Acidification provides the thrust
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.
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.
The project was funded by a European Research Council (ERC) Advanced Investigator Grant (EVOKES) and further supported by an LMUexcellent Research Professorship awarded to Donald Dingwell. (göd/PH)
Kimberlite ascent by assimilation-fuelled buoyancy
J.K. Russell, L.A. Porritt, Y. Lavallée, D.B. Dingwell
Nature Advanced Online Publication 18. January 2012
Professor Donald B. Dingwell
Department of Geo- and Environmental Sciences, LMU Munich
Phone: +49 89 2180 4136
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