Analyses show 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.
Scientists have long suspected that an area of the Earth’s mantle, somewhere between the crust and the core, contains a vast reservoir of rock, comparatively undisturbed since the planet’s formation. Until now, there has been no firm proof if or where it exists. Now an international group of scientists has measured helium isotopes contained in superdeep diamonds brought to the surface by violent volcanic eruptions, to detect the footprints of this ancient reservoir. This work will be presented to scientists for the first time on Friday 23rd August at the Goldschmidt conference in Barcelona, after publication (15 August) in the journal Science.
After the formation of the Earth, violent geological activity and extra-terrestrial impacts disrupted the young planet, meaning that almost nothing of the Earth’s original structure remains. Then in the 1980’s geochemists noted that in some basalt lavas from particular locations the ratio of the helium 3 to helium 4 isotope was higher than expected, mirroring the isotope ratio found in extremely old meteorites which had fallen to Earth. This indicates that the lava had carried the material from some kind of reservoir deep in the Earth, with a composition which hasn’t changed significantly in the last 4 billion years. “This pattern has been observed in “Ocean Island Basalts,” which are lavas coming to the surface from deep in the Earth, and form islands such as Hawaii and Iceland” said research leader Dr. Suzette Timmerman, from the Australian National University. “The problem is that although these basalts are brought to the surface, we only see a glimpse of their history. We don’t know much about the mantle where their melts came from.”
To address this problem, Timmerman’s team looked at helium isotope ratios in superdeep diamonds. Most diamonds are formed between 150 to 230 km below the Earth’s crust, before being carried to the surface by melts. Very occasionally some ‘superdeep’ diamonds (created between 230 and 800 km below the Earth’s surface) are brought to the surface. These superdeep diamonds are recognizably different from normal diamonds.
Superdeep diamonds confirm ancient reservoir deep under Earth’s surface. (2019). Phys.org. Retrieved 23 August 2019, from https://phys.org/news/2019-08-superdeep-diamonds-ancient-reservoir-deep.html