Discovery of the presence of 'fossil' gases trapped in the earth's mantle which keep traces of the evolution of our planet. These are the results of a study, signed by INGV, CNRS and Université de Lorraine, of gases of magmatic origin emitted in the volcanic region of the Eifel in Germany, recently published in Nature
The birds trapped in the earth's mantle keep traces of the evolution of our planet, a part of which comes from asteroids, ie from planetary bodies whose remains are now located between Mars and Jupiter. Conversely, the noble gases in the atmosphere have a different cosmochemical origin, probably from comets. These are the results that emerged from the study, Chondritic xenon in the Earth's mantle,
(http://www.nature.com/nature/journal/v533/n7601/full/nature17434.html), conducted by the National Institute of Geophysics and Volcanology (INGV), Center National de la Recherche Scientifique (CNRS) and Université de Lorraine, on magmatic gases sampled in the Eifel volcanic complex (Germany), helping to reveal the origin of Volatile Elements on Earth, recently published in Nature.
“The origin of volatiles (eg gases) on Earth”, explains Antonio Caracausi, a researcher at INGV, “is a subject of considerable debate in planetary sciences. The atmosphere and oceans may have been severely perturbed both by the loss of volatiles into space and by their addition from volatile-rich bodies (such as asteroids, comets): which means that the current compositions are not representative of the original components. However, gases trapped in the Earth's mantle may have been isolated from these events, keeping track of Earth's evolution."
The team of researchers studied the isotopic composition of xenon (Xe) in gas samples of magmatic origin. The noble gases, including xenon and its nine isotopes, are chemically inert elements that can be used to trace the physical processes, without even having to consider chemical reactions, which take place inside the Earth (core, mantle, crust) and in the atmosphere.
“This study also gave the opportunity to highlight that the volcanism of the Eifel region has its origin precisely in the deep mantle which has been isolated, for most of the earth's history, despite convective processes in the earth's mantle. The discovery of the origin of these birds, which can also be defined as fossils, contributes to a better understanding of the origin of water and other life-supporting species on Earth”, continues Caracausi.
The Earth formed about 4,54 billion years ago. Since that time, our planet has been continuously evolving, experiencing numerous events in its history, including core formation, mantle differentiation, oceanic and continental crust production, crustal recycling into the mantle, new additions of volatiles from comets and/or asteroids.
Because of these many processes, the origin of the Earth's volatile elements (hydrogen, water, carbon, nitrogen, and noble gases) remains enigmatic, and it is difficult to determine whether they originated from gas, which remained trapped after the formation of the system. solar (solar nebula), or from differentiated bodies, such as asteroids, which may have collided with the proto-Earth.
In this study, the isotopic abundances of the noble gas xenon were analyzed in samples of CO2-rich gas (99,8%), emitted in the volcanic region of the Eifel (Germany) (photos 1 and 2). Thanks to advanced purification techniques, it has been possible to separate Xe from CO2 and other volatile components, thus allowing its abundance and isotopic composition to be measured with high precision. The researchers ascertained that the light isotopes of Xe (124-128Xe) in the Earth's mantle were brought to Earth by asteroid-related bodies. Instead, the Xe in the atmosphere originates from a different source (still not well identified). Hence the idea that at least two distinct sources have contributed to the origin of volatile elements on Earth, one for the mantle, another for the atmosphere. The heavy isotopes of Xe (129Xe and 131-136Xe), the excesses of which are produced by radioactive decay, in the gas samples taken in the Eifel region show that this gas is "very old" and that their source remained isolated from the rest of the mantle about 4,45 billion years ago. Consequently, the mantle in this region was able to preserve the original isotopic signal, despite the convective processes.
"Thanks to a numerical calculation model", concludes Caracausi, "it was possible to quantify the different contributions from radioactive decay and the results obtained are in agreement with previous studies on volatiles emitted in other oceanic volcanic regions, characterized by the presence of Plume in the earth's mantle . Identified as rising portions of the deep mantle to reach the surface of the Earth, these plumes are responsible for important volcanic edifices such as the Hawaiian Islands and Iceland. The new results suggest that a mantle plume is present in central Europe and has influenced the magmatism of the Eifel.
Links to images
Photo 1: Victoriaquelle site (Germany)
Photo 2: view of the inside of the well. Intense emission of gas bubbles in water and intense CO2 outgassing can be observed. The gases are sampled through a funnel immersed in water, which is connected to a pipe and in turn to pre-evacuated steel samplers
Photo 3: A picture of the forest near the sampling site in Germany