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Attrition of mantle cargo during kimberlite ascent : insights from analogue experiments Sasse, David


Kimberlite magmas transport mantle cargo in the form of xenoliths and xenocrysts to the surface of the Earth. Due to the lack of recent kimberlite eruptions and unanswered questions concerning melt composition and magma rheology, the mechanisms supporting efficient ascent of these cargo-rich magmas remains enigmatic. Although olivine is the dominant mineral phase in kimberlite, given the polymineralic nature of mantle xenoliths, xenocrysts are transported as a multi-mineral mixture. Within the ascending dyke, high particle concentrations and high velocities resulting in turbulent flow provides an environment for frequent particle-particle interaction. Xenocryst morphologies, textures and size distributions observed in kimberlite deposits are not reflective of how they occur in xenoliths and are therefore modified during ascent to the surface. The degree of modification of each mineral varies and is a function of its chemical and physical properties. Here I present a series of analogue attrition experiments on the mantle minerals: olivine, orthopyroxene, clinopyroxene, garnet and diamond designed to inform on the ascent of kimberlite magmas. Data is collected on particle size distributions, particle morphologies and particle velocities. Natural xenocrysts extracted from coherent kimberlite reveal remarkably similar surface features and morphologies to that of the experiments suggesting that attrition indeed operates during kimberlite ascent. Kimberlite ascent velocities are estimated by using a scaling analysis of the experiment conditions and by investigating impact pits observed on the surfaces of kimberlitic olivine and garnet xenocrysts. Both methods result in calculated ascent velocities of ~ 4 m s-1. In mineral mixtures, cleavage is shown to be a controlling factor in determining attrition rates whereby minerals with cleavage undergo accelerated breakdown. I suggest that the accelerated breakdown of orthopyroxene increases assimilation rates, contributing to the onset of turbulent ascent.

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