Noble gas dissolves atmospheric theory in air

Melt and volcanism are not enough to release argon

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How did the primeval atmosphere of the earth come about? In the early days of the planet, have all the gases escaped from the then molten rock, or were they 'caught' in the depths as they slowly cooled? An answer to these questions is now provided by an analysis published in "Nature" by American researchers who studied the behavior of the noble gas argon in mantle rock. Their result refutes at least one of the two competing theories about the primeval outgassing.

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Shortly after its formation, the earth was covered by a glowing layer of molten rock that reached several hundred kilometers in depth. As this layer cooled, this process began on the surface and then progressively deepened. But what happened to the gases that the earth still possessed at birth? Were they all repelled at once during the molten stage and formed the primordial atmosphere? Or was this outgassing incomplete, as some geoscientists believe, and some of the so-called primordial gases have been trapped in the rock to this day?

Until recently, it was almost "undecided" between these two theories. But E. Bruce Watson and his colleagues from the Rensselaer research center have now discovered new evidence that at least the noble gas argon is still bound in the minerals of the mantle. It also travels much slower in the rock than previously thought and even volcanic activity is not enough to extract the Argon atoms from their "resting places" in the Earth's mantle. However, the continued presence of such a gas contradicts in any case the theory of the complete outgassing in the youth of the earth and confirms rather that of a continuing "captivity" of the gases in the depths.

"For the theory of deep sequestration to be correct, certain gases would have to remain in the ground despite mantle convection and volcanism, " Watson explains. "Our data suggests that argon actually remains trapped in the mantle, even at extremely high temperatures. This makes it difficult for the underground to get rid of the constant amount of argon produced by the radioactive decay of potassium. "Display

Valuable indicator gases

Argon and other noble gases are valuable indicator gases for the researchers because they are particularly stable and hardly change over long periods of time. Carbon and oxygen, on the other hand, are constantly reconnecting with each other and with other elements, and their concentrations are therefore difficult to understand in the long term. "By measuring the behavior of argon in minerals, we can understand the formation of the Earth's atmosphere and understand how and if complete outgassing has occurred, " Watson said. "We noticed from our first results that the consequences could cause excitement, so we wanted to double and triple secure."

The scientists heated magnesium silicate minerals from the Earth's mantle in an argon atmosphere, simulating the extremely high temperatures inside the Earth. They then examined whether and how quickly the argon atoms migrated from the atmosphere into the rock. The rock took up an unexpected amount of argon, but in extremely slow speed. "Therefore, we can no longer assume that a partially melted region of the shell automatically loses all argon and ultimately all other noble gases, " explains Watson.

Weathering of the upper crust provides argon

But if the melt and later volcanic eruptions are not the source of argon, where does one percent of the noble gas in our atmosphere come from? "We suggest that the release of argon into the atmosphere is due to the weathering of the upper crust and not the melting of mantle rocks, " said the researcher. "The oceanic crust, for example, is constantly being eroded by seawater." And what about the primeval argon that was trapped in the depths of the earth billions of years ago? "Part of it is probably still there, " Watson said.

The new results could also allow us to draw conclusions from our sister planets Mars and Venus. Because both have a mantle rock, which resembles in its composition the earth. This would also allow them to reconstruct how the respective atmospheres formed. We have to fundamentally reconsider our ideas about atmosphere formation, according to the Rensselaer researchers. They will now continue their analysis with other noble gases to further their results.

(Rensselaer Polytechnic Institute, 20.09.2007 - NPO)