- 3695 - DIAMONDS - how they form on Earth? Before diamonds can begin growing deep underground in Earth's mantle, they need a little zap from an electric field. In lab-based experiments, scientists mimicked conditions in the mantle, the layer just beneath Earth's crust, and found that diamonds grew only when exposed to an electric field, even a weak one of about 1 volt.
--------------------- 3695 - DIAMONDS - how they form on Earth
- Diamonds are made of carbon atoms aligned in a particular crystal structure. They form more than 90 miles under Earth's surface, where pressures reach several giga-pascals and temperatures can soar upward of 2,732 degrees Fahrenheit.
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- But many factors behind the "birth" of diamonds prized for its polished beauty and extreme hardness are a mystery.
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- Researchers gathered the starting ingredients needed to make a diamond, carbonate and carbonate-silicate powders that are similar to carbonate-rich melts abundant in the mantle. They put these powders in an artificial mantle in their lab and subjected them to pressures of up to 7.5 giga-pascals and temperatures of up to 2,912 F, and electrode-powered electric fields ranging from 0.4 to 1 volt. After varying periods lasting up to 40 hours, diamonds and their softer carbon-based cousin, graphite, formed, but only when the researchers set up an electric field of about 1 volt..
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- The diamonds and graphite formed only at the cathode, the negative part of the electric field. This spot provides electrons to jumpstart a chemical process so that certain carbon-oxygen compounds in the carbonates can undergo a series of reactions to become carbon dioxide and, eventually, the carbon atoms that can form a diamond.
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- The synthetic diamonds were small, with diameters no larger than 0.007 inches, but they were surprisingly similar to natural diamonds. They have an octahedral shape and tiny amounts of other elements and compounds, including a relatively high nitrogen content and silicate-carbonate inclusions.
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- These experiments suggest that local electrical fields play a pivotal role in diamond formation in Earth's mantle. This local voltage is likely created by rock melts and fluids in the mantle that have high electrical conductivity, but it's unclear how strong these electrical fields are.
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- These rare type of diamond may suggest that water can penetrate deeper into Earth's interior than scientists previously thought. Though more than 70% of our planet is covered with water, there is also water in minerals more than 200 miles underground, including in the upper mantle, the semi malleable layer that the crust "floats" on top of. Scientists have long thought that as the upper mantle transitions into the hotter, denser lower mantle, minerals can hold far less water.
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- Researchers found that a diamond contained inclusions, or tiny bits of other minerals, that can hold more water and seem to have existed on the boundary between the upper and lower mantle. The results suggest that there may be water deeper in the Earth than scientists thought, which could affect our understanding of the deep water cycle and plate tectonics.
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- “Type IaB” diamonds are a rare type of diamond from the Karowe mine in Botswana that form deep underground and are often in the Earth for a long time. To study the diamond, they used "nondestructive" forms of analysis, including Raman micro-spectroscopy, which uses a laser to noninvasively reveal some of a material’s physical properties, and X-ray diffraction to look at the diamond's internal structure without cutting it open.
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- Inside the diamond's inclusions, the researchers found a mineral called “ringwoodite“, which has the same chemical composition as “olivine“, the primary material of the upper mantle but forms under such intense temperature and pressure.
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- Ringwoodite is typically found in the transition zone between the upper and lower mantle, between around 255 and 410 miles below Earth's surface and can contain much more water than the minerals “bridgmanite and ferropericlase“, which are thought to dominate the lower mantle.
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- But instead of minerals usually found in the transition zone, surrounding this ringwoodite were forms of minerals typical of the lower mantle. Because the encasing diamond preserved these minerals' properties as they appeared in the deep Earth, the researchers could find the temperatures these the minerals endured and the pressures they were under; they estimated the minerals' depth to be around 410 miles below the surface, near the outer boundary of the transition zone.
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- Analysis further revealed that the ringwoodite was likely in the process of breaking down into more typical lower mantle minerals in a hydrous, or water-saturated, environment, hinting that water might penetrate from the transition zone into the lower mantle.
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- Although previous research has found some forms of minerals from the lower mantle in diamond inclusions, the combination of materials in this inclusion is unique. It was also unclear from prior findings if these minerals hinted at the presence of water-containing minerals in the lower mantle.
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- Because no one has directly sampled rock deeper than around 7 miles beneath the planet's surface, diamond inclusions are one of the few sources of minerals from Earth's mantle. The results could have implications for understanding the deep water cycle, or the cycle of water between the planet's surface and deep interior.
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- The findings also might affect models of plate tectonics. How water in the mantle might influence processes such as Earth's internal convection current. This current powers plate tectonics by unevenly heating the Earth’s mantle, causing hotter parts to rise and shift the Earth’s plates over millions of years.
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September 26, 2022 DIAMONDS - how they form on Earth? 3695
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