Accessing the dynamic evolution of terrestrial magma plumbing systems by linking kinetic and thermodynamic modelling

Abstract:

Deciphering the evolution of the internal dynamics of magmatic plumbing systems and identifying the key parameters that drive the dynamics are major goals of volcanology. We present a novel petrological approach that combines kinetic modelling of the diffusive relaxation of chemical zoning patterns in olivine crystals with thermodynamic modelling (MELTS) to constrain the thermodynamic parameters (e.g. P, T, water content, oxygen fugacity and bulk composition of melt) of magma storage beneath Mt. Etna between 1991 and 2008. For this purpose we have studied the compositional zoning patterns in 180 olivine crystals, and the mineral chemistry of associated clinopyroxene, plagioclase and Fe-Ti oxides. The minerals span a wide compositional range and show a large variability (e.g. a single thin section can contain olivines showing four different compositional ranges and zoning patterns). The olivine zoning data has been organized using the tool of Systems analysis to identify five different magmatic environments (MEs) characterized by different core- and rim-plateau olivine compositions: M0 (=Fo79-83), M1 (=Fo75-78), M2 (=Fo70-72), M3 (=Fo65-69) and mm1 (=Fo73-75). We developed a forward modeling approach using thermodynamic calculations with the MELTS software aiming to identify the key intensive variables associated with the different magmatic environments. In this approach the observed populations of mineral compositions (e.g. Fo79-83), rather than individual compositions, are associated with thermodynamic parameters (P, T, H₂O, fO₂ and bulk composition of melt) to identify the most plausible set corresponding to each ME. We found that temperature, water content, and oxidation state, are the main distinguishing features of the different MEs. Combined with kinetic modelling the duration of residence and the time of magma transfer between these MEs can be determined. The method is applicable to active as well as temporarily remote (dormant) volcanoes and its application allows identification of different points of residence of magma along certain pathways in a plumbing system as well as the quantification of their environmental (e.g. water content, oxidation state) and temporal (duration and timing of recharge of different MEs) parameters to reconstruct the dynamics of the plumbing system.