Bubbles Accumulation And Their Role On The Eruptability Of Melt-Rich Silicic Lenses In Upper Crustal Magma Reservoirs

Abstract:

A first-order observation in magmatic rocks is that highly evolved rhyolites are relatively abundant in the volcanic realm, but their plutonic counterparts (granites sensu stricto) are rarer, when ratioed to dacitic/granodioritic compositions. As eruptability is a function of the buoyancy of magmas in upper crustal reservoirs, the presence of exsolved gas (bubbles) plays a fundamental role on eruptability by lowering the bulk density of magmas. Then, if exsolved gas content can accumulate in certain areas of magma reservoirs, it follows that such areas might be more prone to erupt.

Magma reservoirs in the upper crust likely have relatively stable, sharp transitions in crystallinity between crystal-rich regions and crystal-poor regions. With this framework in mind, in this presentation, by means of theoretical considerations, numerical modelling and laboratory experiments, we suggest that the storage capacity of exsolved gas in magma reservoirs is a function of the relative abundance of melt respect to crystals present; crystal-poor regions (high melt to crystal volume ratio) tend to act as sponges, accumulating bubbles, while crystal-rich regions (mush zones with low melt to crystal ratio) tend to degas efficiently, leading to upward percolation of volatiles. Hence, melt-rich cupolas accumulating in upper parts of crystal-rich upper crustal reservoirs are particularly eruptible and dominate the volume of volcanic deposits in silicic magmatic provinces.