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Kimberlite magmas have the deepest origin of all terrestrial magmas and are exclusively associated with cratons. 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). Kimberlite magmas also reputedly have higher ascent rates than other xenolith-bearing magmas. 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. 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 the increasingly kimberlitic magma. Our model provides an explanation for continuous ascent of magmas laden with high volumes of dense mantle cargo, an explanation for the chemical diversity of kimberlite, and a connection between kimberlites and cratons.

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The northern Cordilleran volcanic province encompasses a broad area of Neogene to Quaternary volcanism in northwestern British Columbia, the Yukon Territory, and adjacent eastern Alaska. Volcanic rocks of the northern Cordilleran volcanic province range in age from 20 Ma to ca. 200 yr B.P. and are dominantly alkali olivine basalt and hawaiite. A variety of more strongly alkaline rock types not commonly found in the North American Cordillera are locally abundant in the northern Cordilleran volcanic province. These include nephelinite, basanite, and peralkaline phonolite, trachyte, and comendite. The most MgO-rich nephelinites, basanites, and alkaline basalts from throughout the northern Cordilleran volcanic province show trace element abundances and isotopic compositions that are consistent with an asthenospheric source region similar to that for average oceanic island basalt and for post-5 Ma alkaline basalts from the Basin and Range. Our petrologic observations help constrain the origin of northern Cordilleran volcanic province magmatism as well as lithosphere changes between the four major basement terranes that underlie the province. Results from phase equilibria calculations and the spatial distributions of volcanic rock types and magmatic inclusions are more consistent with the existence of thicker lithosphere beneath Stikinia, which underlies the southern part of the northern Cordilleran volcanic province, than beneath the Cache Creek and Yukon-Tanana terranes, which underlie the northern part of the northern Cordilleran volcanic province. Our results support a model for initiation of northern Cordilleran volcanic province magmatism due to incipient rifting of the northern Cordillera, driven by changes in relative plate motion between the Pacific and North American plates ca. 15-10 Ma.

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Welding of pyroclastic deposits involves flattening of glassy pyroclasts under a compactional load at temperatures above the glass transition temperature. Progressive welding is recorded by changes in the petrographic (e.g., fabric) and physical (e.g., density) properties of the deposits. Mapping the intensity of welding can be integral to studies of pyroclastic deposits, but making systematic comparisons between deposits can be problematical. Here we develop a scheme for ranking welding intensity in pyroclastic deposits on the basis of petrographic textural observations (e.g., oblateness of pumice lapilli and micro-fabric orientation) and measurements of physical properties, including density, porosity, point load strength and uniaxial compressive strength. Our dataset comprises measurements on 100 samples collected from a single cooling unit of the Bandelier Tuff and parallel measurements on 8 samples of more densely welded deposits. The proposed classification comprises six ranks of welding intensity ranging from unconsolidated (Rank I) to obsidian-like vitrophyre (Rank VI) and should allow for reproducible mapping of subtle variations in welding intensity between different deposits. The application of the ranking scheme is demonstrated by using published physical property data on welded pyroclastic deposits to map the total accumulated strain and to reconstruct their pre-welding thicknesses.