CO 2 and the eruptibility of picrite and komatiite

Abstract

A tholeiitic picrite erupted from the summit region of Kilauea volcano, Hawaii in 1959. Work by Wright, Helz and Schwindinger and Anderson shows that the 1959 magma was a mixture of new, hot, MgO-rich magma that rose through and mixed with cooler stored magma causing crystallization of abundant olivine phenocrysts. CO 2 dissolved in melt inclusions in olivine phenocrysts from the eruption of 1959 reveals that most of the olivines grew at pressures less than 1 kbar (100 MPa). Therefore, the new magma probably was buoyant relative to stored magma at 1 kbar. The MgO-rich magma is characterized by a relatively dense melt and could only be buoyant, if it contained more gas than the stored magma. Compositions of inclusions indicate that the gas was rich in CO 2. Because gas is highly compressible, the mass fraction of CO 2-rich gas that is required for magma buoyancy increases at greater pressures. A critical pressure is that at which gassy, MgO-rich, parental magma has the same density as stored magma. If the parental magma invades stored magma at a sufficiently great pressure, it will be negatively buoyant and its consequent descent will inhibit its eruption. Kilauea's degassing suggests that Kilauean parental basalt may contain about 0.3 wt.% CO 2. Gassy picritic magma (liquid with 15 wt.% MgO + 0.3 bulk wt.% CO 2) is buoyant with respect to degassed tholeiitic basalt (8 wt.% MgO liquid) at pressures less than about 2 kbar. For Kilauea's 1959 picrite the inferred 1 kbar pressure at which new MgO-rich magma invaded stored magma is consistent with its buoyant rise to the surface and eruption. Although the parental magma of Kilauea is considered to have a picritic composition, erupted picrite is uncommon, particularly in Kilauean summit eruptions. Beneath Kilauea's summit a reservoir of intermittent magma storage extends downward to about 10 km corresponding to a pressure of about 2.7 kbar. Ryan has argued that the base of the magma storage reservoir is controlled by the transition region that separates porosity elimination from mineral compression. The rarity of erupted picrite as a lava on Kilauea probably reflects a combination of two factors: (1) parental Kilauean picritic magma probably has a CO 2 content less than about 0.3 wt.%; (2) most parental picritic magma enters the reservoir near its base where the pressure is sufficiently high to result in the bulk density of parental magma being greater than that of stored magma. The new, 1959 picritic magma ascended through an unusual route and probably first encountered stored magma at an unusually shallow level where the new magma was relatively buoyant. Because of the role of porosity, the base of subvolcanic magma storage reservoirs is dependent only on pressure and rock strength. Therefore, the base of reservoirs of stored magma beneath active volcanoes likely occurred between 2 and 3 kbar during ancient as well as modern times. Because gas-free komatiitic magma is relatively dense, it too is expected to be trapped beneath stored magma. Komatiitic magmas containing more than about 0.6 wt.% of CO 2, however, would be buoyant relative to stored, degassed basalt melt even at the base of magma storage reservoirs. The common occurrence of komatiitic lavas in Archaean times may reflect a greater CO 2 content of mantle-derived Archean parental magmas.