Influence of magma withdrawal on compositional gradients during the AD 79 Vesuvius eruption

Abstract

COMPOSITIONAL variations in pyroclastic deposits have been used as indicators of pre-eruptive chemical gradients in magma chambers and of processes leading to compositional stratification^1,2. Theoretical studies of the fluid dynamics of magma withdrawal from stratified chambers have shown that compositional gradients in the products erupted at the surface are not merely inversions of the gradients that had existed in the magma chamber. These studies predict that complex mixing can occur across stratified layers, and that withdrawal of dense underlying layers through overlying low-density layers can occur given certain combinations of discharge rate and magma density contrast^3–7. These predictions have yet to be quantitatively verified, however, because simultaneous measurements of eruption dynamics and compositional gradients for a specific eruption have not been collected. Here we explore the relationship between chemical gradients observed in pyroclastic deposits and the dynamics of magma withdrawal, by modelling the temporal evolution of magma discharge rate during the AD 79 eruption of Vesuvius in Italy. The results provide evidence that compositional gradients in zoned deposits cannot be properly interpreted without consideration of the dynamics of magma withdrawal.