MOON ROCKS

Highland rocks, the lunar magma
ocean, and maybe a cataclysm

Strange as it may seem, the first highland rocks were collected during the first lunar landing, the Apollo 11 mission, which landed on a mare, Mare Tranquillitatis. Although most of the rocks collected were, indeed, basalts, some millimeter-sized rock fragments were quite different. They were composed chiefly of the mineral plagioclase feldspar; some fragments were composed of nothing but plagioclase. [See the 'Rock ABCs Fact Sheet' on Page 19.] Such rocks are called anorthosites. Some scientists suggested that these fragments were blasted to the Apollo 11 landing site by distant impacts on highland terrain. Thus, they argued, the highlands are loaded with plagioclase. This was a bold extrapolation confirmed by subsequent Apollo missions to highland sites.

But this was not enough for some scientists. If the highlands are enriched in plagioclase, how did they get that way? One way is to accumulate it by flotation in a magma (molten rock). This happens in thick subterranean magma bodies on Earth. So, plagioclase floated in a magma. But if ALL the lunar highlands are enriched in plagioclase, then the magma must have been all over the Moon. The early Moon must have been covered by a global ocean of magma, now commonly referred to as the lunar magma ocean. Although some scientists still remain unconvinced about the veracity of the magma ocean hypothesis, nothing we have learned since has contradicted the idea that 4.5 billion years ago the Moon was covered by a layer of magma hundreds of kilometers thick. The idea has been extended to the young Earth as well, and even to Mars and some asteroids. And all this sprung forth because creative and bold scientists saw special importance in a few dozen white fragments of anorthosite strewn about in a pile of charcoal gray lunar regolith.

The magma ocean concept was tested by the 1994 U.S. Clementine Mission to the Moon. Clementine was in a pole-to-pole orbit for two months, during which it took thousands of photographs in several wavelengths. Scientists at the University of Hawaii developed a method to determine the iron content of the lunar surface from ratios of the intensity of light reflected in different wavelengths. The magma ocean hypothesis predicts that the lunar highlands should have low iron contents, less than about 5 wt. % (when recorded as iron oxide, FeO). According to Clementine measurements, the highlands average slightly under 5 wt. % FeO, consistent with the magma ocean idea. Further refinement of this test is underway using data from Clementine and the forthcoming U. S. Lunar Prospector Mission, scheduled for launch in early 1998.
The highlands also contain other types of igneous rocks. The most abundant are called norites and troctolites, rocks composed of equal amounts of plagioclase and either olivine or pyroxene (both silicate minerals containing iron and magnesium). Age dating suggests that these rocks are slightly younger than the anorthosites and formed after the magma ocean had crystallized.

Highland rocks are difficult to work with because all that cratering, so evident in photographs of the highlands, has taken its toll on the rocks. Most highland rocks are complex mixtures of other rocks. The original igneous rocks have been melted, mixed, smashed, and generally abused by impacts during the Moon’s first half billion years. We call these complicated rocks breccias. Some are so mixed up that they contain breccias within breccias within breccias. Most of the anorthosites, norites, and troctolites are actually rock fragments inside breccias. Separating them out is painstaking work.

An interesting thing about highland breccias, especially those we call impact melt breccias (rocks partly melted by an impact event), is that most of them fall into a relatively narrow span of ages, from about 3.85 to 4.0 billion years. This has led some scientists to propose (boldly againÐlunar scientists don't seem to be timid!) that the Moon was bombarded with exceptional intensity during that narrow time interval. If it happened, it probably affected Earth as well, perhaps leading to production of the first sedimentary basins, and possibly inhibiting the formation of the first life on this planet or harming whatever life had developed by four billion years ago. This idea of a cataclysmic bombardment of the Moon is not yet proven. It could be that the apparent clustering in rock ages reflects poor sampling—we may only have obtained samples from one or two large impact basins. The idea can be tested by obtaining samples from many more localities on the Moon.

Many highland breccias and a few igneous rocks are enriched compared to other lunar samples in a set of elements not familiar to most of us. The elements are those that tend not to enter the abundant minerals in rocks. The result is that as a magma crystallizes the part that is still liquid becomes progressively richer in these special elements. The rocks that contain them are called KREEP, for potassium (chemical symbol K), rare-earth elements (abbreviated REE), and phosphorus (P). Most Moon specialists believe that KREEP represents the last dregs from the crystallization of the magma ocean. Huge impacts dug down to the lower crust of the Moon and excavated it, mixing it with other debris to form KREEPy breccias.

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