Degassing and microlite crystallization during pre-climactic events of the1991 eruption of Mt. Pinatubo, Philippines

Dacite tephras produced by the 1991 pre-climactic eruptive sequence at Mt. Pinatubo display extreme heterogeneity in vesicularity, ranging in clast de nsity from 700 to 2580 kg m(-3). Observations of the 13 surge-producing bla sts that preceded the climactic plinian event include radar-defined estimat es of column heights and seismically defined eruptive and intra-eruptive du rations. A comparison of the characteristics of erupted material, including microlite textures, chemical compositions. and H2O contents, with eruptive parame ters suggests that devolatilization-induced crystallization of the magma occurred to a varying extent prior to at least nine of the explosive events. Although volatile lass progressed to the same approximate level in all of the clasts analyzed (weight percent H2O = 1.26-1.73), microlite crys tallization was extremely variable (0-22%). We infer that syn-eruptive vola tile exsolution from magma in the conduit and intra-eruptive separation of the gas phase was facilitated by the development of permeability within mag ma residing in the conduit. Correlation of maximum microlite crystallinity with repose interval duration (28-262 min) suggests that crystallization oc curred primarily intra-eruptively, in response to the reduction in dissolve d H2O content that occurred during the preceding event. Detailed textural c haracterization, including determination of three-dimensional shapes and cr ystal size distributions (CSD), was conducted on a subset of clasts in orde r to determine rates of crystal nucleation and growth using repose interval as the time available for crystallization. Shape and size analysis suggest s that crystallization proceeded in response to lessening degrees of feldsp ar 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 prev ious explosive event. If plug thickness grew proportionally to the square r oot of time, and if magma pressurization increased during the eruptive sequ ence, the frequency of eruptive pulses may have been modulated by degassing of magma within the conduit. Dense clasts in surge deposits probably repre sent plug material entrained by each subsequent explosive event.