Rare blue diamonds form deep, deep, deep inside Earth | Science News for Students

Rare blue diamonds form deep, deep, deep inside Earth

Their recipe includes boron that may have ridden to great depths aboard a sinking tectonic plate
Sep 5, 2018 — 6:45 am EST
a blue diamond

A blue diamond gets its color from the element boron. Tiny bits of minerals inside the gem suggest that diamonds of this hue form at very great depths. 

ROBISON MCMURTRY/© 2018 GIA

Blue diamonds are among the rarest of gems. Earth’s mantle is that deep, hot layer between Earth’s crust and core. And it’s here that these diamonds are born. Yet their blue hue comes from boron, an element not commonly found there. Scientists now think they’ve figured out how the boron got there. Their analyses suggest that sinking pieces of Earth’s crust ferried the boron deep down from the crust. If true, the blue gems’ birthplace is among the deepest of any diamonds.  

Evan Smith and his colleagues made the new findings. Smith is a geologist at the Gemological Institute of America, based in New York City. That institute has studied millions of diamonds from around the world. Only a small share of them — about two in every 10,000 — are blue. And only a few blue diamonds have tiny bits of non-diamond material, called mineral inclusions, inside their crystal structure.

A diagram showing the Earth's layers. The innermost layer is the inner core, then outercore, lower mantle, upper mantle and crust.
This artist’s depiction shows the relative depths of Earth’s inner layers. Blue diamonds appear to form deep within the mantle.
ttsz/iStockphoto

Those inclusions trap minerals that were in the vicinity when the diamonds formed. They therefore can offer clues to the gem’s origins. Whenever the researchers came across a blue diamond, they checked for those inclusions. Altogether, Smith and his colleagues found and analyzed 46 blue diamonds with these inclusions.

Their structure and chemical recipe point to an origin more than 660 kilometers (410 miles) deep. That’s below the boundary between the mantle’s upper and lower layers. All diamonds form within Earth’s mantle, but most form above that boundary layer.

Smith’s team described its findings August 1 in Nature.

Telltale chemical hitchhikers

Gem experts refer to inclusions as a type of flaw. Blue diamonds are not just extremely rare, but also very pure, Smith reports. “They’re quite often completely flawless.” That made it difficult for researchers to identify enough blue diamonds with mineral inclusions to study. 

The team probed the inclusions using something known as Raman spectroscopy.  This didn’t require cutting into the stones to gain clues on how deep their birthplace had been. They merely shone a laser beam onto the inclusions in each of the 46 stones. Then they measured the wavelengths of light scattered by these inclusions. Diamonds are made of carbon. Like chemical fingerprints, those wavelengths told scientists what other elements the inclusions held.

Certain minerals, such as bridgmanite (BRIDJ-maan-eit), are stable only at the high temperatures and pressures found deep inside Earth. Such minerals change in structure as the diamonds holding them rise through the Earth in molten rock. But the inclusions also keep traces of the deep, high-pressure forms. The researchers used these molecules to better understand the blue diamonds’ origins.

“It’s a bit of detective work to trace back what you see in the diamond,” Smith says. For example, a diamond might hold both bridgmanite and ferropericlase (Fair-oh-PAIR-ih-klaes). Both minerals come from the lower mantle. (Indeed, bridgmanite becomes common at about 660 kilometers down.) Seeing the two minerals together means the diamond formed very deep. That combo would not have been stable at shallower depths, Smith says. “It’s like oil and water. Under certain conditions they mix, but under others, they don’t.”

And now, about that boron . . .

The element boron usually prefers to stay in Earth’s crust. So scientists were puzzled how it could have gotten deep enough to end up inside a blue diamond. The likeliest explanation, Smith’s team says, is that the boron hitched a ride on a tectonic plate.

These large slabs of rock, which make up the planet’s surface, are always on the move. Some slowly push toward one another. Others are pulling away from each other. Still others grind sideways past each other. When two plates collide, the plates may scrunch up to form mountains. Or one plate may slide under another. The process where a collision sends one plate deep into Earth’s is called subduction

Much plate pulling and pushing takes place beneath the oceans. Seawater contains dissolved boron. As this water moves along the seafloor, it reacts with certain minerals to form a water-rich mineral called serpentinite (Sur-PEN-tih-neit). If this mineral is in a tectonic plate that subducts during a collision, that sinking plate will carry the mineral deep into the mantle. That’s one way to enrich the mantle with boron.

The hypothesis also points to one way that water may travel deep into the mantle: hitchhiking within serpentinite.

a photo of the Hope diamond
The deep blue Hope Diamond, the largest gem in this setting, is part of the gem collection held by the Smithsonian Institution’s National Museum of Natural History.
David/Flickr (CC BY 2.0)

Scientists had long wondered just how far down water can go. They have suspected that water might help keep tectonic plates moving by lubricating surfaces deep within the planet. Finding serpentinite, Smith says, is “not proof, but a glimmer of hope that we’ve got actual water being transported from the ocean down to the lower mantle.”

Using diamonds to study how boron moves through Earth is clever, says Graham Pearson. He’s a geochemist at the University of Alberta in Canada and wasn’t connected with the study. “Virtually nothing is known about the boron cycle in the deeper parts of the mantle,” Pearson adds.

He and other researchers are now studying boron isotopes — forms of the element with different masses — inside blue diamonds. This might help them pin down whether the boron in the crystals came from the crust or from the mantle.

Because blue diamonds in the new study all contained inclusions, they’re not perfect. They would be less expensive than flawless blue diamonds, such as the world famous Hope Diamond. Still, the research value of flawed blue diamonds, as seen in this new study, may boost their value, Smith says. “I think it makes them a little more special.”

Power Words

(for more about Power Words, click here)

boron     The chemical element having the atomic number 5. Its scientific symbol is B.

bridgmanite      A mineral made of magnesium, iron, and silicate that occurs only in the extreme pressures and temperatures inside Earth’s lower mantle. By volume, bridgmanite is Earth’s most abundant mineral, accounting for about 38 percent of the planet.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

colleague     Someone who works with another; a co-worker or team member.

core     Something — usually round-shaped — in the center of an object. (in geology) Earth’s innermost layer.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

diamond     One of the hardest known substances and rarest gems on Earth. Diamonds form deep within the planet when carbon is compressed under incredibly strong pressure.

Earth’s crust     The outermost layer of Earth. It is relatively cold and brittle.

element     A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.(in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

hue     A color or shade of some color.

hypothesis     (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.

inclusion     (in geology) Something trapped inside a mineral.

isotopes     Different forms of an element that vary somewhat in mass (and potentially in lifetime). All have the same number of protons in their nucleus, but different numbers of neutrons.

laser     A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

mantle     (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.

mineral     Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns).

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

molten     A word describing something that is melted, such as the liquid rock that makes up lava.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.

pressure     Force applied uniformly over a surface, measured as force per unit of area.

Raman spectroscopy     A chemical tool that helps provide a chemical fingerprint of some sample.

seawater     The salty water found in oceans.

subduction      (v. to subduct) The process by which tectonic plates sink or slide back from Earth’s outer layer into its middle layer, called the mantle.

tectonic plates     The gigantic slabs — some spanning thousands of kilometers (or miles) across — that make up Earth’s outer layer.

wavelength     The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

Citation

Journal: E.M. Smith et al. Blue boron-bearing diamonds from Earth’s lower mantleNature. Vol. 560, August 1, 2018, p. 84. doi: 10.1038/s41586-018-0334-5.