Highly explosive basaltic eruptions driven by CO(2) exsolution

The most explosive basaltic scoria cone eruption yet documented (>20 km high plumes) occurred at Sunset Crater (Arizona) ca. 1085 AD by undetermined eruptive mechanisms. We present melt inclusion analysis, including bubble contents by Raman spectroscopy, yielding high total CO(2) (approaching 6000 ppm) and S (~2000 ppm) with moderate H(2)O (~1.25 wt%). Two groups of melt inclusions are evident, classified by bubble vol%. Modeling of post-entrapment modification indicates that the group with larger bubbles formed as a result of heterogeneous entrapment of melt and exsolved CO(2) and provides evidence for an exsolved CO(2) phase at magma storage depths of ~15 km. We argue that this exsolved CO(2) phase played a critical role in driving this explosive eruption, possibly analogous to H(2)O exsolution driving silicic caldera-forming eruptions. Because of their distinct gas compositions relative to silicic magmas (high S and CO(2)), even modest volume explosive basaltic eruptions could impact the atmosphere.