Plagioclase textural and compositional parameterization: A tool for tracking magma dynamics at Stromboli

Abstract

The Present-day (<1.2 kyr) activity of Stromboli (Aeolian Islands, Southern Italy) is characterized by periodic and mildly explosive “Strombolian” eruptions alternating with episodic lava effusion and more violent eruptive events (i.e., major explosions and paroxysms). The plumbing system controlling the eruptive behavior is fed by a vertically-extended mush column in which the shallow magmatic reservoir (highly porphyritic or Hp-magma) is continuously refilled with mafic magmas (low porphyritic or Lp-magma) rising from depth. Currently, we are investigating the textural and compositional attributes of plagioclase phenocrysts and microlites from nineteen scoria clasts ejected during mild to violent explosions at Stromboli over a timespan of ~18 years, from 2003 to 2021. The morphological stability of large-sized, euhedral phenocrysts is superimposed on an internal textural heterogeneity due to growth-dissolution phenomena associated with the input rate of hot, H2O-rich recharge magmas rising from depth. As a result, the volumetric plagioclase proportion, dominant size, and number of phenocrysts per unit volume decrease from mild to violent explosions, responding to a more efficient magma mixing process via sustained injections of mafic magmas into the shallow reservoir. Crystallization of hybridized recharge basaltic melts is faithfully recorded by intracrystalline major-trace element and Sr-isotope variations in plagioclase phenocrysts, providing temporal and spatial constraints on crystal recycling and mush remobilization phenomena. On the other hand, the formation of anhedral plagioclase microlites is controlled by fast growth kinetics taking place in the uppermost part of the conduit during magma acceleration towards the surface. Under such highly dynamic crystallization conditions, the microlite number density closely depends on the increase of melt liquidus temperature via magma decompression and H2O exsolution. This mutualism allows to model the degassing rate and ascent velocity of magma under open-conduit flow regimes for the different eruptive styles, thereby supporting the idea that violent explosions at Stromboli are driven by sustained influxes of recharge magmas leading to strong acceleration, decompression, and H2O exsolution before magma discharge at the vent.