H2O phase separation in FeO-free phonolitic melt

Melt degassing is an important driving force for accelerated magma ascent from storage regions toward the surface and volcanic activity. Hence, the investigation of degassing processes of H2O as the most abundant volatile in magmas is important. One of the key properties of eruptive products used in reconstructing volcanic degassing processes is the vesicle number density (VND) of volcanic glasses. The formation of such vesicles can be explained by nucleation in the metastable region of the melt-H2O system (e.g. Hurwitz and Navon, 1994). The nucleation rate of vesicles in rhyolitic melts increases exponentially with increasing supersaturation and VND increases exponentially with decompression rate (dP/dt) (e.g. Hamada et al. 2010; Hajimirza et al. 2019). However, for phonolitic melt with 2.71 wt% FeO (Vesuvius white pumice, VAD79) independence of VND on dP/dt has been observed (Allabar and Nowak, 2018). Therefore, these authors proposed spinodal decomposition, a non-activated mechanism at the thermodynamic limit of stability, as a phase separation mechanism.

Aiming to derive a possible effect of FeO on the phase separation mechanism in phonolitic melt, we performed decompression experiments with iron-free hydrous VAD79 melt and decompression rates of 0.17, 1.7, and 5 MPa/s. Our first results suggest a dependence of VND on dP/dt which can be approximated by the VND – dP/dt relation of Toramaru (2006). This indicates that the FeO melt component substantially affects the type of phase separation mechanism of hydrous phonolitic melt.