Microlite crystallization during eruptions at Mt. Mazama: implications for magma ascent

Abstract

Large silicic eruptions can be preceded by small eruptions of different styles and volumes. The Holocene Llao Rock, Cleetwood, and climactic eruptions of Mt. Mazama, OR, USA, were sourced from the same magma and followed this pattern. The Llao Rock and Cleetwood eruptions are both relatively small, have pyroclasts with microlites and a wide range of vesicularities, and each consisted of an explosive phase followed by an effusive phase. The climactic eruption had no effusive phase and created highly vesicular pyroclasts with no microlites. We analyzed microlite crystallization using phase equilibria and decompression experiments. Comparing the results to the pyroclasts from the Llao Rock and Cleetwood eruptions, we find that the differences between the small and climactic eruptions are likely caused by different magma ascent dynamics. Our experiments show that plagioclase and pyroxene microlites crystallize only during decompressions that are most likely too slow to result in explosive eruptions. The Llao Rock magma likely stalled at shallow depths before continuing fast ascent, which allowed for microlite crystallization that might have also caused explosive eruptions. The Cleetwood magma likely took two separate ascent paths, a majority fraction that ascended quickly from high pressure without stalling and a minority fraction that stalled at shallow depths before continuing ascent along with the majority fraction. These ascent dynamics of the Llao Rock and Cleetwood magmas led to the creation of obsidian pyroclasts from sidewall sintering of fragmented majority-fraction ash. The climactic magma did not stall at shallow depths and instead ascended from depth quickly to the surface, creating the conditions necessary for caldera collapse.