

□ Apollo rj high titanium basalts

4.56

Billions of Years before Present

Their increasing density means that the last minerals to crystallize from the magma ocean, near the top, are far denser than those below them. These dense minerals are driven by gravity to sink as blobs through the more buoyant minerals underneath.These cumulate minerals sink and rise as solids, flowing slowly by human standards, but completing their overturn in about 1 million years (for more, see the sidebar "Rheology, or How Solids Can Flow" on page 38).

As deeper cumulates rise toward the surface through buoyancy, they can also melt through decompression as they come closer to the surface (see the sidebar "What Is Pressure?" on page 52).This may be the process that forms these younger anorthosites: Melting during magma ocean overturn. Overturn may also produce some other enigmatic rocks found in the lunar highland crust, known as the mafic- and alkali-rich suites (the word suite is used for a group of related rocks). These are also intrusive igneous rocks; that is, they cooled within the crust rather than erupting onto the surface as volcanics.The mafic-rich suite has more magnesium- (ma) and iron-rich (fic, fromferrous, meaning iron-rich) minerals than do the anorthosites, and the alkali-rich suite is enriched in sodium and potassium relative to other highlands rocks. Many of these are younger than the anorthositic highlands, as shown in the figure on page 134. Many of the mafic and alkali suit rocks have ages far younger than the likely completion of magma ocean crystallization, and may therefore be another indicator for melting during magma ocean overturn.

No matter what their age, the lunar anorthosites are compelling evidence for the existence of a magma ocean early in lunar evolution. They may have formed through flotation from the crystallizing magma ocean, or they may have formed by melting during magma ocean overturn. By 150 million years after formation of the Earth and Moon, at the latest, the Moon had a fully formed plagioclase-rich crust, much of which can be seen today.

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