The Depth of Mid-Ocean Ridges Through Earth's Evolution and a Two-Phase Study of Melt Focusing at Mid-Ocean Ridges

摘要

Mid-ocean ridges are prominent features of plate tectonics, stretching for more than 60,000 km in the major ocean basins. Organisms thrive at mid-ocean ridges making it a unique system that connects life, water and plate tectonics. This thesis evaluates the evolution of the global ridge system and considers the processes of melt focusing beneath mid-ocean ridges using two phase flow models. First, I use whole Earth mantle convection models to understand how the global ridge system might have changed over Earth's evolution along with the depths of ocean. I show that mid-ocean ridges have remained submerged over geologic time and that its average depths have not varied by more than 500 m. Mid-ocean ridges contribute to 90% of global magmatism and yet melt generation and extraction are not well understood due to the difficulty in surveying these systems submerged under a few kilometers of ocean. Active seismic and magnetotelluric surveys suggest that melt is generated in a wide region beneath mid-ocean ridges (Forsyth et al., 1998; Key et al., 2013) and yet the oceanic crust is formed within a narrow neo-volcanic zone at the ridge axis (Macdonald, 1984). Several mechanisms to focus melt have been proposed (Spiegelman and McKenzie, 1987; Phipps-Morgan, 1987; Sparks and Parmentier, 1991; Aharonov et al., 1995). I present new open source two phase models, Melt in the Mantle beneath Mid-ocean ridges (M3LT), based on TerraFERMA, the Transparent Finite Element Rapid Model Assembler (Wilson et al., 2017), a software for coupled multi-physics problems. Our multi-phase flow models incorporate realistic viscosities and thermal feedbacks. To ensure reproducibility, the models are openly available in the form of TerraFERMA mark up language files. I present a suite of models varying the half spreading rates. The melt generated in our models produce oceanic crustal thicknesses within geophysical observations. We use these models to review and illustrate three mechanisms that are responsible for melt focusing namely, ridge suction, decompaction layers and melting rate focusing (recently observed mechanism). I show that decompaction layers and melting rate focusing are the dominant mechanisms for focusing melt at mid-ocean ridges. Model results show that the melting rate focusing persists regardless of half spreading rates, while the proportion of melt focused by decompaction layers increases with half spreading rate.