Experimental investigation of H2O degassing in silicate melts during magma ascent: A closer look at decompression experiments

Abstract

Magma degassing during ascent can be simulated by decompression experiments with volatile-bearing silicate melts. Decompression-induced volatile supersaturation in the melt results in bubble nucleation and growth. H2O-bearing silicate melts were decompressed at super-liquidus conditions over a wide range of nominal decompression rates to investigate homogeneous bubble nucleation and further H2O degassing. The quenched samples document that the onset and extent of H2O degassing are highly sensitive to the experimental protocol. This study provides essential guidelines for the conduction of degassing experiments and the interpretation of the bubble-bearing samples. The bubble number density, porosity and residual H2O content in the melt are influenced by the starting material (glass powder or cylinder), decompression method (step-wise or continuous), sample size and run time of the experiment. A fundamentally important aspect is the consideration of the bubble volume reduction due to decreasing molar volume of the exsolved H2O during isobaric rapid quench. This quench effect must also be considered for the interpretation of bubble-bearing volcanic rocks in terms of magma porosity and ascent velocity. The bubble number densities of samples from optimized experiments are up to 5 orders of magnitude higher than modeled values. This may be attributed to the usage of (1) the macroscopic surface tension and/or (2) the diffusivity of total H2O instead of the diffusivity of network formers in models to describe the nucleation of bubbles at the molecular level. Improved models for homogeneous bubble nucleation on the basis of optimized experiments will contribute to a better understanding of the dynamic melt degassing processes that can trigger explosive volcanic eruptions.