Modeling heat transfer from a convecting, crystallizing, replenished silicic magma chamber at an oceanic spreading center

Abstract

Most hydrothermal systems at oceanic spreading centers are underlain by basaltic magma bodies; however, some are underlain by higher-silica magmas such as andesite or dacite. The different viscosity of the latter magmas, which results from their higher SiO 2 content, lower liquidus and solidus temperatures, and higher water contents, affects the rate of heat transport from these magmas, and the behavior of the overlying hydrothermal system. We construct viscosity models for andesite and dacite melts as a function of temperature and water content and incorporate these expressions into a numerical model of thermal convective heat transport from a high Rayleigh number, crystallizing, replenished axial magma chamber (AMC) beneath a hydrothermal circulation system. Simulations comparing the time-dependent heat flux from dry basalt, 0.1 wt.% H 2 O andesite, 3 wt.% H 2 O andesite, and 4 wt.% H 2 O dacite indicate that higher-viscosity magmas convect less vigorously, resulting in lower heat flux, possibly lower vent temperatures, and a slower decay rate of the heat flux. Hydrothermal systems driven by unreplenished high-silica melts may have slightly longer lifetimes than those driven by basalt; however, vent temperature and heat output decay on decadal time scales. Magma replenishment at a rate of ∼10 -8 -10 -7 m/s across the base of the AMC can maintain relatively stable heat output between ∼10 7 -10 9 Watts and typical hydrothermal vent temperatures. Such replenishment rates are not likely to result from buoyancy-driven melt transport by porous flow through the lower crust, especially for high-viscosity magmas such as andesite and dacite. Copyright © 2011 by the American Geophysical Union.