Cryptic magma recharge associated with the most voluminous 20th century eruptions (1921, 1948 and 1971) at Villarrica Volcano

Villarrica volcano is one of the most active volcanoes of the Andes (rest cycles of 5.4 ± 3.9 years) and erupts products of basaltic to basaltic-andesite compositions and in styles from Hawaiian to Vigorous Strombolian. We compare whole-rock geochemistry and mineral abundances, textures and compositions of the most voluminous basaltic lavas of the 20th century, which were erupted in 1921, 1948 and 1971. The analyzed lavas show a restricted range in whole-rock compositions (51.7–52.4 wt% SiO₂), but there is an increasing trend of MgO content from 1921 (~5.3 wt% MgO) to 1948 (~7.0 wt% MgO) and a decreasing trend from 1948 to 1971 (~6.0 wt% MgO). Two groups of plagioclase compositions are observed in all lavas: An-poor (~An₆₀) and An-rich (~An₈₀). An-poor compositions are the most abundant in all lavas and were commonly formed in both the earliest (cores) and latest stages (external rims and small and nearly unzoned phenocrysts) of plagioclase crystallization, whereas An-rich plagioclase formed around An-poor cores during intermediate stages. Additionally, precipitation of An-poor plagioclase in both rims and cores could simultaneously have occurred in highly dissolved An-rich antecrysts. Equilibrium temperatures of plagioclase-olivine-clinopyroxene of about 1090 °C were obtained in crystal clots of the three lavas. Thermometry calculations obtained from MELTS simulations of plagioclase formation indicate a heating (thermal mixing) of the reservoir at ~0.5 kbar of up to 100 °C with respect to the crystal clots temperatures. Incorporation of small amounts of volatile-rich and hotter mafic magma of similar composition (cryptic mixing) into the Villarrica reservoir, could heat it, probably in a more efficient way than heat conduction alone. A longer interaction of the hot mafic magma within the reservoir could cause crystallization of An-rich plagioclase, dissolution of An-poor plagioclase (i.e. increase in An-rich/An-poor modal content ratios) and compositional modifications toward MgO-rich and volatile-rich compositions, that ultimately result in higher intensity of eruption.