Kimberlite pyroclasts - what and why? (original) (raw)

Abstract

The dominant type of pyroclasts found in kimberlite deposits are crystals of rounded to subrounded olivine that have no apparent quenched melt or magma attached. We refer to these as free-olivine pyroclasts. Rounded to sub-rounded juvenile lapilli comprising olivine grains mantled by a thin rim of (micro) crystalline coherent kimberlite are also common but they are generally subordinate to free olivine grains. Within the juvenile lapilli vesicles are rare to absent, and the rims of quenched kimberlite can show alignment of microlites. The rounded nature of these juvenile pyroclasts is generally considered to be a manifestation of surface tension processes operating on a very low viscosity melt (i.e. kimberlite melt) although there are no direct measurements of physical properties for kimberlite melt. We suggest that the properties of kimberlite pyroclasts can be used to constrain the physical properties of kimberlite magmas and the styles of volcanic eruption. Specifically, we consider the implications that the morphology and internal structure of these pyroclasts has for the melt properties of kimberlite magmas. We also explore how pyroclast shape and size distributions can be used to constrain eruption dynamics and depositional processes. Lastly, we assess the significance of the highly abundant free olivine crystals in terms of the transport and eruption processes assumed to be responsible for liberating these crystals from their host melt.

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