Constraints on the nature and evolution of the magma plumbing system beneath Mt. Etna (1991 – 2008) from a combined thermodynamic and kinetic modelling of the compositional record of minerals

Abstract:

We present a novel approach that combines kinetic (diffusion) and thermodynamic (MELTS) modelling of the compositional zoning preserved in 180 olivines crystals to constrain the nature and evolution of the plumbing system beneath Mt. Etna over the last 17 years (1991-2008).

Though the observed olivine compositions span a wide range and show an apparently chaotic heterogeneity we were able to identify five distinct populations. These can be explained as the result of multistage magma mixing and magma exchange between five different magmatic environments (MEs): M0 (=Fo79-83), M1 (=Fo75-78), M2 (=Fo70-72), M3 (=Fo65-69) and mm1 (=Fo73-75). Application of diffusion modelling to the zoned olivine populations allowed us to access typical timescales and durations over which crystals (and melt) are transferred between the different MEs.

We could identify pre-and syn-eruptive recharge of multiple pulses of primitive M0 (=Fo79-83)-type magma into different sections of the plumbing system. A detailed analysis of all possible connectivity patterns revealed that three dominant magma migration pathways connecting the MEs M0:M1, M2:mm1 and M1:M2 with each other have been active for most of the eruptive episodes. Because these migration routes are common to all eruption products we infer that they represent robust and long-term features of Etna’s plumbing system. We also find minor migration routes connected to more evolved MEs such as M3 (=Fo65-69). We found that two characteristic timescales describe the transfer of magma along the three dominant migration routes: M0:M1, M2:mm1 and M1:M2.

The recharge of the MEs M1 and M2 commence with minor pulses that begin months or even years (up to 2 years) before an eruptive event. This recharge activity gets more frequent in the weeks and days prior to eruptive activity. The recharge of the ME mm1 is characterized by short timescales of less than 40 days. These short timescales underpin the transient character of the ME mm1.

Combining these results with thermodynamic modelling using the MELTS software [1, 2], we found that water content, and possibly oxidation state, are the main distinguishing features of the different MEs.

[1] Ghiorso, M.S. & Sack, R.O. (1995) Contrib Mineral Petrol 119, 197-212. [2] Asimow, P.D., Ghiorso, M.S. (1998) Am Mineral 83, 1127-1131.