Qualitative models of spreading-center processes, including hydrothermal penetration

Abstract

Observations on active geothermal areas suggest that water can penetrate rapidly into hot rock, and this is confirmed by semi-quantitative theory. Upwelling of magma material at an oceanic spreading center places hot rock close to an ample supply of cold water. Clues to the nature of the interaction can be found in the structure of ophiolite suites, and in simple physical induction on the nature of the upwelling. The subsurface dike injection zone seems to be dominated by conductive cooling and to provide a barrier against water penetration as long as it is heated by liquid magma in a chamber below. As soon as the magma has crystallized into a cumulate, water penetration proceeds rapidly to the base of the crust. It is probably stopped by meeting an ultramafic layer of olivine phenocrysts separated from the upwelling melt. The olivine crystals may be oriented by a fluidized-bed process near the spreading axis, forming a thin acoustically anisotropic layer. Below, mantle material depleted by partial melting grades back to primitive composition at depth. The mantle material should remain viscous enough to upwell in a broad zone, and the coupling of distributed upwelling to a thick rigid crust formed close to the spreading axis is responsible for the central valley and block tectonics characteristic of slow-spreading ridges. At fast spreading rates, the larger magma chamber decouples a thinner rigid surface layer and smooth topography results.