Melt inclusions are tiny dots of magma frozen within olivine crystals. The crystals lock in volatile elements that may have otherwise escaped from the magma. Researchers 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. Credit: Saal Lab / Brow University
Scientists have found traces of carbon in volcanic glass collected from the Apollo missions to the Moon. The finding may not only explain the driving force behind ancient ‘fire fountain’ eruptions on the Moon but also suggest that some volatile elements on the Moon and Earth have a common origin.
Fire fountains, a type of eruption that occurs frequently in Hawaii, require the presence of volatiles mixed in with the erupting lava. Volatile compounds turn into gas as the lavas rise from the depths. Expansion of gas causes lava to blast into the air once it reaches the surface. “The question for many years was what gas produced these sorts of eruptions on the Moon,” said A/prof Alberto Saal, “The gas is gone, so it hasn’t been easy to figure out.”
The research suggests that lava associated with lunar fire fountains contained significant amounts of carbon. As it rose from the lunar depths, C combined with O to make substantial amounts CO gas >> responsible for the fire fountains that sprayed volcanic glass over parts of the lunar surface.
Saal and his colleagues carefully analyzed 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. “This breakthrough depended on the ability of Carnegie’s NanoSIMS ion probe to measure incredibly low levels of carbon, on objects that are the diameter of a human hair,” said Hauri.
Using secondary ion mass spectroscopy they calculated the samples contained initially 44 to 64 parts per million carbon. They then came up with a model which showed carbon, as it combines with oxygen to form CO gas, would have degassed before other volatiles.”Most of the carbon would have degassed deep under the surface,” Saal said. “Other volatiles like hydrogen degassed later, when the magma was much closer to the surface and after the lava began breaking up into small globules. That suggests carbon was driving the process in its early stages.”
The amount of carbon detected in the melt inclusions was found to be very similar to the amount of carbon found in basalts erupted at Earth’s mid-ocean ridges. Saal and his colleagues have shown previously that Earth and the Moon have similar concentrations of water and other volatiles + hydrogen isotope ratios from lunar samples are similar to Earth.
Scientists believe the Moon formed when Earth was hit by a Mars-size object very early in its history. Debris from that impact accreted to form the Moon. “The volatile evidence suggests that either some of Earth’s volatiles survived that impact and were included in the accretion of the Moon or that volatiles were delivered to both the Earth and Moon at the same time from a common source – perhaps a bombardment of primitive meteorites,” Saal said. https://news.brown.edu/articles/2015/08/lunar
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