CRYSTALLIZATION HISTORY OF THE 1984 MAUNA-LOA LAVA FLOW

During a 3-week eruption in 1984, Mauna Loa produced vent lavas that i ncreased in crystallinity from <1 to 30%, and 27-km-long flows that in creased in crystallinity as they moved downstream. We examined the cry stallization history of these lavas using crystal size distribution (C SD) analysis to study the rates of crystallization, viscosity increase , and latent heating. Typical average growth and nucleation rates were 5 x 10(-9) cm s-1 and 5 cm-3 s-1 for microphenocrysts (20- to 500-mum size crystals nucleated in the rift zone) and 5 x 10(-8) cm s-1 and 5 x 10(4) cm-3 s-1 for microlites (1- to 20-mum size crystals nucleated in the channel). These crystallization rates are high compared with t hose found in other CSD studies of igneous rocks, probably due to high ly nonequilibrium conditions brought on by rapid degassing in the rift zone and cooling in the lava channel. Growth and nucleation rates dec reased with time at the vent and with distance downstream. The maximum downstream total crystallinity measured is 39% (25% microlites, 14% m icrophenocrysts) in a quenched sample 14 km from the vent. Growth and nucleation rates cannot be calculated for postemplacement samples, but they place upper limits of 53-58% on the amount of crystallization in the channel 9-20 km from the vent. Crystallization could have been mo stly responsible for the 10(5)-fold downstream increase in apparent vi scosity, although degassing and increasing incorporation of solid lava fragments also contributed. Another effect of crystallization on the lava flow was the sizeable latent heating (0.01 J g-1 s-1 over the fir st half of the flow length, if the crystallinity of downstream quench samples is representative of the hot fluid core), which may have been counteracted by entrainment of cooler material. Measurements of crysta llization are shown to be crucial in the study of lava flow emplacemen t dynamics.