Origin of volatiles emitted by Plinian mafic eruptions of the Chikurachki volcano, Kurile arc, Russia: Trace element, boron and sulphur isotope constraints

Abstract

Chikurachki is a 1816-m high stratovolcano on Paramushir Island, Kurile arc, Russia, which has repeatedly produced highly explosive eruptions of mafic composition. The present work is aimed at constraining the origin of volatile components (CO2, H2O, F, S, and Cl), along with B and S isotopic compositions in a series of phenocryst-hosted melt inclusions and groundmass glasses from basaltic andesite pyroclasts of the 1853, 1986, and prehistoric Plinian eruptions of the volcano. The ranges of volatile concentrations in melt inclusions (47–1580μg/g CO2, 0.4–4.2wt% H2O, 399–633μg/g F, 619–3402μg/g S and 805–1240μg/g Cl) imply a sudden pressure release from ~460 through ~35MPa that corresponds to ~1.2–16-km-depth range of magma ascent upon decompression. We conclude that rapid ascent of the volatile-rich basaltic magmas from ~16-km initial depth accompanied by near-surface bubble nucleation and growth, and subsequent magma fragmentation appear to be a primary reason for the Plinian character of the Chikurachki eruptions. Significant negative correlations of S with K, Zr, Nb, Ba, La, Ce, Pr (R=−0.8 to −0.9), no clear relationships of S with H2O, CO2 and Cl, but strong positive correlations of S/K2O with H2O/K2O, Cl/K2O and F/K2O preclude magma degassing to be the only process affecting volatile concentrations dissolved in the melt. The δ34S values of the studied inclusion and groundmass glasses range from −1.6 to +12.3‰, decrease with decreasing S, show significant positive correlations with H2O/K2O, Cl/K2O and F/Zr, and negative correlations with a number of incompatible trace elements. Neither open- nor close-system magma degassing can account for the observed range of δ34S. The δ11B values of the melt inclusions range from −7.0 to +2.4‰ with 13–23μg/g B. The relationships of δ11B with B/K2O and B/Nb are inconsistent with magma contamination at shallow crustal depths. Linear character of 1/S vs. δ34S relationship suggests two-component mixing. The possible mixing end-members could be the magmas having similar major and trace element compositions, but strongly contrasting volatile contents and S isotopes. Based on the behaviour of fluid-mobile vs. fluid-immobile incompatible trace elements, we conclude that the subduction component likely represents a mixture of subduction sediment-derived melt with up to 60% of slab-derived fluid. Admixture of ~1–8% of the inferred subduction component to the depleted mantle wedge source is required to account for the compositional range of the Chikurachki melt inclusions, and ~0.4–10% to constrain the composition of Kurile arc mafic magmas.