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Bubbly melt trapped in Youngest Toba Tuff quartz reveals pre-eruptive magma vesicularity, compressibility, and why the eruption was so big

Tyler Cadena , Michael Manga, Stephen Self

Abstract

The 75 ka eruption of the Youngest Toba Tuff is one of few eruptions documented to be initiated by volatile accumulation, an eruption initiation mechanism much less common than recharge or rejuvenation. We searched for evidence of pre-eruptive exsolved volatiles in the erupted products of the Youngest Toba Tuff by using x-ray microtomography to image six quartz crystals. The imaged quartz crystals contain populous bubbly melt inclusions. Analysis of 59 melt inclusions show an average of 8.6 bubble vol.% and 2.2 bubbles (maximum 11 bubbles) per inclusion. We found that 51% of inclusions have a bubble vol.% greater than 7, the maximum vol.% that could be caused by post-eruptive hydrogen diffusion and host-inclusion thermal volume changes, showing that these bubbly inclusions sampled an at-depth exsolved vapor phase that was co-entrapped with the melt during pre-eruptive crystal growth. Many (56%) of these bubbles are also attached to small oxide crystals, recording evidence of heterogeneous bubble nucleation in the magma reservoir. These physical observations support the chemical analyses that suggest the eruption of the Youngest Toba Tuff was initiated by volatile accumulation, a seldom identified eruption initiation mechanism in large silicic systems. Using our vesicularity measurements and Voigt-Reuss-Hill elastic moduli bounds, we hypothesize that this exsolved vapor phase can increase magma compressibility by up to an order of magnitude. These large changes in magma compressibility can suppress eruption initiation, promoting magma accumulation in the crust,  and lead to especially large eruptions such as that responsible for the Youngest Toba Tuff.