Crystal residence times from trace element zoning in plagioclase reveal changes in magma transfer dynamics at Mt. Etna during the last 400 years

Abstract

Trace element zoning in plagioclase of selected alkaline lavas from the historic (pre- 1971 AD) and recent (post-1971 AD) activity of Mt. Etna volcano has been used to highlight the possible role that volcano-tectonics exert on magma transfer dynamics. The observed textural characteristics of crystals include near-equilibrium textures (i.e.,oscillatory zoning) and textures with variable extent of disequilibrium (patchy zoning, coarse sieve textures and dissolved cores). One of the main differences between crystals of the historic and recent activity is the higher average 28 anorthite content for the post-1971 AD plagioclases, which agrees with the more basic character of the hosting lava. Among the most noticeable variations, recent plagioclases also exhibit higher K, Rb, LREEs and lower Ba abundances than the historic ones, with the largest differences more evident at high anorthite (An) contents. Variations in anorthite content along core-to-rim profiles obtained on crystals with different types of textures for both the historic and recent eruptive periods were evaluated particularly versus Sr/Ba. At comparable average An contents, crystals characterized by oscillatory zoning, which are representative of near-equilibrium crystallization from the magma, display distinct Sr/Ba ratios (~6 in historic and ~17 in recent lavas). We suggest these features are primarily related to recharge of a new, geochemically-distinct magma into the storage and transport system of the volcano since 1971 AD. In addition to distinct trace element and textural characteristics of plagioclase, Sr diffusion calculations for plagioclase suggest that magma residence times are generally shorter for magmas erupted in recent times (i.e., post-1971 AD) compared to those erupted during the historic period. These estimations match well with the enhanced extension rate within the upper 10 km of the crust observed during the last decades, and are also in accordance with the increased eruptive frequency after the 1971 AD eruption.