Models of hydrothermal heat output from a convecting, crystallizing, replenished magma chamber beneath an oceanic spreading center

The temperature and heat output of hydrothermal systems at oceanic spreading centers place strong constraints on the mechanism of heat transfer in oceanic crust. In this paper, we investigate time-dependent heat transfer from a vigorously convecting, crystallizing, and replenished magmatic sill beneath an ocean ridge axis, linked to an overlying hydrothermal system. We first consider two different crystallization scenarios, crystals suspended'' and crystals settling,'' coupled with crystallinity-dependent and temperature-dependent magma viscosity. The large-scale convection is assumed to rapidly homogenize the magma, resulting in a characteristic temperature T-m. In cases without magma replenishment, the simulation results for crystal-suspended models show that heat output and the hydrothermal temperature decrease rapidly and that crystallinity reaches 60% in less than 10 a. In crystal-settling models, magma convection may last for decades, but decreasing heat output and hydrothermal temperatures still occur on decadal timescales. When magma replenishment is included, the magmatic heat flux approaches steady state on decadal timescales, whereas the magma body grows to double its original size. The rate of magma replenishment needed ranges between 5 x 10(5) and 5 x 10(6) m(3) a(-1), which is somewhat faster than that required for seafloor spreading but less than that of fluxes to some terrestrial volcanoes on similar timescales. The heat output from a convecting, crystallizing, replenished magma body that is needed to drive observed high-temperature hydrothermal systems is consistent, with gabbro glacier models of crustal production at mid-ocean ridges.