Acidification provides the thrust

Kimberlites are magmatic rocks that form deep in the Earth’s interior and are brought to the surface by volcanic eruptions. On their turbulent journey upwards magmas assimilate other types of minerals, collectively referred to as xenoliths (Greek for “foreign rocks”). The xenoliths found in kimberlite include diamonds, and the vast majority of the diamonds mined in the world today is found in kimberlite ores. Exactly how kimberlites acquire the necessary buoyancy for their long ascent through the Earth’s crust has, however, been something of a mystery. An international research team led by Professor Donald Dingwell, Director of the Department of Geo- and Environmental Sciences at LMU, has now demonstrated that assimilated rocks picked up along the way are responsible for the providing the required impetus. The primordial magma is basic, but the incorporation of silicate minerals encountered during its ascent makes the melt more acidic. This leads to the release of carbon dioxide in the form of bubbles, which reduce the density of the melt, essentially causing it to foam. The net result is an increase in the buoyancy of the magma, which facilitates its continued ascent. “Because our results enhance our understanding of the genesis of kimberlite, they will be useful in the search for new diamond-bearing ores and will facilitate the evaluation of existing sources,” says Dingwell. (Nature 18. January 2012)

Most known kimberlites formed in the period between 70 and 150 million years ago, but some are over 1200 million years old. Generally speaking, kimberlites are found only in cratons, the oldest surviving areas of continental crust, which form the nuclei of continental landmasses and have remained virtually unchanged since their formation eons ago. </P

Kimberlitic magmas form about 150 km below the Earth’s surface, i.e. at much greater depths than any other volcanic rocks. The temperatures and pressures at such depths are so high that carbon can crystallize in the form of diamonds. When kimberlitic magmas are forced through long chimneys of volcanic origin called pipes, like the water in a hose when the nozzle is narrowed, their velocity markedly increases and the emplaced diamonds are transported upwards as if they were in an elevator. This is why kimberlite pipes are the sites of most of the world’s diamond mines. But diamonds are not the only passengers. Kimberlites also carry many other types of rock with them on their long journey into the light.

In spite of this “extra load”, kimberlite magmas travel fast, and emerge onto the Earth’s surface in explosive eruptions. “It is generally assumed that volatile gases such as carbon dioxide and water vapour play an essential role in providing the necessary buoyancy to power the rapid rise of kimberlite magmas,” says Dingwell, “but it was not clear how these gases form in the magma.” With the help of laboratory experiments carried out at appropriately high temperatures, Dingwell’s team was able to show that the assimilated xenoliths play an important role in the process. The primordial magma deep in the Earth’s interior is referred to as basic because it mainly consists of carbonate-bearing components, which may also contain a high proportion of water. When the rising magma comes into contact with silicate-rich rocks, they are effectively dissolved in the molten phase, which acidifies the melt. As more silicates are incorporated, the saturation level of carbon dioxide dissolved in the melt progressively increases as carbon dioxide solubility decreases. When the melt becomes saturated, the excess carbon dioxide forms bubbles. “The result is a continuous foaming of the magma, which may reduce its viscosity and certainly imparts the buoyancy necessary to power its very vehement eruption onto the Earth’s surface,” as Dingwell explains. The faster the magma rises, the more silicates are entrained in the flow, and the greater the concentration of dissolved silicates – until finally the amounts of carbon dioxide and water vapor released thrust the hot melt upward with great force, like a rocket. The new findings also explain why kimberlites are found only in ancient continental nuclei. Only here is the crust sufficiently rich in silica-rich minerals to drive their ascent and, moreover, cratonic crust is exceptionally thick. This means that the journey to the surface is correspondingly longer, and the rising magma has plenty of opportunity to come into contact with silicate-rich minerals.

Diamonds show depth extent of Earth’s carbon cycle

Scientists have speculated for some time that the Earth’s carbon cycle extends deep into the planet’s interior, but until now there has been no direct evidence. The mantle-Earth’s thickest layer -is largely inaccessible. A team of researchers analyzed diamonds that originated from the lower mantle at depths of 435 miles (700 kilometers) or more, and erupted to the surface in volcanic rocks called kimberlites. The diamonds contain what are impurities to the gemologist, but are known as mineral inclusions to the geologist. Analysis shows compositions consistent with the mineralogy of oceanic crust. This finding is the first direct evidence that slabs of oceanic crust sank or subducted into the lower mantle and that material, including carbon, is cycled between Earth’s surface and depths of hundreds of miles. The research is published in the September 15, 2011, online Science Express.

The mantle extends from as little as 5 to 1,800 miles (10-2,900 kilometers) beneath the Earth’s surface. Most diamonds are free from inclusions and come from depths less than 120 miles (200 km). But in a few localities researchers have found super-deep diamonds from the depths of the convecting upper and lower mantle, as well as the transition zone in between. Whereas inclusions in diamonds from the depths of the upper mantle and transition zone have been consistent with a surface-rock origin, none from the lower mantle have borne this signature until now.

