INTERACTION OF MANTLE PLUMES AND MIGRATING MIDOCEAN RIDGES - IMPLICATIONS FOR THE GALAPAGOS-PLUME-RIDGE-SYSTEM

We investigate the three-dimensional interaction of mantle plumes and migrating mid-ocean ridges with variable viscosity numerical models. N umerical models predict that along-axis plume width W and maximum dist ance of plume-ridge interaction x(max) scale with (Q/U)(1/2), where Q is plume source volume flux and U is ridge full spreading rate. Both W and x(max) increase with buoyancy number Pi(b) which reflects the str ength of gravitational- versus plate-driven spreading. Scaling laws de rived for stationary ridges in steady-state with near-ridge plumes are consistent with those obtained from independent studies of Ribe [1996 ]. In the case of a migrating ridge, the distance of plume-ridge inter action is reduced when a ridge migrates toward the plume because of th e excess drag of the faster moving leading plate and enhanced when a r idge migrates away from the plume because of the reduced drag of the s lower moving trailing plate. Given the mildly buoyant and relatively v iscous plumes investigated here, the slope of the lithospheric boundar y and thermal erosion of the lithosphere have little effect on plume f low. From observed plume widths of the Galapagos plume-migrating ridge system, our scaling laws yield estimates of Galapagos plume volume fl ux of 5-16 x 10(6) km(3) m.y.(-1) and a buoyancy flux of similar to 2 x 10(3) kg s(-1). Model results that the observed increase isochron ag e is due to higher crustal production when the Galapagos ridge axis wa s closer to the plume several million years ago. The anomaly amplitude s can be explained by a plume source with a relatively mild temperatur e anomaly (50 degrees-100 degrees C) and moderate radius (100-200 km). Predictions of the along-axis geochemical signature of the plume sugg est that mixing between the plume and ambient mantle sources may not o ccur in the asthenosphere but, instead, may occur deeper in the mantle possibly by entrainment of depleted mantle as the plume ascends from its source.