WASHINGTON - Researchers at Rice University and Harvard University, US, have developed a new model to explain how noble gases - elements like helium, neon and argon - are lost from the Earth’s interior during mantle convection.
The research takes aim at a question that has vexed geoscientists for years: how to reconcile leading theories about the convection of Earth’s mantle with observations of ancient noble gases in volcanic rocks.
“Most existing models find that convection should have left the mantle extensively depleted in ancient noble gases, unless part or all of the lower mantle has been somehow isolated,” said study co-author Helge Gonnermann, assistant professor of Earth science at Rice.
“We set out to see if there was a mechanism that could both preserve ancient noble gases in the lower mantle and still be consistent with the existing framework for whole mantle convection,” he added.
In the new research, Gonnermann and longtime collaborator Sujoy Mukhopadhyay, a Harvard geochemist, developed a model that could reconcile convection involving the lower mantle with the helium-3 measurements found in ocean island basalts.
The model suggests that both the upper and lower mantle are involved in convection, but it affects them in different ways.
Whereas the upper mantle has been extensively degassed through repeated tectonic cycling, the lower mantle has been recycling approximately once during the past 4.5 billion years.
Continuous mixing of subducted plates into the lower mantle has been diluting the concentrations of ancient noble gases there.
Instead of extracting ancient noble gases at their original concentrations, progressively smaller amounts are extracted at any given rate of tectonic cycling.
Consequently, about 40 percent of the ancient helium-3 can still be present in the lower mantle, even though it may have undergone one complete tectonic cycling over the past 4.5 billion years.
“Contrary to the conventional view that tectonic cycling of the lower mantle should result in extensive mixing between the lower and upper mantle, thereby erasing any differences in helium-3, we find that much of the tectonic cycling of the lower mantle essentially bypasses the upper mantle,” Mukhopadhyay said.
“What goes down must come up: Slabs that subduct and mix into the lower mantle are balanced by mantle plumes, rich in helium-3, which rise from the lower mantle to the Earth’s surface without mixing significantly as they traverse the upper mantle,” he added. (ANI)
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