The guts of a sea-floor spreading system

What goes on beneath constructive plate margins, and ocean ridges has, up to now, been largely a matter of conjecture, blended with the geology of ophiolite complexes obducted onto continents.  Ophiolites are perhaps not such a good model, since the low buoyancy of the basalt capped lithosphere that they represent prevented them from subduction, and stems from unusual conditions.  The bulk of oceanic lithosphere is destined for resorption into the mantle, and it forms at common or garden ridge systems.

One way of modelling magmatism at ridges is through geochemical analysis of mid-ocean ridge basalts matched with topographic and structural detail of the ridge itself, but this is a blurred approach.  It shows that part of the process must involve ponding of magma in chambers at shallow levels beneath the ridges.  The other aspect is the form taken by the mantle that must rise to undergo adiabatic partial melting.  For fast-spreading ridges, such as the East Pacific Rise, there are two such models: constraint of rising mantle in two-dimensional sheets descending from beneath the ridge itself; three-dimensional plumes of mantle from which magma migrates laterally to ridge segments.  Amplifying geochemical-structural models needs a better idea of the actual processes and the geometries that they take.  A means of getting this information is to use a technique well-honed by petroleum exploration; 3-D seismic reflection profiling.

A consortium of geophysicists from the universities of California and Cambridge used this costly method, involving 200 profiles, to look at 400 km2 of the East Pacific Rise at 9°N (Kent, G.M. and 10 others, 2000.  Evidence from three-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities.  Nature, v. 406, p. 614-618).  Melts have about half the seismic velocity of solid rock, and so boundaries between melt and solid show up with better contrast on seismic records than do boundaries in piles of sedimentary rocks.  The surprising result is that instead of vertically extensive magma chambers, expected from either hypothesis, melt occurs in a narrow, continuous sill-like body beneath the ridge.  This connects to a plunging tongue that is probably the path taken by magma from the zone of partial melting in the mantle.  The sill itself occurs at a fixed depth below that predicted from ophiolite studies for the level at which vertical sheeted dykes form the lower part of the petrologically defined crust.  This suggests that the magma simply cannot rise en masse to inject along extensional fissures as the lower crust fails, the sheeted dyke layer acting like a seal in the flow of petroleum in sedimentary basins.  Instead, it seems more likely that magma ekes out as rising rivulets that follow the base of the dyke layer until the reach dilatations at the ridge.

Although results from this study are inconclusive as regards the two models for rising mantle, the detail that it reveals augurs well for further 3-D surveys of ocean magmatism that will complement seismic tomography of the deep mantle.


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