Structure and physical characteristics of pumice from the climactic eruption of Mount Mazama (Crater Lake), Oregon

Abstract

The vesicularity, permeability, and structure of pumice clasts provide insight into conditions of vesiculation and fragmentation during Plinian fall and pyroclastic flow-producing phases of the ~7,700 cal. year B.P. climactic eruption of Mount Mazama (Crater Lake), Oregon. We show that bulk properties (vesicularity and permeability) can be correlated with internal textures and that the clast structure can be related to inferred changes in eruption conditions. The vesicularity of all pumice clasts is 75-88%, with >90% interconnected pore volume. However, pumice clasts from the Plinian fall deposits exhibit a wider vesicularity range and higher volume percentage of interconnected vesicles than do clasts from pyroclastic-flow deposits. Pumice permeabilities also differ between the two clast types, with pumice from the fall deposit having higher minimum permeabilities (~5×10 -13 m 2 ) and a narrower permeability range (5-50×10 -13 m 2 ) than clasts from pyroclastic-flow deposits (0.2-330×10 -13 m 2 ). The observed permeability can be modeled to estimate average vesicle aperture radii of 1-5 µm for the fall deposit clasts and 0.25-1 µm for clasts from the pyroclastic flows. High vesicle number densities (~10 9 cm -3 ) in all clasts suggest that bubble nucleation occurred rapidly and at high supersaturations. Post-nucleation modifications to bubble populations include both bubble growth and coalescence. A single stage of bubble nucleation and growth can account for 35-60% of the vesicle population in clasts from the fall deposits, and 65-80% in pumice from pyroclastic flows. Large vesicles form a separate population which defines a power law distribution with fractal dimension D=3.3 (range 3.0-3.5). The large D value, coupled with textural evidence, suggests that the large vesicles formed primarily by coalescence. When viewed together, the bulk properties (vesicularity, permeability) and textural characteristics of all clasts indicate rapid bubble nucleation followed by bubble growth, coalescence and permeability development. This sequence of events is best explained by nucleation in response to a downward-propagating decompression wave, followed by rapid bubble growth and coalescence prior to magma disruption by fragmentation. The heterogeneity of vesicle sizes and shapes, and the absence of differential expansion across individual clasts, suggest that post-fragmentation expansion played a limited role in the development of pumice structure. The higher vesicle number densities and lower permeabilities of pyroclastic-flow clasts indicate limited coalescence and suggest that fragmentation occurred shortly after decompression. Either increased eruption velocities or increased depth of fragmentation accompanying caldera collapse could explain compression of the pre-fragmentation vesiculation interval.Editorial responsibility: M. Rosi

Simulating bubble number density of rhyolitic pumices from Plinian eruptions: constraints from fast decompression experiments

“…3b and d). The huge BNDs in our experiments at 90 MPa/s are similar to those observed in natural pumices from the Plinian eruptions (e.g., 10 15 -10 16 m −3 : Klug et al 2002).…”

“…In addition, bubbles commonly show a bimodal size distribution with a numerically minor population of large bubbles (≈ 10 9 m −3 ), and a major population of small bubbles, typically from a few μm to a few tens of μm in diameter (e.g., Klug et al 2002;Formenti and Druitt 2003). The origin of bimodal size distribution may be attributed to two successive nucleation events in ascending magmas.…”

“…Compared to crystalfree magmas, magnetite-bearing rhyolitic magmas produce higher BNDs at a given decompression rate; for instance, 2× 10 12 m −3 at ≈ 0.03 MPa/s (Gardner et al 1999). Observed BNDs in natural silicic pumices commonly exhibit BNDs much higher than those measured in experimental pumices: up to ≈ 10 16 m −3 (e.g., Klug et al 2002). Such huge BNDs may be due to faster decompression than those studied in the laboratory and/or to the presence of a high number density of magnetite crystals.…”

Porosity evolution and crystallization-driven fragmentation during weathering of andesite

“…The corresponding slope of approximately À1.0 is comparable to the pore size distribution for the larger vesicle fraction in volcanic rocks [cf. Klug et al, 2002]. In contrast, weathered andesite from the area with onion-skin fractures (see Figure 3b) has a much smaller number of large pores (ignoring the macroscopic onion-skin fractures themselves; Figure 7a).…”

Homogeneous bubble nucleation in rhyolitic melt: Experiments and non-classical theory

“…It is therefore possible to constrain magma ascent and decompression rates from the number of bubbles that nucleated in a given volume of melt upon eruptive magma ascent. To this end, a substantial body of research has been aimed at characterizing the size and abundance of bubbles (vesicles) in pyroclasts [e.g., Sparks and Brazier, 1982;Toramaru, 1989Toramaru, , 1990Mangan et al, 1993;Klug and Cashman, 1994;Gardner et al, 1996;Klug and Cashman, 1996;Mangan and Cashman, 1996;Blower et al, 2001;Polacci et al, 2001;Blower et al, 2002;Klug et al, 2002;Polacci et al, 2003;Gaonac'h et al, 1996aGaonac'h et al, , 1996bGaonac'h et al, , 2005Polacci, 2005;Polacci et al, 2006Polacci et al, , 2009Gurioli et al, 2005;Adams et al, 2006;Lautze and Houghton, 2007;Piochi et al, 2008;Giachetti et al, 2010;Houghton et al, 2010;Alfano et al, 2012].…”