Fragmentation in kimberlite: products and intensity of explosive eruption

Abstract

The explosive eruption of kimberlite magma is capable of producing a variety of pyroclast sizes, shapes, and textures. However, all pyroclastic deposits of kimberlite comprise two main types of pyroclasts: (1) pyroclasts of kimberlite with or without enclosed olivine crystals and (2) olivine crystals which lack coatings of kimberlite. Here, we propose two hypotheses for how kimberlite magmas are modified due to explosive eruption: (1) olivine crystals break during kimberlite eruption, and (2) kimberlite melt can be efficiently separated from crystals during eruption. These ideas are tested against data collected from field study and image analysis of coherent kimberlite and fragmental kimberlite from kimberlite pipes at Diavik, NT. Olivines are expected to break because of rapid pressure changes during the explosive eruption. Disruption of kimberlite magma, and pyroclast production, is driven by ductile deformation processes, rather than by brittle fragmentation. The extent to which melt separates from olivine crystals to produce kimberlite-free crystals is a direct consequence of the relative proportions of gas, melt and crystals. Lastly, the properties of juvenile pyroclasts in deposits of pyroclastic kimberlite are used to index the relative intensity of kimberlite eruptions. A fragmentation index is proposed for kimberlite eruption based on: (a) crystal size distributions of olivine and on (b) ratios of selvage-free olivine pyroclasts to pyroclasts of kimberlite with or without olivine crystals.

Silicate glass micro and nanospherules generated in explosive eruptions of ultrabasic magmas: Implications for the origin of pelletal lapilli

“…Furthermore, for a gas phase to separate melt from a solid surface, the dispersive pressure applied to the outer surface of the melt-wetted solid must overcome the work of adhesion that couples the melt to the solid (Bangham and Razouk, 1937). The relative volumetric properties of gas, melt and solids determine how readily melt can separate from crystals in low-viscosity magmas (Moss and Russell, 2012), with atomization and stripping of melt from crystals occurring more easily in the case of dense and low viscosity melts. Also, if the gas fraction is markedly greater than liquid + solids, this will lead to a fully fluidized magma and the stripping of liquid from solids (Moss and Russell, 2012).…”

“…One indication of high velocity contrast is the size distribution of the droplets created by atomization, as the size of droplets sprayed off from a liquid jet decreases with the velocity contrast Wu and Faeth, 1995) and scales as a function of energy input (Hinze, 1959). If the velocity contrast between large melt clots, blebs or droplets and ambient gas is enough, the pieces of magma could efficiently disaggregate into much smaller droplets (Moss and Russell, 2012). Furthermore, for a gas phase to separate melt from a solid surface, the dispersive pressure applied to the outer surface of the melt-wetted solid must overcome the work of adhesion that couples the melt to the solid (Bangham and Razouk, 1937).…”

“…The relative volumetric properties of gas, melt and solids determine how readily melt can separate from crystals in low-viscosity magmas (Moss and Russell, 2012), with atomization and stripping of melt from crystals occurring more easily in the case of dense and low viscosity melts. Also, if the gas fraction is markedly greater than liquid + solids, this will lead to a fully fluidized magma and the stripping of liquid from solids (Moss and Russell, 2012). The total work required to separate liquid from a solid surface increases proportionally with the total surface area (i.e., crystal surfaces).…”

Modification of Mantle Cargo by Turbulent Ascent of Kimberlite

“…Our experimental conditions (i.e., gas flux and host fluid) were chosen to ensure that the dynamical regime (turbulent flow) and parameter space cover a similar range as expected during kimberlite magma ascent (Re ∼10 2 -10 5 ). Specifically, as conservative estimates we use a magma density ρ m of 2,800 kg m −3 , a magma viscosity µ range of 0.1-50 Pa s (Sparks et al, 2006;Moss and Russell, 2011), a dyke width x of 1 m (Sparks et al, 2006), and an ascent velocity υ of 4 m s −1 (Sparks et al, 2006), to estimate the Reynolds number of kimberlite ascent by: Re = ρ m xυ µ . The Reynolds number range expected for natural systems (2 × 10 2 to 1 × 10 5 ) matches our experimental conditions (8 × 10 2 < Re < 4 × 10 3 ).…”

Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania)

“…Large juvenile bombs (>50 cm in diameter) also common in proximal settings during monogenetic eruptions are similarly absent in the IHV. The small juvenile pyroclast size (coarse ash to fine lapilli) probably reflects the explosive disruption of predominantly low viscosity magma: this is backed-up by the droplet shapes of many kimberlite pyroclasts (e.g., Mitchell 1986;Moss et al 2011). The absence of large lithic ballistic clasts is puzzling, because even weak explosions eject coarse lithic clasts over proximal areas.…”