The investigation of lithium in plagioclase is becoming an important tool to track and quantify the temporal development of degassing processes of magmas ascending toward surface. We present SIMS data of Li in plagioclase crystals from products of Mount Etna (Italy) in an attempt to investigate the late-stage ascent and degassing processes leading to or accompanying the most energetic, paroxysmal eruptions occurred at the volcano in the period 2011-2013. The investigated plagioclase crystals have shown trends of decreasing lithium concentrations from the crystal interior toward the edge, with localized changes to higher Li contents. The general rim-ward decline of Li in plagioclase implies significant diffusive disequilibrium during the final stage of crystal growth, which reflects in turn Li loss from the melt through sin-eruptive degassing driven by decompression. In contrast, the occurrence of localized lithium spikes/enrichments along the plagioclase rims may be evidence of transient, dynamic processes associated with gas exsolution from deep portions of the volcano plumbing system and consequent flushing at shallow depths. Modeling the Li diffusion between plagioclase and its hosting melt allows determination of the timescales of decompression-driven magma degassing just before and during the eruption culminating with the paroxysmal phenomenology. Diffusion calculations yield timescales on the order of minutes or less. Our improved knowledge on depths of storage levels beneath the volcano, based on seismic signals, has led us to quantify extremely fast final magma ascent rates for these paroxysmal eruptions at Etna. Importance of our results consists in the application of methods, recently used exclusively for closed-system volcanoes producing violent eruptions, also to open-conduit systems that have generally quiet eruptive periods of activity sometimes interrupted by sudden re-awakening and production of anomalous energetic eruptions.
Ultra-fast sin-eruptive degassing and ascent of Etna magmas from Li gradients in plagioclase
Luisa Ottolini;
2017
Abstract
The investigation of lithium in plagioclase is becoming an important tool to track and quantify the temporal development of degassing processes of magmas ascending toward surface. We present SIMS data of Li in plagioclase crystals from products of Mount Etna (Italy) in an attempt to investigate the late-stage ascent and degassing processes leading to or accompanying the most energetic, paroxysmal eruptions occurred at the volcano in the period 2011-2013. The investigated plagioclase crystals have shown trends of decreasing lithium concentrations from the crystal interior toward the edge, with localized changes to higher Li contents. The general rim-ward decline of Li in plagioclase implies significant diffusive disequilibrium during the final stage of crystal growth, which reflects in turn Li loss from the melt through sin-eruptive degassing driven by decompression. In contrast, the occurrence of localized lithium spikes/enrichments along the plagioclase rims may be evidence of transient, dynamic processes associated with gas exsolution from deep portions of the volcano plumbing system and consequent flushing at shallow depths. Modeling the Li diffusion between plagioclase and its hosting melt allows determination of the timescales of decompression-driven magma degassing just before and during the eruption culminating with the paroxysmal phenomenology. Diffusion calculations yield timescales on the order of minutes or less. Our improved knowledge on depths of storage levels beneath the volcano, based on seismic signals, has led us to quantify extremely fast final magma ascent rates for these paroxysmal eruptions at Etna. Importance of our results consists in the application of methods, recently used exclusively for closed-system volcanoes producing violent eruptions, also to open-conduit systems that have generally quiet eruptive periods of activity sometimes interrupted by sudden re-awakening and production of anomalous energetic eruptions.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
