Degassing and microlite crystallization of basaltic andesite magma erupting at Arenal Volcano, Costa Rica

Volcanoes can erupt explosively in Plinian style or effusively as lava flows. Most models for such ranges in activity are based on silicic magma, which may not be appropriate for less viscous basic magma. Although basic magma erupting at Arenal volcano has not varied significantly in bulk composition, the volcano has exhibited a full range in eruptive style, from Plinian activity in 1968 to Strombolian bursts to lava flows. We examined groundmass textures of samples erupted over that range of activity to investigate the controls on the variability. Microlite textures in lavas collected both hot (rapid quenched) and cold show that most samples have textures that are overprinted by crystallization as a result of cooling. Despite that overprint, microlites in the Plinian sample have unique crystal morphologies and vesicles that are much smaller and more spherical than those in the other samples. We interpret those differences as recording a change in degassing style as a result of changing ascent rate in the conduit. To constrain the potential changes in ascent rate, a limited number of decompression experiments were run at rates from 0.0013 to 0.25 MPa/s. Crystal textures and morphologies vary greatly as decompression rates change, and compared to our observed differences in the natural groundmass, it appears that magma erupted in the Plinian event decompressed between 0.0013 and 0.025 MPa/s, whereas magma erupted in non-Plinian events decompressed slower than 0.0013 MPa/s. The change in eruptive style from explosive Plinian to lava effusion thus resulted from an order of magnitude decrease in magma ascent rate. Plinian magma probably rose too quickly to allow bubbles to coalesce and allow the magma to degas efficiently, whereas at other times magma rose more slowly, which allowed bubbles to coalesce and gas to escape leading to less explosive activity.