The lunar crust formed from frozen magma

Detailed study of the Moon by scientists dates back to the Apollo missions when astronauts brought rock samples from the lunar surface back to Earth for analysis. Apollo 11 collected samples from the lunar mountainous regions, the pale areas on the Moon’s surface easily visible from Earth. The highlands are made of a relatively light rock called anorthosite, which formed early in the Moon’s history, between 4.3 and 4.5 billion years ago.

There is a mystery involved in the formation of anorthosite on the Moon. The age of the anorthosite uplands does not match the time it took for the Moon’s magma ocean to cool. But the scientists behind a new study believe they have solved that mystery.

Geologists are interested in lunar anorthosites because of the uncertainty surrounding their formation. Their formation involves fractional crystallization, where anorthite mineral crystals are pulled out of the magma as they form, lighter crystals rising to the surface. But some details of their training are still unclear.

(Note: Anorthosite is the rock that forms the uplands. Anorthite is a mineral (plagioclase). Anorthosites are therefore rocks that are very rich in anorthite.)

A collision between two protoplanets created the Moon. One of the protoplanets became the Earth, and the smaller one became the Moon. After the crash, the Moon warmed up so much that the entire mantle melted. Geologists call it an ocean of magma.

When scientists studied the Apollo 11 samples from the lunar highlands, the evidence seemed to confirm that the lunar highlands anorthosites had formed by fractional crystallization. Light anorthite crystals rose to the top of the magma oceans comprising the highlands, and heavier crystals sank. The Highlands are over 90% anorthite.

But there are problems with this explanation, and two scientists think they have a better answer. They presented their work in a paper titled “Formation of lunar primary crust from an ocean of long-lived melting magma.” The Geophysical Letters of the AGU published the article by researchers Chloé Michaut and Jerome A. Neufeld.

“Since the Apollo era, the lunar crust has been thought to have been formed of lightweight crystals of anorthite floating on the surface of the ocean of liquid magma, with heavier crystals solidifying on the ocean floor,” said the co-author Chloé Michaut from the Ecole Normale Supérieure. from Lyon. “This ‘flotation’ model explains how the Lunar Highlands may have formed.”

But that conclusion is based on samples from Apollo11, and current researchers have more tools and evidence at their disposal. Analysis of lunar meteorites and further analysis of the Moon’s surface contradict the previous conclusion. Lunar anorthosites appear to be more heterogeneous rather than highly fractionated. Anorthites are distributed throughout the rock, but the surface is particularly rich in anorthites. These findings suggest that our understanding of the lunar magma ocean is not complete.

So what happened in the ancient ocean of magma to create these heterogeneous anorthosites in the lunar highlands?

One clue to what happened is the disparity between the age range of the anorthosites and the time it took for the magmatic ocean to cool. The anorthosites are over 200 million years old, but the ocean solidified in about 100 million years.

“Given the range of ages and compositions of anorthosites on the Moon, and what we know about how crystals are deposited in solidifying magma, the lunar crust must have formed by some other mechanism. said co-author Professor Jerome Neufeld of the Cambridge department. Applied Mathematics and Theoretical Physics.

Michaut and Neufeld developed a mathematical model to identify this mechanism. Their model shows that it is difficult for heavier crystals to settle to the bottom of the Moon’s lower gravity. Convective currents in the magmatic ocean also discourage sedimentation. The research duo discovered that the ocean could have formed a sort of mush, where the crystals remain in suspension rather than settling or rising. They also found that there was a critical threshold. When the crystal content in the grout is above this threshold, the grout becomes more viscous and the deformation slows down.

In their paper, the authors state that “upon reaching this critical crystal fraction, the viscosity of the mixture increases dramatically, which can result in a prolonged oceanic stage of mushy magma.”

In this scenario, the surface of the slurry cools faster than the interior. The result is the anorthite-rich crust we see in the lunar highlands and a more well-mixed, muddy interior.

“We believe it was in this stagnant ‘lid’ that the lunar crust formed, as anorthite-enriched light melt seeped in from the convective crystalline suspension below,” Neufeld said. “We suggest that the cooling of the early magma oceans led to such vigorous convection that the crystals remained suspended as mud, much like the crystals in a mud machine.”

The press release goes on to say that “enriched lunar surface rocks likely formed in magma chambers within the lid, which explains their diversity.”

This research explains the discrepancy between the age of anorthosites and the understood time it took for the ocean of magma to solidify. Rather than taking 100 million years to cool, the magma ocean was a muddy mixture that took more than 200 million years to cool, matching the age of the lunar highland anorthosites.

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