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

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.