Convection in a volcanic conduit recorded by bubbles

Microtextures of juvenile pyroclasts from Kilauea's (Hawai'i) early A.D. 2008 explosive activity record the velocity and depth of convection within the basaltic magma-filled conduit. We use X-ray microtomography (XRT) to document the spatial distribution of bubbles. We find small bubbles (radii from 5 µm to 70 µm) in a halo surrounding larger millimeter-size bubbles. This suggests that dissolved water was enriched around the larger bubbles - the opposite of what is expected if bubbles grow as water diffuses into the bubble. Such volatile enrichment implies that the volatiles within the large bubbles were redissolving into the melt as they descended into the conduit by the downward motion of convecting magma within the lava lake. The thickness of the small bubble halo is ~100-150 µm, consistent with water diffusing into the melt on time scales on the order of 10³ s. Eruptions, triggered by rockfall, rapidly exposed this magma to lower pressures, and the haloes of melt with re-dissolved water became sufficiently supersaturated to cause nucleation of the population of smaller bubbles. The required supersaturation pressures are consistent with a depth of a few hundred meters and convection velocities of the order of 0.1 m s^-1, similar to the circulation velocity observed on the surface of the Halema'uma'u lava lake.