Reduced carbonic fluid and possible nature of high-K magmas of Tolbachik

Abstract

Abstract Historical basaltic eruptions of Tolbachik volcano (Kamchatka) are of a medium to high potassic type. The potassic character of magmatism can be attributed to the influence of CO2–CO-rich fluid at or near the magma generation depths. Decarbonatization reactions in the mantle under Tolbachik producing a column of the carbonic fluids may be connected with the recent accretion of Kronotsky paleoarc with carbonates dragged under the mantle wedge. With thermodynamic modeling, we show that reduced carbonic fluid at fO2 < {NNO} may be a good carrier of nickel transported in the form of Ni(CO)4. This carbonyl is expected to become thermally stable near the magmatic temperatures at pressures above 1 GPa. In the crust, it is predicted to be thermally stable within the {PT} field of the amphibolite facies. We connect the particles of native Ni and Ag–Pt alloy observed in the volcanic aerosols from the 2012–13 Tolbachik eruption with flushing of the ascending Tolbachik magma with reduced carbonic fluids enriched with {PGE} and Ni. Native metals may form by the thermal decomposition of the carbonyls and other carbon-bearing compounds dissolved in the fluid.