Constraints on the timescales of magma degassing from chemical heterogeneity in rhyolites

Abstract:

Magma degassing is one of the fundamental processes in volcanology. Volatile exsolution, leading to pressure build-up, is a major driving force for explosive volcanic eruptions, whereas gas escape has the ability to diffuse volcanic crises. Understanding the dynamics of magma ascent, volatile exsolution and gas escape is thus a crucial objective of volcanology.

The behaviour of volatile elements in ascending magma is controlled by solubilities, partition coefficients and diffusivities. Once exsolved, gas moves through magma in bubbles and fractures. During this transport (i.e. open system degassing, gas fluxing or streaming), elements are continuously redistributed between melt and gas. However, this redistribution is time-dependent. Hence, magma can be spatially heterogeneous with respect to volatile elements. The extent of this heterogeneity is determined by the volatilities and the diffusivities of the elements, the type of pathway used, and time. Studying the spatial distribution of different volatile elements in volcanic glass can thus provide information on the dynamics of magma degassing.

Here, we show that tuffisite veins, formed when magma fragments during deformation and degassing within the conduit, are associated with both enrichments and depletions of certain volatile elements (H₂O, Cl, Li, As, Pb, Mo, Zn and others). From the spatial distribution of the chemical variations we are able to estimate total gas flux through the veins showing that tuffisite veins form highly efficient gas pathways, which can channel pre-exsolved volatiles to the surface. We also infer significant vertical transport of clasts within interconnected tuffisite vein networks, indicating that degassing pathways may extend to hundreds of meters below the surface.