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Superdeep diamonds confirm ancient reservoir deep below the earth's surface

Barcelona: Analyzes show that gases found in microscopic inclusions in diamonds come from a stable underground 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 with the Earth's mantle, somewhere between the Earth's crust and the core, contains a huge reservoir of rock, relatively undisturbed since the formation of the planet. So far, there has been no solid evidence of or where it exists. Now, an international group of scientists have measured helium isotopes in superdeep diamonds that were brought to the surface by violent volcanic eruptions to detect the footsteps of this ancient reservoir. This work will be presented to researchers for the first time on Friday, August 23 at the Goldschmidt Conference in Barcelona, ​​after publication today (August 1

5) in the journal Science *.

After the formation of the earth, violent geological activity and extraterrestrial effects, the young planet disturbed, which means that almost nothing of the earth's original structure remains. In the 1980s, the geochemists noted that the ratios of the helium 3 to the helium 4 isotope in some basalt lava from some sites were higher than expected, reflecting the isotope ratio found in extremely ancient meteorites that had fallen to the earth. This indicates that the lava had transported the material from some type of reservoir deep into the earth, with a composition that has not changed significantly in the last 4 billion years. "This pattern has been observed in 'Ocean Island Basalts', which are lavor that come to the surface deep in the earth, forming islands such as Hawaii and Iceland," says research leader Dr. Suzette Timmerman, from the Australian National University. "The problem is that even though 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 conditions in superdeep diamonds. Most diamonds are formed between 150 to 230 km below the earth's crust before being transported to the surface by melting. Very occasionally some & # 39; superdeep & # 39; diamonds (created between 230 and 800 km below the earth's surface) to the surface. These superdeep diamonds differ significantly from normal diamonds.

Suzette Timmerman said "Diamonds are the hardest, most indivisible natural substance known, so they form a perfect time capsule that gives 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 isotope composition that we can expect from a very ancient reservoir, which confirms that the gases are remnants of a time at or even before the moon and earth collide. From the geochemistry of diamonds, we know that they were formed in an area that is called & # 39; the transition zone & # 39; which lies between 410 and 660 km below the earth's surface, which means that this invisible container, still from the beginning of the earth, must be in this area or below it.

Questions remain about this reservoir form, is it a large simple container, or are there several smaller ancient reservoirs? where exactly is the container? what is the complete chemical composition of this reservoir? But with this work we start at home in what is probably the oldest remaining undisturbed material on earth. "

Comment, Professor Matthew Jackson (University of California, Santa Barbara) said:

" There has been a lot of the 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" where elevated 3He / 4He domains are in the deep interior of the earth. Helium can diffuse rapidly under mantle conditions, so it will be important to evaluate whether the ancient helium signature reflects compositions captured at diamond formation depth, or the host lava composition transported to diamonds to the surface. This work is an important step towards understanding these reservoirs and points the way to further research. "


This is an independent comment. Professor Jackson was not involved in this work.

* The embargo on this press release matches the publication embargo applied by Science . Copies of the paper are available available from Goldschmidt or Science press office – see Notes for editors.

Conference website: https: / / goldschmidt. info / 2019 / index

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