The influence of carbonate assimilation and deformation regime on the multiphase rheology of a phonotephritic melt from Vesuvius

The emplacement of magma chambers within a carbonate basement promotes a sequence of thermochemical reactions that progressively evolve from the carbonate wall-rock towards the magma, altering the chemical composition of the overall system (crystal + melt). Recent petrological and experimental studies highlight that varying degrees of limestone/dolostone assimilation controls the liquid line of descent of magmas, promoting or hindering the crystallization onset of distinct mineral phases.

In this study, we investigate how differing degrees of limestone/dolostone assimilation and deformation regimes impact the rheology of a leucite-bearing phonotephrite magma from Somma-Vesuvius (Italy). Using starting materials doped with 0, 10, or 20 wt.% of CaO and CaO+MgO, mimicking the effects of limestone/dolostone assimilation, we conducted two sets of crystallization experiments at 1180°C under static and dynamic conditions (shear strain rate of 1 and 5 s^-1).    

We observe distinct rheological behaviours among melts as a function of composition and applied shear rate, displaying significant differences in terms of crystallizing mineral phases (±clinopyroxene±melilite±leucite±nepheline in the CaO-doped samples and ±clinopyroxene±melilite±olivine±leucite±nepheline, in the CaO+MgO-doped ones) and final crystal contents. Increased alkaline earth content (both Ca and Ca+Mg) alongside higher shear rates foster crystallization, leading to heightened crystal fractions (up to 51%) and larger crystals. Consequently, in heavily doped samples and under high shear rates, viscosity increased of up to 1.5 Log Pa s due to crystallization, causing the rheological transition from coherent flow to shear localization, culminating in physical separation (i.e., viscous rupture).  

This study underscores the significant influence of deformation on magma, affecting both mineralogical assemblages and crystallization efficiency. These effects compound the pivotal role played by changing magma composition due to carbonate assimilation, governing magma’s crystallization capability, transport properties, and flow behaviour during its ascent from depth to surface.