Degassing of hydrous trachytic Campi Flegrei and phonolitic Vesuvius melts: Experimental limitations and chances to study homogeneous bubble nucleation

Abstract

Melt degassing by bubble nucleation and growth is a driving mechanism of magma ascent. Therefore, decompression experiments with hydrous silicate melts were used to investigate the onset and the dynamics of H 2 O degassing. Nominally H 2 O-undersaturated trachytic Campi Flegrei and phonolitic Vesuvius melts representative for the magma compositions of the Campi Flegrei volcanic system were decompressed at a super-liquidus temperature of 1050 °C from 200 MPa to final pressures ( P final ) of 100, 75, and 60 MPa using continuous decompression rates of 0.024 and 0.17 MPa/s. Experiments started from either massive glass cylinders or glass powder to demonstrate the influence of the starting material on melt degassing. Glass powder can be used to shorten the equilibration time ( t eq ) prior to decompression for dissolution of H 2 O in the melt. The decompressed samples were quenched and compared in terms of bubble number density ( N V ), porosity, and residual H 2 O content in the melt. Decompression of all glass cylinder samples led to homogeneous bubble nucleation with high N V of ~10 5 mm −3 . The supersaturation pressures for homogeneous bubble nucleation were estimated to be 2 O-N 2 bubbles throughout the samples prior to decompression. Degassing of these glass powder samples was facilitated by diffusive growth of these pre-existing bubbles and thus did not require significant H 2 O supersaturation of the melt. This is evidenced by several orders of magnitude lower N V and lower residual H 2 O contents at correspondingly higher porosities compared to the glass cylinder samples. However, a significant extension of t eq to 96 h in the glass powder experiments led to degassing results comparable to the glass cylinder samples. This effect is probably due to Ostwald ripening, coalescence, and the ascent of the pre-existing bubbles during the extended t eq prior to decompression. The N V of the glass cylinder samples were used to test the applicability of the vesiculation model provided by Toramaru (2006). For the applied decompression rates, the experimental N V are up to 5 orders of magnitude higher than the values predicted by the model. This may be mainly attributed to the usage of the macroscopic surface tension and the total H 2 O diffusivity in the model to describe the molecular process of bubble nucleation. A significant increase in modeled N V can be achieved by application of a reduced surface tension in combination with the lower diffusivity of network formers as a limiting parameter for the formation of a bubble nucleus. This study demonstrates that the investigation of homogeneous bubble nucleation necessitates an optimized experimental protocol. We strongly recommend performing experiments with massive glass cylinders as starting material. The timescale of decompression is a limiting parameter and must be short enough to minimize the opportunity for a reduction of N V by bubble coalescence. Considering our comparably high N V , the samples of many previous experimental studies that were used to calibrate models for homogeneous bubble nucleation were probably subject to significant N V reduction. Newly derived data from optimized experiments will require improved models for homogeneous bubble nucleation during magma ascent.