Diamond is one of the most treasured minerals, both for its unique properties and for its scarcity. It is no surprise that there are very deep diamond reserves in the Earth’s mantle; in fact, a study published just over two months ago in the journal Science Advances concluded that most diamonds are made from recycled marine material. But what is striking about this new discovery is its age: a reserve of diamonds that has been practically intact since the formation of the planet.
Scientists already suspected that there would have to be an area of the Earth’s mantle, somewhere between the crust and the core, that should contain a large reservoir of rock. Now, an international team of scientists has managed to measure helium isotopes contained in super- deep diamonds brought to the surface by violent volcanic eruptions, to detect the traces of this ancient deposit, hidden more than 410 kilometers below sea level in the mantle of the earth.
After the formation of Earth, violent geological activity and extraterrestrial impacts disrupted the young planet, meaning that almost nothing of Earth’s original structure remains.
But in the 1980s, some geochemists found basalt lavas with helium 3 and helium 4 isotope ratios much higher than they expected; that is, these basalt lavas reflected the same proportion of isotopes as very old meteorites that had changed the structure of the Earth. Stable isotopes do not decompose, since their measurement is a geological marker, which indicates the age of these basaltic lavas: they were ancient materials, from the violent stage of bombardment of extraterrestrial rocks that hit the Earth in its early stages of formation. ; and whose composition had not changed in the last four billion years.
Research leader Dr Suzette Timmerman of the Australian National University explains in a press release: “This pattern has been observed in” Ocean Island Basalts, which are the lavas that rise to the surface from the depths. of the Earth and form islands like Hawaii and Iceland. The problem is that, although these basalts do surface, we can only take a small look at their history. We don’t know much about the mantle from which their melts come. “
Electron microscope images of diamonds from the Juina area, Brazil / Credit: Suzette Timmerman.
To address this problem, Timmerman’s team looked at helium isotope ratios in super-deep diamonds. Most diamonds form between 150 and 230 km below the earth’s crust. Very occasionally, some ‘super-deep’ diamonds (created between 230 and 800 km below the Earth’s surface) rise to the surface. These super deep diamonds are recognizably different from normal diamonds.
Diamonds are the hardest and most indestructible natural substance known , so they form a perfect time capsule that provides us with a window into the characteristics of the deep Earth. As Timmerman recounts, his team was able to extract helium from 23 super-deep diamonds in Juina, an area of Brazil. These showed the characteristic isotopic composition that we would expect from a very old deposit, confirming that they are remnants of a time equal to or even before the formation of the Moon.
A note: about four and a half billion years ago, when the Earth was newly formed, another planet called Teia, presumably the size of Mars, collided with the newborn Earth , amid the violent chaos of rocky planet formation. Of the solar system. This caused the materials of Earth and Teia to melt, reshaping our planet; and the debris remained orbiting the Earth until it formed the body that we know today as the Moon.
Where exactly is this reserve located? From the geochemistry of these diamonds, we also know that they formed in an area called the “transition zone”, which is between 410 and 660 kilometers below the Earth’s surface. This means that this deposit, left over from this violent beginning of the Earth, must be in this area or below it.
However, some questions remain about this ancient diamond reserve: is it a single large reserve, or are there other smaller and equally old ones? What is its exact chemical composition? Suzette Timmerman and her team have no answer at this time. But at least this work allows them to focus on what researchers have called “the oldest material that remains relatively intact on Earth.”
This work will be presented to scientists for the first time on Friday, August 23 at the Goldschmidt conference in Barcelona, after its publication (August 15) in the journal Science.
Source : S. Timmerman el al., Primordial and recycled helium isotope signatures in the mantle transition zone , Science (2019). DOI: 10.1126 / science.aax5293
