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.
Diamonds from the Juina area: most of these are superdeep diamonds [Credit: Graham Pearson] |
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".
A selection of electron microscope images of diamonds from the Juina area, Brazil [Credit: Suzette Timmerman] |
Suzette Timmerman said "Diamonds are the hardest, most indestructible natural substance known, so they form a perfect time capsule that provides us a window into the deep Earth. We were able to extract helium gas from twenty-three super-deep diamonds from the Juina area of Brazil. These showed the characteristic isotopic composition that we would expect from a very ancient reservoir, confirming that the gases are remnants of a time at or even before the Moon and Earth collided. From the geochemistry of the diamonds, we know 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 this unseen reservoir, left over from the Earth's beginnings, must be in this area or below it.
Diamond from Collier kimberlite [Credit: Antony Burnham] |
Commenting, Professor Matthew Jackson (University of California, Santa Barbara) said: "There has been a lot of work focused on identifying the location of primordial reservoirs in the deep Earth. So this is an interesting result, with a lot of potential to "map out" where elevated 3He/4He domains are located in the Earth's deep interior. Helium can diffuse rapidly at mantle conditions, so it will be important to evaluate whether the ancient helium signature reflects compositions trapped at diamond-formation depths, or the composition of the host lava that transported to diamonds to the surface. This work is an important step towards understanding these reservoirs, and points the way to further research".
Source: Goldschmidt Conference [August 15, 2019]
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