Subduction - the process whereby a tectonic plate slides beneath another and into the Earth's mantle as they converge - could hold clues to the evolution of the planet's atmosphere, as a route for noble gases to reach the Earth's interior. Deep oceanic trenches are common in subduction zones. This image shows the Japan Trench, east of Honshu Island.
Credit: NOAA National Geophysical Data Cente
SYDNEY: A newly discovered mechanism by which gases can travel to the Earth's mantle could lead to a re-evaluation of how the Earth's atmosphere first formed, Australian researchers suggest.
The findings, published this week in the journal Nature Geoscience, detail how noble gases are unexpectedly trapped in rocks, which are then injected into the mantle via subduction - a process that takes place at convergent plate boundaries.
The presence of these noble gases in the mantle, especially neon, defy the previous notion that such elements could have only arrived via a meteorite impact with Earth - a view that has recently been put forth as the sole causal factor for our atmosphere.
Lead author and biologist Mark Kendrick from the University of Melbourne said this was an unexpected result, with subduction having previously been dismissed as a mechanism for the injection of neon into the mantle due to its low solubility in sea-water.
“This finding is important because it was previously believed that inert gases inside the Earth had primordial origins and were trapped during the formation of the solar system,” said Kendrick.
“Our findings throw into uncertainty a recent conclusion that gases throughout the Earth were solely delivered by meteorites crashing into the planet.”
Multiple atmospheres explained
The Earth's first atmosphere was blown away by what is known as the 'giant impact': a collision with another forming planet, the debris of which is said to have formed the Moon and contributed to Earth's metallic core.
Our planet's second atmosphere came when it had cooled enough to form a crust. It is thought that while volcanoes and fissures would have released gases from within the Earth's mantle, most of its atmosphere was delivered by hundreds of thousands of impacts by meteorites, comets, and even small protoplanets.
This theory is supported by the fact that the composition of neon in the Earth's mantle is very similar to that in meteorites.
Challenging old views with plate tectonics
This new study has provided an additional mechanism by which gases can be injected into the Earth's mantle and the evidence may require a re-evaluation of the previous measurements of noble gases in the mantle, which constrain our understanding of how our planet's atmosphere formed.
It was the combination of two processes that lead to the study's intriguing results. The first was the geological phenomenon known as subduction - the process that occurs when two tectonic plates meet and one of the plates slides under the other, driving it down into the Earth's mantle.
The second was the unexpected ability of a special kind of rock known as serpentinite to trap noble gases in its structure. Noble gases are a group elements including helium, neon, argon, krypton, xenon and radon whose distinguishing property is a very low level of chemical reactivity.
This lack of reactivity means that they do not readily form compounds with other chemicals, leading to a difficulty in studying their movement through the Earth's mantle.
'Rubbish' rocks hold clues
Serpentinite accomplishes the tricky task of incorporating the noble gases into its structure due to the comparatively large spaces within its lattice.
Combine the two and you have a new mechanism by which noble gases can be trapped within minerals at the seafloor and subducted into the mantle.
