Oceanic Lithosphere 4. The origin and evolution of oceanic lithosphere: Magmatic processes at oceanic spreading centres

Primary melts generated in the oceanic mantle migrate upward and pond at ma jor discontinuities to form magma chambers. Such chambers have thus far bee n identified only on fast- and intermediate-spreading ridges but probably a lso exist at slow-spreading ridges. The size, shape and longevity of subrif t chambers reflect the magma supply rate, the extent of hydrothermal coolin g, and the regional stress field. Ophiolite studies suggest small, ephemera l chambers rather than large, long-lived bodies. At fast-spreading ridges t he chambers probably consist largely of crystalline mush, possibly with som e melt sills, and a thin melt zone at the top. At slower-spreading ridges, magmatic activity is more episodic and seafloor spreading may be punctuated by periods of mainly tectonic extension. Fractionating melts in the chambe rs are buffered by injections of more primitive melt from depth to produce the relatively uniform composition of MORB. The gross structural uniformity of the ocean crust must reflect extensive interplay and feedback of magmat ic, hydrothermal and tectonic processes, resulting in a self-ordered system .