The link between convection and crystallization in a sub-axial magma chamber and heat output in a seafloor hydrothermal system

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Liu, Lei
Lowell, Robert P.
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In this thesis, I present a simple time-dependent model of heat transfer between a turbulently convecting and crystallizing magma body and the overlying hydrothermal circulation. Most of the known seafloor hydrothermal sites on faster-spreading ridges are dominated by basalt. The hydrothermal fields within parts of the Lau Basin in the Southwest Pacific are driven by andesitic magma. To determinate the different characteristics of magma-driven hydrothermal system, two types of magma material, basaltic and andesitic magma are considered. Two different crystallization scenarios are considered¡ªcrystals in suspension and crystals settling. In either case, I assume that large-scale convection within the magma chamber is homogenous. Also, the effect of crystallinity and water content-dependent magmatic viscosity is considered. Based on the proposed models, the total heat output from the upper surface of the magma chamber and the temperature in hydrothermal system are derived numerically. The simulation results show that without magma replenishment, the heat output and hydrothermal temperature decay rapidly within about ten years. For two different crystallization distribution cases, such rapid decay is not consistent with observations. The conflict between the simulation results and the field observations shows the need to develop more accurate magma convection models. Different from the existing modeling methods, I propose to model the magma convection with replenishment. The replenishment model can be classified into two categories in terms of status of magma chamber size. To replenish the magma system without changing the magma chamber size, the heat flux decaying rate is slowed down and hydrothermal system lifetime is extended for a little longer. Although this model is more accurate than existing ones in terms of slow decaying rate of heat flux, it does not achieve a steady state as is observed. This leads us to model replenishment with variant magma chamber size. I model the replenishment rate as a constant and exponential decay, respectively. Thus, I assume the magma chamber size is time-varying. Simulation results show that magma heat flux approaches a steady state over a time scale of decades. This result is consistent with the observations, which indicates the effectiveness of proposed modeling methods.
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