Disequilibrium Rheology of Basalts from Campi Flegrei and Derivation of Empirical Solidification Criteria.

Large volcanic eruptions are often triggered by intrusion of hot primitive magma into an evolved magma-chamber or -mush zone. During intrusion into the cooler magma chamber, the basalt crystallizes, supplying heat and volatiles to the evolved magma. This can cause a drop in mush-viscosity and mobilize the mush zone, leading to eruption. These non-linear changes in the transport properties of both magmas alter how they accommodate deformation during interaction and ascent. They represent complex, disequilibrium phenomena, during which the process-guiding material properties (dominantly viscosity) constantly evolve. This highlights the critical importance of non-isothermal and sub-liquidus processes for the understanding of natural systems and the need for accurate rheological data to understand and model the interaction process.

Here we present new experimental data on the disequilibrium rheology of the basaltic end member thought to be involved in both mixing and eruption triggering processes in the Campi Flegrei magmatic system. The experiments map the melts' sub liquidus rheological evolution and systematic shifts in their "cut-off temperature; T_cutoff", i.e. the point where flow ceases as a function of cooling rate and oxygen fugacity. The data show that during the sub-liquidus rheology experiments:

1) The rheological evolution and solidification behaviour fundamentally depends on the imposed cooling-rate.

2) Decreasing oxygen fugacity decreases the onset of crystallization and modifies the crystallization kinetics of both melts.

3) Dynamic cooling produces different crystallization kinetics/sequences and phase-dynamics than equilibrium or near-equilibrium conditions.

Based on the experimental data we derive empirical relationships between the environmental parameters and the melts' Tcutoff", i.e. the point where flow ceases. These empirical descriptions of flow and solidifications may be employed in numerical models aiming to reconstruct the thermomechanical interaction between basalts and rhyolites at magmatic conditions.