New insights from plumbing system below composite mafic volcanoes: Post-glacial volatile contents and magmatic fluids from Villarrica magmas

Abstract

Villarrica volcano, in the southern Andes, is a composite mafic volcano whose persistent open-vent activity is punctuated by frequent Strombolian/Hawaiian eruptions and, more rarely, by more energetic (sub-Plinian) events. Here, we investigate the volatile composition of the parental melts that sustain this activity, and the conditions of pre-eruptive magma storage, by characterizing the composition of olivine-hosted melt and fluid inclusions. We concentrate on inclusions entrapped in minerals from pyroclastic materials erupted from both Villarrica summit and from its flank Minor Eruptive Centers (MECs) post the 14.5–13.5 kyr caldera collapse event that formed the Licán ignimbrite. Our micro-FTIR and SIMS measurements indicate that the Pucón eruption records the highest volatile contents, with 6.0 wt% H₂O, >1500 ppm CO₂, 1330 ppm S, 1556 ppm Cl, and 2055 ppm F. These volatile contents imply a volatile-saturated magma originating from a depth of 14.4 to 17 km below Villarrica. Results for other flank eruptions highlight a similarly deep (17–21 km depth) source for basaltic CO₂-rich mafic magmas erupted at regional MECs (Los Nevados, Caburgua). Melt inclusion results also reveal that deep rising mafic magma batches, when temporarily stored at 1–5 km depth, produce the more differentiated and degassed magma batches that sustain the decadal-old persistent effusive-explosive eruptive activity at Villarrica. Helium isotope ratios (³He/⁴He; Rc/Ra when corrected for atmosphere) measured in bulk noble gases from olivines (Fo₇₅–₈₈) indicate that the parental magmatic fluid signature (Rc/Ra = 6.7–7.6; CO₂/³He = 4.7–7.5E⁺⁰⁸) is only recorded during central paroxysmal sub-Plinian eruption, and that this primitive gas signal is diluted in lateral MECs (Rc/Ra < 6.5; CO₂/³He = 1.4 × 10⁺⁹–3.1E⁺¹⁰).