Degassing and microlite crystallization during pre-climactic events of the 1991 eruption of Mt. Pinatubo, Philippines

Abstract

Dacite tephras produced by the 1991 pre-climactic eruptive sequence at Mt. Pinatubo display extreme heterogeneity in vesicularity, ranging in clast density from 700 to 2580 kg m -3 . Observations of the 13 surge-producing blasts that preceded the climactic plinian event include radar-defined estimates of column heights and seismically defined eruptive and intra-eruptive durations. A comparison of the characteristics of erupted material, including microlite textures, chemical compositions, and H 2 O contents, with eruptive parameters suggests that devolatilization-induced crystallization of the magma occurred to a varying extent prior to at least nine of the explosive events. Although volatile loss progressed to the same approximate level in all of the clasts analyzed (weight percent H 2 Op1.26-1.73), microlite crystallization was extremely variable (0-22%). We infer that syn-eruptive volatile exsolution from magma in the conduit and intra-eruptive separation of the gas phase was facilitated by the development of permeability within magma residing in the conduit. Correlation of maximum microlite crystallinity with repose interval duration (28-262 min) suggests that crystallization occurred primarily intra-eruptively, in response to the reduction in dissolved H 2 O content that occurred during the preceding event. Detailed textural characterization, including determination of three-dimensional shapes and crystal size distributions (CSD), was conducted on a subset of clasts in order to determine rates of crystal nucleation and growth using repose interval as the time available for crystallization. Shape and size analysis suggests that crystallization proceeded in response to lessening degrees of feldspar supersaturation as repose interval durations increased. We thus propose that during repose intervals, a plug of highly viscous magma formed due to the collapse of vesicular magma that had exsolved volatiles during the previous explosive event. If plug thickness grew proportionally to the square root of time, and if magma pressurization increased during the eruptive sequence, the frequency of eruptive pulses may have been modulated by degassing of magma within the conduit. Dense clasts in surge deposits probably represent plug material entrained by each subsequent explosive event.

Constraining pre-eruptive magma conditions and unrest timescales during the Monte Nuovo eruption (1538 ad; Campi Flegrei, Southern Italy): integrating textural and CSD results from experimental and natural trachy-phonolites

“…N a and crystal area fraction (ϕ) of alkali feldspar were calculated on a vesicle-free basis. The relevant area (A r ) in each BSE image consists of only glass and crystal phases, thus, all abundances refer to phase proportions in glass plus crystals (vesicle-free basis; e.g., Hammer et al 1999). N a of alkali feldspar was obtained through the relation (Hammer et al 1999):…”

Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by X-ray computed microtomography

“…We note, however, that small vesicles in El Hierro xeno-pumice are more often tortuously shaped (V small ) and may reflect viscous deformation during conduit flow or rapid growth of large vesicles in their vicinity. Large vesicles, on the other hand, appear to be less affected by shearing and viscous deformation probably because of the continuous supply of volatiles into these larger vesicles and the increasing stiffness of the surrounding melt films that would become strongly volatile depleted and therefore have higher viscosity (Schmincke and Swanson 1967;Spark 1978;Hammer et al 1999; Fig. 3e).…”

Textures of plagioclase microlite and vesicles within volcanic products of the 1914-1915 eruption of Sakurajima Volcano, Kyushu, Japan

“…The ascent rate may be larger than this value in the case of the early to middle stage Plinian eruption. Microlite free pumices observed from the large pyroclastic eruption at Mt Mazana (Klug et al, 2002) or large Plinian eruptions such as the 1980 1986 Mt St. Helens eruption and 1991 Mt Pinatubo eruption (Cashman, 1992;Hammer et al, 1999) are also interpreted as resulting from high ∆P/∆t. Extremely high values of ∆P/∆t result in a low number density, indicating disequilibrium in crystallization.…”

“…Extremely high values of ∆P/∆t result in a low number density, indicating disequilibrium in crystallization. Type 1 pumices had little time to crystallize plagioclase microlite during the rapid ascent of magma because of a time lag between H 2 O exsolution and accompanying crystallization (approximately < 40 min or 1 4 h) (Hammer et al, 1999;Couch et al, 2003). The intermediate number density of microlite in Taisho 1 Lava is due to the low decompression rate of magma despite the large magnitude of overall decompression.…”