The team,* which included Carnegie scientists, was led by former Carnegie postdoctoral fellow Michael Walter, now a professor at the University of Bristol, UK. The scientists analyzed minute (one to two hundredths of a millimeter) mineral grains from six diamonds from the Juina region in Brazil. The analysis showed that diamond inclusions initially crystallized as a single mineral that could form only at depths greater than 435 miles (700 km). But the inclusions recrystallized into multiple minerals as they were carried up to the surface-first probably from a mantle upwelling known as a plume, then as they erupted to the surface in kimberlites

The diamonds were analyzed for carbon at Carnegie. Four of the diamonds contained low amounts of carbon-13, a signature not found in the lower mantle and consistent with an ocean-crust origin at Earth’s surface. “The carbon identified in other super-deep, lower mantle diamonds is chiefly mantle-like in composition,” remarked co-author Steven Shirey * at Carnegie. “We looked at the variations in the isotopes of the carbon atoms in the diamonds. Carbon originating in a rock called basalt, which forms from lava at the surface, is often different from that which originates in the mantle, in containing relatively less carbon-13. These super-deep diamonds contained much less carbon-13, which is most consistent with an origin in the organic component found in altered oceanic crust.”

“I find it astonishing that we can use the tiniest of mineral grains to show some of the motions of the Earth’s mantle at the largest scales,” concluded Shirey.

Diamond impurities bonanza for geologists studying Earth’s history

This is an optical photomicrograph of a sulfide-inclusion-bearing rough diamond from Botswana. -  Steven Shirey
This is an optical photomicrograph of a sulfide-inclusion-bearing rough diamond from Botswana. – Steven Shirey

Jewelers abhor diamond impurities, but they are a bonanza for scientists.

Safely encased in super-hard diamond, impurities are unaltered, ancient minerals that tell the story of Earth’s distant past.

Researchers analyzed data from more than 4,000 of these mineral inclusions to find that continents started the cycle of breaking apart, drifting, and colliding about three billion years ago.

The research results, published in this week’s issue of the journal Science, pinpoint when this so-called Wilson cycle began.

Lead author Steven Shirey of the Carnegie Institution’s Department of Terrestrial Magnetism says that the Wilson cycle is responsible for the growth of the Earth’s continental crust, the continental structures we see today, the opening and closing of ocean basins through time, mountain building, and the distribution of ores and other materials in the crust.

“But when it all began has remained elusive until now,” Shirey says.

“We used the impurities, or inclusions, contained in diamonds, because they are perfect time capsules from great depth beneath the continents.

“They provide age and chemical information for a span of more than 3.5 billion years that includes the evolution of the atmosphere, the growth of the continental crust, and the beginning of plate tectonics.”

Co-author Stephen Richardson of the University of Cape Town says that it’s “astonishing that we can use the smallest mineral grains that can be analyzed to reveal the origin of some of Earth’s largest geological features.”

“The tiny inclusions found inside diamonds studied by this team have recorded the chemistry and evolution of the Earth over 3.5 billion years,” says Jennifer Wade, program director in the National Science Foundation (NSF)’s Division of Earth Sciences, which funded the research. “They help pinpoint when the cycle of plate tectonics first began on Earth.”

The largest diamonds come from cratons, the most ancient formations within continental interiors that have deep mantle roots or keels around which younger continental material gathered.

Cratons contain the oldest rocks on the planet, and their keels extend into the mantle more than 125 miles where pressures are sufficiently high, but temperatures sufficiently low, for diamonds to form and be stored for billions of years.

Over time, diamonds have arrived at the surface as accidental passengers during volcanic eruptions of deep magma that solidified into rocks called kimberlites.

The inclusions in diamonds come in two major varieties: peridotitic and eclogitic.

Peridotite is the most abundant rock type in the upper mantle, whereas eclogite is generally thought to be the remnant of oceanic crust recycled into the mantle by the subduction or sinking of tectonic plates.

Shirey and Richardson reviewed the data from more than 4,000 inclusions of silicate–the Earth’s most abundant material–and more than 100 inclusions of sulfide from five ancient continents.

The most crucial aspects, they say, looked at when the inclusions were encapsulated and the associated compositional trends.

Compositions vary and depend on the geochemical processing that precursor components underwent before they were encapsulated.

Two systems used to date inclusions were compared. Both rely on natural isotopes that decay at exceedingly slow but predictable rates–about one disintegration every ten years on the scale of an inclusion–making them excellent atomic clocks for determining absolute ages.

The researchers found that before 3.2 billion years ago, only diamonds with peridotitic compositions formed, whereas after three billion years ago, eclogitic diamonds dominated.

“The simplest explanation,” says Shirey, “is that this change came from the initial subduction of one tectonic plate under the deep mantle keel of another as continents began to collide on a scale similar to that of the supercontinent cycle today.

“The sequence of underthrusting and collision led to the capture of eclogite in the subcontinental mantle keel along with the fluids that are needed to make diamond.”

Concludes Richardson, “This transition marks the onset of the Wilson cycle of plate tectonics.”