LAUSR

Abstract This study reports the first measurements of sulphur and boron isotopic compositions in olivine-hosted melt inclusions (MI) from basaltic lava and scoria deposits from the 1941 Tolbachik eruption, Kamchatka, Russia. The primary aim of the present study is to constrain the origin of sulphur in relation to silicate-sulphide immiscibility that occurred in the Tolbachik mafic magmas. Melt inclusions fall into two compositionally distinct groups based on their S- and B-isotope systematics, the first is hosted by olivine from lava and the second in olivine from lapilli and scoria. The upper ends of the concentration ranges for CO2 (3660–4200 μg/g) and H2O (4.5–5.9 wt%) imply the onset of magma crystallization between ~25 and ~ 30 km for both MI groups, i.e., close to the Moho discontinuity. After filtering the compositions of melt inclusions experienced H2O-loss due to the diffusion of H from olivine hosts, both lava- and scoria-related melt inclusions are characterized by similar fractionation depths between ~15 and ~ 30 km. The inclusions from scoria show no signs of sulphur degassing at depth and, along with the most S-rich inclusions from lava, exhibit enrichment in S. Strong positive correlations of S/K2O with H2O/K2O, Cl/K2O and F/K2O, as well as the correlations of these ratios with δ34S also preclude significant S-loss by degassing and point towards two-component mixing of magmas containing contrasting concentrations of volatile components. As follows from B isotopic data, slab-derived 11B-enriched fluids and, to a minor extent, 11B-depleted melts could be major agents controlling the composition of the mantle wedge and, consequently, the chemical and isotopic variability of its derivative melts. A subducted serpentinized peridotite could serve as a possible source of the 11B- and probably the 34S-rich signature of the studied primitive Tolbachik magmas.