Origin of sulphur in relation to silicate-sulphide immiscibility in Tolbachik primitive arc magma (Kamchatka, Russia): Insights from sulphur and boron isotopes

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