Kimberlite ascent by assimilation-fuelled buoyancy

Assimilation of mantle minerals is proposed as a cause of deep-seated exsolution of dissolved volatiles and the driver of kimberlite magma ascent. What gives kimberlites a lift Kimberlites are volcanic rocks sourced from the deep mantle underlying the oldest portions of Earth's crust, called cratons. They provide geologists with precious samples of the deep mantle and are of economic importance as a source of diamonds. The ascent of kimberlites is thought to be anomalously fast even though they carry substantial mantle 'cargo'. The exsolution of dissolved volatiles, such as carbon dioxide and water, is widely seen as crucial to providing sufficient buoyancy, and here James Russell and colleagues present a novel explanation for how this occurs. In their model, silica-undersaturated melts assimilate mantle minerals, driving the melt to more silicic compositions and causing a marked drop in carbon dioxide solubility and hence foaming. This reduces magma density, increasing buoyancy, and drives the rapid and accelerating ascent of the magma. Kimberlite magmas have the deepest origin of all terrestrial magmas and are exclusively associated with cratons 1 , 2 , 3 . During ascent, they travel through about 150 kilometres of cratonic mantle lithosphere and entrain seemingly prohibitive loads (more than 25 per cent by volume) of mantle-derived xenoliths and xenocrysts (including diamond) 4 , 5 . Kimberlite magmas also reputedly have higher ascent rates 6 , 7 , 8 , 9 than other xenolith-bearing magmas 10 , 11 . Exsolution of dissolved volatiles (carbon dioxide and water) is thought to be essential to provide sufficient buoyancy for the rapid ascent of these dense, crystal-rich magmas. The cause and nature of such exsolution, however, remains elusive and is rarely specified 6 , 9 . Here we use a series of high-temperature experiments to demonstrate a mechanism for the spontaneous, efficient and continuous production of this volatile phase. This mechanism requires parental melts of kimberlite to originate as carbonatite-like melts. In transit through the mantle lithosphere, these silica-undersaturated melts assimilate mantle minerals, especially orthopyroxene, driving the melt to more silicic compositions, and causing a marked drop in carbon dioxide solubility. The solubility drop manifests itself immediately in a continuous and vigorous exsolution of a fluid phase, thereby reducing magma density, increasing buoyancy, and driving the rapid and accelerating ascent of t