14 DAY TRIAL // Ever since the creation of the Earth's convection model, which basically holds that rocks in the planet's mantle constantly move and interact on account of the large temperatures and pressures they are subjected to, geologists have had a hard time combining it with the fact that they found noble gases in volcanic rocks. According to existing convection theories, the core of the planet should have long ago been depleted of noble gases, such as helium, neon and argon. Yet, they are found in volcanic rock. A new study, published in this week's issue of the journal Nature, attempts to explain the paradox.
According to researchers at the Rice University and the Harvard University, the gases should have been lost from the Earth's center during convection processes. “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. 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,” Rice University Assistant Professor of Earth Sciences Helge Gonnermann, who is also the co-author of the Nature paper, explained.
The entire surface of the planet is covered with a thin layer of rock on the outside, known as the lithosphere, which basically floats above the upper mantle. The layer is made up of inter-locking tectonic plates, which constantly interact with each other, forming either volcanoes, mountain ranges and volcanic islands, or new crust, depending on whether they collide, or pull apart, respectively. When one plate goes under another, a process known as subduction, the one that sinks may dive vertically towards the core, a find that was made less than 14 years ago.
“This was a real problem because the prevailing view in geoscience was that only the upper mantle was involved in this plate tectonic recycling process. One reason people believed this was because there appear to be relatively high concentrations of ancient noble gases in ocean island basalts, volcanic rocks found at volcanic island chains, such as Hawaii,” the expert added.
“The presence of ancient noble gases in these basalts implies that they have remained locked inside the lower mantle since the Earth formed about 4.5 billion years ago. In contrast, most of these ancient noble gases appear to have leaked out of the upper mantle, because the plate tectonic recycling process allows noble gases to escape with the basalt magma as it continuously forms new ocean crust at mid-ocean ridges,” Gonnermann shared.
According to the scientist, and to collaborator Sujoy Mukhopadhyay, a geochemist at Harvard University, both the upper and the lower mantles are involved in the convection processes, but each of the two layers is affected differently. The upper mantle has been losing gas since the forming of the planet, while the lower one has been recycling it about once in 4.5 billion years, the new model holds.
“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. 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,” Mukhopadhyay concluded.