Volatile Bubble Resorption in Silicate Melts & Magmas via Diffusive Mass Transfer

Abstract

Bubble growth in magmas is a first-order control on volcanic eruption style, with changes typically resulting from re-equilibration in non-isothermal-isobaric conditions, or diffusive mass transfer of volatiles across the bubble interface. The latter process is well understood for bubbles coupled to high-viscosity liquids such as rhyolitic magmas associated with Plinian and Dome-forming eruptions, but two significant gaps remain: (1) changes to decoupled bubbles in lower viscosity fluids like basaltic magmas typical of Hawaiian or Strombolian eruptions, and (2) bubble resorption and magma regassing resulting from reverse volatile mass transfer into the magma.
Using two new definitions of Péclet number for coupled and decoupled bubbles (Pes and Peb), and Sherwood number (Sh), these two complexities are explored through the relative timescales of diffusion and advection in analogue and magmatic bubble- melt. Numerical simulations find that in basaltic systems, spherical bubbles are almost always decoupled with resorption limited by diffusion (Peb ≫ 105 or Sh>10), meaning they resorb at the rate of diffusive mass transfer. By contrast, spherical bubbles in rhyolitic melts have restricted buoyancy making them coupled and their resorption limited by the high melt viscosity (Pes ≪ 105). In both melt compositions, resorption of the smallest bubbles (R0 < 1μm) becomes limited by surface tension effects.
Experimental observations of decoupled bubbles show that larger bubbles described by high values of Peb resorb at a faster rate than smaller bubbles in lower Peb systems. This is attributed to larger bubbles rising faster to continually encounter new melt with a renewed concentration gradient.
These findings have great significance to the modelling of eruptive volcanic processes, providing support for theories on the formation of bubble-free material, or dynamic magma regassing. Whilst a numerical model for decoupled bubble resorption in magmas is not yet complete, this thesis quantifies the onset of different bubble regimes for magmas of contrasting compositions.