Some hard rock constraints on the supply of heat to mid-ocean ridges
In this paper, we examine many aspects of how heat from melts and from the mantle is transferred to the oceanic lithosphere at mid-ocean ridges, focusing on constraints that can be derived from rock specimen and geological observations at mid-ocean ridges and in ophiolites. These range from experimental constraints on the specific heat and latent heat of crystallisation of oceanic crustal materials, to geological constraints on how melt is distributed throughout the axial lithosphere, and partitioned between gabbros, dykes, and lava. We also discuss oceanic magma chamber processes and the episodicity of magma input at fast and slow-spreading ridges, and test four different ways of estimating the melt supply to mid-ocean ridges (seismic crustal thickness; regional axial depth; sodium and REE contents in MORB). We find that these four methods yield consistent results at East Pacific Rise sites, but that determination of the melt (and heat) supply to the crust of slow-spreading ridges is not as straightforward and requires a good integration of local scale geological, geophysical, and geochemical data. Many of our conclusions concern the tectonically and lithologically complex slow-spreading ridges. We discuss these ridges under two crustal end-members: a magmatically-dominated, segment centre type, end-member, and a magma-poor, segment end type, end-member. In this magma-poor end-member mantle-derived peridotite must be a significant source of heat, and tectonic heat advection is expected as deeply-derived rocks are tectonically uplifted through the axial lithosphere and emplaced into the crust. We estimate that the total heat supply per unit area of young crust in this magma-poor end-member setting should not be much less than the heat supply per unit area of young crust formed in magmatically-dominated segment centres. This heat would not, however, be available in the same way to axial hydrothermal systems. We also show that there is substantial evidence for partial crystallisation of magma in the form of gabbros (and for the production of latent heat of crystallisation) at sub-crustal depth below slow-spreading ridges.