RECRYSTALLIZATION OF BASALTIC TEPHRA THROUGH REHEATING EXPERIMENTS

During explosive eruptions of subaerial volcanoes, ejected tephra is rapidly cooled and quenched. The explosivity, eruption rate and particle size control the distance that materials are ejected from the vent. Low mass eruption rates often do not have enough energy to completely eject tephra from the vent and some material falls back down into the vent, potentially to be reheated or even mix with the juvenile magma, i.e. recycled. The extent of crystallization and microlite textures in tephra are often used to determine the eruption dynamics and crystallization histories of an eruption. Reheating/recycling of tephra within a vent may induce additional crystallization complicating our ability to determine the magma’s ascent and eruption history, but can potentially provide a signature of the reheating event.

We have conducted a series of experiments to determine the timescales and temperatures at which reheated tephra will undergo additional groundmass crystallization. We heated basaltic tephra samples (4-8mm diameter) from a cinder cone quarry south of Mt. Bachelor in central Oregon in a Deltech vertical tube furnace in time sets of 5, 10, 30, and 60 minutes, and at temperatures (T) ranging from 650 - 1100° C. Photos using a stereo microscope were taken before and after to look at differences in external color and texture. We then imaged the heated samples and controls (unheated half of tephra) with a scanning electron microscope to observe the differences in microcrystalline textures. No obvious changes in external tephra color or groundmass crystallization was observed at temperatures <1000°C. However at experimental runs of 1000°C and 1100°C there is obvious additional crystallization demonstrated as rapid growth microlite textures and the appearance of new mineral phases. New microlite crystallization occurs in localized areas in runs of 1000°C and ≤ 30 minutes but extensive new crystallization, affecting all groundmass glass, at T > 30 minutes for 1000°C runs and T ≥ 5 minutes and 1100°C. This suggests that for basaltic tephra falling back into the vent, the tephra must be reheated close to eruption temperatures to undergo extensive microcrystallization. These experiments support the hypothesis that groundmass crystallization can be created by the recycling of tephra and will help identify recycling textures.