Fluid regime of magma reservoirs of large-scale caldera-forming eruptions by example of pleistocene-holocene calderas at the Iturup island (Kurile islands)

Large caldera eruptions are generally confined to subduction zones. They are featured by colossal released energy and frequent catastrophic consequences. They are fast and recover from tenths to thousands km3 of magmatic substance from depth. Large caldera eruptions have notable and sometimes significant climatic effect, affecting the compositions of the Earth atmosphere. Postcaldera hydrothermal processes lead to formation of epithermal and porphyry mineral deposits. All this means that fluids play important role in magmatic processes prior to large eruptions. Nevertheless the details of volatile behavior in the shallow magma reservoir prior to catastrophic volcanic explosions are largely unknown. The paper represents the results of the study of pre-eruption volatile and fluid behavior in magma chambers beneath two large calderas at the Iturup Island (Great Kuril island row). Wide areas at the north and south of the Iturup Island are covered by Pleistocene-Holocene ` tuff and pumice deposits. The southern area is related to formation of Lvinaya Past’(LP) 7x8 km caldera. The estimations of volume of the erupted material vary from 20 to 170 km3. The northern area is located at the lowland Vetrovoy Isthmus (VI) and the volume of erupted material is estimated at 100 km3. Both dacitic magmas were similar in compositions were crystal rich and contained rhyolitic melts. They crystallized at 850°C under strongly oxidized conditions (NNO+2). Magma storage of the VI eruption was shallow (~1 kbar) and increase of the water content lead to degassing and release of the H2O-CO2 fluid. The storage of the LP eruption was located deeper (>1.5 kbar) and in contrast to VI similar increase in water content resulted in crystallization of Mg-hornblende. The presence of CO2 was documented only for fluids released on degassing from the VI reservoir. Rhyolitic melts of both eruptions were strongly depleted in S. Small amounts of SO2 were released on degassing of the VI magma, but the most was redistributed between apatite and sulfide. Similarly S was distributed between minerals: apatite and sulfide in the LP magma. Despite the degassing of the VI reservoir Cl was mostly distributed between the melt and apatite. Probably small amounts left it with aqueous fluids. In the LP reservoir Cl was distributed between the melt, apatite and amphibole. Similarly to the most of subduction-zone felsic magmas the melts of both eruptions are depleted in F. However F plays important role in the magma evolution. In both reservoirs apatite is a major sink for F, while in VI reservoir some fluorine could be lost with fluids. The study showed that in the shallow magma reservoirs of large caldera eruptions at the Iturup Island the behavior of H2O and CO2 strongly depends on pressure and degassing. Other major volatiles S, Cl and F tend to rest in the crystallizing melts or to be distributed in the crystallized minerals