Melt expulsion from crystal-rich mushes via induced ephemeral anisotropic permeability

Abstract:

Catastrophic eruptions associated with rhyolitic super-volcanoes, require large volumes of silicic magma to be extracted from a crystal-rich source mush at high enough rates to prevent freezing. However, rapid assembly of a large eruptible silicic reservoir is thought to be difficult because of the low permeability of mushes and the high viscosities of silicic melts. Here, we show that dynamic redistribution of melt pockets, resulting in ephemeral high permeability pathways may be critical. During uniaxial compression, perhaps driven by the impingement of recharging magmas from blow, melt within the mush can be readily redistributed due to the initially low cost of energy dissipation. This redistribution of melt causes dilation of vertical melt pockets and contraction of horizontal melt pockets, increasing vertical permeability by a factor of 10-100 times, which in turn allows for efficient expulsion of interstitial melt through the anisotoropic network. As melt is expelled, high-angle melt pockets undergo thinning, causing permeability rapidly decline back to background levels. We show that this fast, but short-lived pulse of melt extraction is sufficient to grow a 1 km thick silicic magma reservoir in <10 kyr. Our analysis suggests that super-eruptions may be triggered by the incursion of deep-seated magmas, which drive a temporary increase in vertical permeability. If so, understanding the cause of super-eruptions will require an understanding of the entire transcrustal magmatic system.