Clinopyroxene and titanomagnetite cation redistributions at Mt. Etna volcano (Sicily, Italy): Footprints of the final solidification history of lava fountains and lava flows

Abstract

For a better understanding of the final solidification history of eruptions at Mt. Etna volcano (Sicily, Italy), we have investigated cation redistributions at the interface between sub-millimetre-sized clinopyroxene and titanomagnetite crystal rims and coexisting melts. The studied products were scoria clasts from lava fountains and rock samples from pahoehoe and aa lava flows. Our data indicate that scoria clasts from lava fountaining were rapidly quenched at the contact with the atmosphere, preserving the original crystal textures and compositions inherited during magma dynamics within the plumbing system. Kinetics and energetics of crystallization were instantaneously frozen-in and post-eruptive effects on mineral chemistry were negligible. The nearequilibrium compositions of clinopyroxene and titanomagnetite indicate that lava fountain episodes were supplied by high-temperature, H2O-rich magmas ascending with velocities of 0.01–0.31 m/s. In contrast, magmas feeding lava flow eruptions underwent a more complex solidification history where the final stage of the crystal growth was mostly influenced by volatile loss and heat dissipation at syn- and post-eruptive conditions. Due to kinetic effects associated with magma undercooling, clinopyroxenes and titanomagnetites formed by crystal attachment and agglomeration mechanisms leading to intricate intergrowth textures. The final compositions of these minerals testify to closure temperatures and melt–water concentrations remarkably lower than those estimated for lava fountains. Kinetically-controlled cation redistributions at the crystal–melt interface suggest that the solidification of magma was driven by degassing and cooling processes proceeding from the uppermost part of the volcanic conduit to the surface.