The input of volatile-charged mafic magma into chemically evolved felsic magma reservoirs in the shallow crust leads to the exchange of heat and chemical components that may serve as the trigger for subsequent volcanic eruption. The exchange of volatiles, which occurs via diffusion and potentially through fluid movement, is particularly important to this process.
In order to simulate the magma mingling process and the role of volatiles within this process, we have conducted a series of hydrothermal experiments involving a hydrous basaltic-andesite glass containing trace olivine (prepared from a natural sample of Augustine volcano in Alaska’s Aleutian Arc) with crystal-deficient, anhydrous or hydrous Los Posos rhyolite glass (also natural) at 100 MPa. Some experiments were conducted under fluid-absent conditions while others involved O-H-Cl±S-bearing fluids. Temperature was varied during these experiments, with most beginning at 1100°C. Temperature was subsequently reduced to 800°C in several steps, where it remained until an isobaric quench. The run durations ranged from 25 to 125 hours.
The experimental products exhibit interesting textural and chemical features. The rhyolitic run product glass, whether hydrous or anhydrous initially, exhibits minimal crystallization throughout the experiments, whereas the starting basaltic-andesite glass shows significant crystallization of iron and titanium oxides, plagioclase, and pyroxene. The exchange of Cl between the two melts was significant for fluid-present and fluid-absent conditions, and the concentration of Cl in the more felsic melt was strongly controlled by well established Cl solubility relationships involving the cations Ca, Al, Na, and Mg and the presence or absence of S.