On the Connection between Sulfide Inclusions in Olivine from Tolbachik Volcano and Fluids from Mafic Cumulates beneath the Klyuchevskoy Group Volcanoes

Abstract

The high activity of the Klyuchevskoy group volcanoes in the Holocene suggests that considerable volumes of partly solidified magma (cumulates) and mafic–ultramafic intrusions have accumulated in the crust. Together with extensive fluid flow typical the zones of rapid subduction of an old oceanic plate, this provides conditions for the formation of a fluid–magma ore-forming system. Olivine with sulfide inclusions was found in the eruption products of Tolbachik Volcano. Its investigation may provide insight into the composition of crustal fluid of such ore-magmatic systems. The interaction of reduced water-poor fluid with oxidized basaltic melt (NNO + 1.5) containing 2000–3000 ppm sulfur was theoretically modeled. It was shown that at a local fluid content higher than ~1–2 wt %, sulfur in the melt is reduced and sulfide droplets are formed. Sulfur reduction in the melt can also be caused by the dissolution of SO₂, which is the main sulfur species in fluid at log fO₂ ≥ NNO + 1.5. This effect is related to the higher degree of sulfur oxidation (S⁺⁶) in melt, where \({\text{SO}}_{4}^{{2 - }}\) is the only oxidized sulfur species, compared with SO₂ (S⁺⁴) in fluid. According to calculations, sulfide formation begins after dissolution of approximately 2000–3000 ppm sulfur in the SO₂ form in melt at log fO₂ ≥ NNO + 1. Interaction with fluid with small contents of precious metals (PM) produces sulfide melt droplets with PM contents corresponding to the background values in the melt. According to experimental evidence, Pt and Pd are highly soluble in reduced water-poor fluids in the form of carbonyls, whereas Au is low soluble; in contrast, Au solubility is very high in oxidized fluids (NNO + 1 to NNO + 1.5). Reaction with mineralized fluid containing up to tens of ppm PM produces sulfide melt enriched in Au (oxidized fluid) or Pt (reduced fluid). Interaction of melt with water-poor fluid causes local dehydration and an increase in liquidus temperature, which results in rapid olivine crystallization at high overcooling. The localization of phase transitions at the boundary of fluid bubbles facilitates the entrapment of sulfide droplets by olivine. The rare occurrence of sulfide inclusions in olivine from Tolbachik Volcano can be related to the rapid dissipation of the local effect of magma interaction with small amounts of fluid and dissolution of the precipitated sulfide phase in the melt.