The Interplay of Magmatic and Hydrothermal Convection: Insights From Numerical Modelling

Abstract:

At fast spreading mid-ocean ridges, the heat released by an axial magma chamber (AMC) is the main driver of hydrothermal circulation at the ridge axis. Seismic studies at the East Pacific Rise show that the AMC is continuous along-axis and has relatively small depth variations. In contrast, slow spreading ridges have short-lived, discontinuous melt lenses and a much sparser abundance of axial hydrothermal fields. These observations point at a strong link between the abundance of on-axis hydrothermal fields and the spatial and temporal variations of magmatic energy input.

To better understand the interplay of magmatic and hydrothermal processes we developed 2D and 3D numerical models that simultaneously solve for crustal accretion processes and hydrothermal convection. Our models cover the oceanic crust from depths below the AMC to the seafloor. The “magmatic" model part simulates the processes within the AMC such as convection of the viscous melt, crystallization and the associated release of latent heat. The “hydrothermal" model part is restricted to the permeable regions of the crust at temperatures below the brittle-ductile transition at ~700 ºC. Here we assume Darcy flow of a super-critical single-phase fluid and account for the thermodynamic properties of water. Boundary conditions allow for free venting of hydrothermal plumes at the top of the model domain. Magmatic and hydrothermal parts are coupled by the crustal temperature field, leading to two dynamic convective systems that are connected by a relatively thin, impermeable conductive boundary layer between ~700 ºC and 1000 ºC.

First results indicate that the balance between the rate of energy input from magmatic processes and the rate of heat removal by hydrothermal flow controls the along-ridge depth of the AMC. Hydrothermal upflow and associated venting preferentially forms above “highs” of the AMC roof. Recharge flow surrounds these hot thermal plumes, because the thermodynamic properties of water (i.e. its fluxibility) make these warm regions more efficient for recharge flow than colder parts of the crust. Above local depressions of the AMC roof we observe less vigorous hydrothermal flow, so that along-ridge variations in AMC depth could be linked to locations of hydrothermal vent sites.