The multiphase rheology of magmas from Monte Nuovo (Campi Flegrei, Italy)

Abstract: We present a study of high-temperature, uniaxial deformation experiments of natural, partially-crystallized magma from the Monte Nuovo (1538 AD) trachytic eruption. The experiments were performed using a high-temperature uniaxial Geocomp LoadTrac II press at dry atmospheric conditions and under controlled deformation rates. Each experiment involved deforming cores of natural (i.e., crystal- and vesicle-bearing) scoriaceous samples isothermally (600 to 800° C) at constant displacement rates (CDR) corresponding to strain rates between 10-7 and 10-4 s-1. Measured viscosities vary between 1010 and 1013 Pa s. The flow behavior of these complex natural materials are fully described by a simplified Herschel-Bulkely equation in terms of consistency K and flow index n. We estimate the combined effects of crystals and pores on the rheology of these multiphase suspensions. Our results demonstrate that the presence of pores has a major impact on the rheological response of magmas and may produce a marked decrease of their viscosity. At the same time, the presence of pores leads to a strong decrease in the strength of the magma inducing local and temporal variation in the deformation regimes (ductile vs. brittle). Brittle failure was in fact observed at T=600°C and strain rates of 10-5 s-1 and at T=800°C for the highest applied strain rate (10-4 s-1), respectively. This study constitutes an important step toward the estimation of multiphase rheological evolution of Monte Nuovo magmas and toward the general understanding of the full complexities governing the dynamics of magma transport in natural systems.