There exists a vast reservoir of primordial magma, undisturbed for more than 4 billion years, near the Earth’s core, according to a study reported in October 2019.
Scientists have hypothesised in the past that a vast reservoir of rock lies in an area of the Earth’s mantle somewhere between the crust and the core, and it has not been disturbed since the creation of the Earth. But they have not known anything about it. The new research measured helium isotopes contained in diamonds from below the Earth brought to the surface by violent volcanic eruptions, to detect the footprints of this ancient reservoir. Helium gas was extracted from 23 super-deep diamonds (claimed to have been formed between 150 and 230 km below the Earth’s crust) from the Juina area of Brazil. It has concluded that gases found in microscopic inclusions in diamonds come from a stable subterranean reservoir at least as old as the Moon, hidden more than 410 km below sea level in the Earth’s mantle. The gas showed the characteristic isotopic composition that was expected from a very ancient reservoir, confirming that the reservoir was a remnant of a very early period.
Scientists, however, still do not know where these ancient domains are in the Earth’s deep interior. No one knows the size of the reservoir, according to the study published in Science. What is the form of this reservoir: is it one large single reservoir, or are there many smaller ones? Nothing is known about the basalts, nor about the mantle from where the melts come from.
Scientists have held that violent geological activity and extra-terrestrial impacts occurred on the Earth in the past, so that almost nothing of the Earth’s original structure remains. But it had also been suspected that there was an area of the mantle somewhere between the crust and core which had been relatively undisturbed. Until now, there was no proof it existed.
However, in the 1980s, geochemists observed that the ratio of the helium 3 isotope to helium 4 isotope was very high (more than what was expected) in some basalt lavas from particular locations; and, interestingly, this isotope ration was to be seen in extremely old meteorites which had fallen on the Earth. This suggested that the lava had carried the material from some kind of ancient reservoir deep in the Earth, with its composition probably remaining unchanged to a large extent in the last 4 billion years. The latest study indicates, on the basis of the geochemistry of the diamonds, that they formed in an area called the ‘transition zone’, which is between 410 and 660 km below the surface of the Earth. This means that the reservoir, left over from the Earth’s beginnings, must be in this area or below it.
The pattern was also observed in Ocean Island Basalts, which are lavas coming to the surface from deep in the Earth, and from islands such as Hawaii and Iceland, said research leader Dr. Suzette Timmerman, from the Australian National University.
Studies have been busy identifying the location of primordial reservoirs in the deep Earth. So now there is a lot of potential to map out where elevated helium 3 isotope/helium 4 isotope domains are located in the Earth’s deep interior. As helium can diffuse rapidly at mantle conditions, a task will be to evaluate whether the ancient helium signature reflects compositions trapped at diamond-formation depths, or the composition of the host lava that transported the diamonds to the surface.
Diamonds
Diamonds are considered the hardest and most indestructible natural substance known. They are therefore seen as a ‘time capsule’ that can provide information about the deep Earth: its content and changes over time deep below the Earth’s surface. They are mostly formed between 150 to 230 km under the crust of the Earth; they are carried to the surface by melts. As compared to diamonds in general, ‘super-deep’ diamonds have been created between 230 and 800 km below the Earth’s surface. They are rarely brought to the surface. The superdeep diamonds can be recognised as different from normal diamonds.