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Kimberlite ascent by assimilation-fuelled buoyancy

Abstract

Kimberlite magmas have the deepest origin of all terrestrial magmas and are exclusively associated with cratons1,2,3. 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 rates6,7,8,9 than other xenolith-bearing magmas10,11. 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 specified6,9. 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 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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Figure 1: Transmitted-light optical microscope images of thin sections of kimberlite, peridotite and orthopyroxene.
Figure 2: CO2 solubilities in silicic to carbonate melts (after ref. 19).
Figure 3: High-temperature analogue experiments.
Figure 4: Model for assimilation-fuelled ascent of kimberlite.

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Acknowledgements

We acknowledge the laboratory support of W. Ertel-Ingritsch and S. Laumann at Ludwig Maximilian University. Funding for this research is from the Natural Sciences and Engineering Research Council (J.K.R.), a Marie Curie outbound fellowship (L.A.P.) and an ERC Advanced Researcher Grant (D.B.D.). The original manuscript benefitted from review by S. Sparks.

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Contributions

J.K.R. conceptualized the original idea, performed the static experiments at the University of Munich, and wrote the original draft paper. L.A.P. provided kimberlite expertise and context and drafted the figures. Y.L. performed the transient experiments and processed the experimental data. D.B.D. hosted, provided and repaired the experimental facility, and advised on the optimal experimental methods. All co-authors contributed to producing a final draft for review and in revising the manuscript after review.

Corresponding author

Correspondence to James K. Russell.

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Russell, J., Porritt, L., Lavallée, Y. et al. Kimberlite ascent by assimilation-fuelled buoyancy. Nature 481, 352–356 (2012). https://doi.org/10.1038/nature10740

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