Melt Extraction From The Mantle Beneath Mid-Ocean Ridges

As the oceanic plates move apart at midocean ridges, rocks from Earth’s mantle, far below, rise to fill the void, mostly via slow plastic flow. As the rock approaches the top of its journey, however, partial melting occurs, so that the upper 6 kilometers of oceanic crust are composed of melts, which both erupt on the seafloor as lava and crystallize beneath the surface to form what are known as “plutonic” rocks. Melt forms in tiny “pores” along the boundaries between different crystals, and the pores form an interconnected network of tubes 1 to 10 microns in diameter. Since the melts are less dense than the residual solids, they are relatively buoyant and move upward faster than the rest of the upwelling mantle. The exact process by which very small amounts of melt rise to form voluminous lava flows is the subject of some controversy. We do know that: 1) melts must rise through the upper 30 kilometers of the mantle without equilibrating with the surrounding rocks, and 2) melt forms in a region several hundred kilometers wide and then coalesces into a narrow band just a few kilometers wide—the mid-ocean ridge—where igneous crust is formed by crystallization of magma. Studying processes that take place within the earth is not easy, but plate motions occasionally thrust pieces of crust to the surface that provide windows to the interior: Outcrops of Earth’s lower crust and upper mantle are exposed in ophiolites, slices of oceanic plates that are found on land (see map overleaf). They generally occur in mountain belts along continental margins and are usually tilted, so that in the largest ophiolites the erosional surface exposes rocks that ordinarily reside 10 or even 20 kilometers below the seafloor. We turned to ophiolites to address the question of how slow, porous flow preserves disequilibrium between melts and the upper 30 kilometers of residual mantle rocks. The process by which ophiolites are thrust onto the continental margins is uncertain. This raises a basic issue: The rocks in ophiolites are certainly formed at submarine spreading centers similar to the mid-ocean ridges, but they may be exceptional in their composition or structure—after all, “normal” oceanic crust is under water! Furthermore, Melt Extraction From The Mantle Beneath Mid-Ocean Ridges