LABORATORY INVESTIGATION OF THE INTERACTION OF OFF-AXIS MANTLE PLUMESAND SPREADING CENTERS

MANTLE plumes and mid-ocean ridge spreading centres are the dominant p henomena through which mass and heat are transported from the mantle t o the Earth's surface. It now seems that the dispersion of near-ridge plumes beneath the lithosphere is modulated strongly by mid-ocean ridg es; in particular, geochemical and geophysical observations have sugge sted that rising plumes are diverted towards and feed nearby ridges(1- 7). Here we confirm the feasibility of this model with laboratory expe riments that incorporate the essential physical and fluid dynamic aspe cts of a plume-ridge upper mantle system. Our results indicate that an off-axis plume may communicate thermally and chemically with a spread ing ridge through a narrow, sub-horizontal conduit instead of a broade r, radially spreading plume head. A necessary condition for this commu nication Is the presence of a lithospheric or theological boundary lay er that thickens away from the ridge axis owing to conductive cooling. Interestingly, we find that for high plume temperatures, increasing t he plume thermal buoyancy may inhibit rather than enhance plume-ridge interaction, as a result of increased erosion of the overlying lithosp here.