Research may solve Moon’s volcanic fire-fountain mystery

Credit: NASA/GSFC/Arizona State University

Some regions of the Moon exhibit dark mantling deposits that were formed by fire-fountain style eruptions, similar to Strombolian or some Hawaiian eruptions. This image is centered on a small impact crater (~170 meters in diameter) located within a larger regional pyroclastic deposit (5.470°N, 352.014°E) south of Sinus Aestuum. Since the crater has excavated fresh pyroclastic material from below, the freshly-exposed pyroclastic appear darker than the surrounding surface, possibly as a result of eject emplacement from impacts (both near and far afield) that deposited higher-reflectance material.
Fire fountains, a kind of eruption that happens frequently in Hawaii, need the presence of volatiles mixed in with the erupting magma. Volatile compounds turn into vapor as the lavas rise from the depths. That expansion of that gas initiates lava to blast into the air once it touches the surface, a bit like taking the lid off a shaken bottle of soda.
Alberto Saal, associate professor of Earth, environmental, and planetary sciences at Brown University and corresponding author of the new research said “The question for many years was what gas produced these sorts of eruptions on the Moon. The gas is gone, so it hasn’t been easy to figure out.”
The study, published in Nature Geo-science, submits that lava correlated with lunar fire fountains contained important amounts of carbon combined with oxygen to make considerable amounts of carbon monoxide (CO) gas. CO gas was responsible for the fire fountains that sprayed volcanic glass over parts of the lunar surface.

Credit: Saal Lab / Brown University

Melt inclusions are tiny dots of magma solidified within olivine crystals. The minerals lock in volatile elements that may have otherwise escaped from the lava. Scientists have shown that melt inclusions within volcanic glasses from the Moon contain carbon. They conclude that gas-phase carbon likely drive the “fire fountain” eruptions the produced the glass.
For this research, Saal and his coworkers gently examined glass beads brought back to Earth from the Apollo 15 and 17 missions. In particular, they looked at samples that contained melt inclusions, tiny dots of molten magma that became trapped within crystals of olivine. The crystals trap gases present in the magma before they can escape.


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