Kimberlite ascent by assimilation-fuelled buoyancy

Abstract

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.

The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

“…(1) Whether the primary kimberlite is carbonatitic (Dawson 1971;Dawson and Hawthorne 1973;Russell et al 2012;Kamenetsky and Yaxley 2015;, a carbonated silicate melt (e.g., Nielsen and Sand 2008;Brey et al 2008), or a highly saline carbonatite (e.g., Kamenetsky et al 2012), and (2) the mechanism for generating the calcite-bearing kimberlites observed in the crust-all experimental evidence suggests that a primary melt formed from the melting of a peridotitic source must produce Mgrich carbonate, namely dolomite (e.g., Wyllie and Huang 1975;Dalton and Presnall 1998a).…”

“…The idea that kimberlites start off as end-member carbonatites that then react with mantle phases, mostly orthopyroxene (opx), to create a more Si-and Mg-rich melt has recently gained ground following experimental studies (Russell et al 2012;Kamenetsky and Yaxley 2015). However, so far attention has focused on opx dissolution as a dominant process in kimberlite magma evolution and eruption from the base of the lithosphere, although this has not yet been validated through experiments at upper mantle conditions (e.g., Stone and Luth 2016;Sokol et al 2016), and could be reproduced only for a limited pressure range (Kamenetsky and Yaxley 2015).…”

Origin and ascent history of unusually crystal-rich alkaline basaltic magmas from the western Pannonian Basin

“…This suggests a change in the style (and possibly in the 561 rate) of the magma ascent. The main driving force of magma ascent is the process of magma-562 filled crack propagation (e.g., Spera 1984; Russell et al 2012). Change in the style and rate of 563 ascent can be caused by the variations in volatile solubility in the melt, by the change in the 564 physical state of magma and wall rocks along the ascent path, and by varying dyke widths.…”

“…An explanation for this effective interaction could be some cryptic processes. In 550 the case of kimberlitic magmas a recent discovery (Russell et al 2012) suggests that 551 continuous assimilation of foreign minerals (especially orthopyroxene) -that can modify the 552 composition of the host melt toward more silicic compositions -causes changes in the 553 volatile solubility in the host melt. The result is volatile exsolution and due to this process the 554 magma can fracture more effectively the wall rock.…”

Unravelling the internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) using 3D models of gravity and magnetic data

“…Solid Earth Discuss., doi:10.5194/se-2017Discuss., doi:10.5194/se- -3, 2017 Manuscript under review for journal Solid Earth Sparks et al, 2006;Russell et al, 2012) and thus it is unlikely that diamond would be preserved during the transport to the surface (e.g., Andersson et al, 2013). However, to confirm this, temperature-pressure studies are required, which we highly recommend.…”