DYNAMICS OF MANTLE FLOW AND MELTING AT A RIDGE-CENTERED HOTSPOT - ICELAND AND THE MID-ATLANTIC RIDGE

The dynamics of mantle flow and melting of a ridge-centered plume were investigated with three-dimensional variable-viscosity numerical mode ls, focusing on three buoyancy sources: temperature, melt depletion, a nd melt retention. The width, W, to which a plume spreads along a ridg e axis, depends on plume volume flux, Q, full spreading rate, U, buoya ncy number, B, and ambient/plume viscosity contrast gamma. When all me lting effects are considered, our numerical results are best parameter ized by W=2.37(Q/U)(1/2)(B gamma)(0.04). Thermal buoyancy is first-ord er in controlling along-axis plume spreading while latent heat loss du e to melting, and depletion and retention buoyancy forces contribute s econd-order effects. We propose two end-member models for the Iceland plume beneath the Mid-Atlantic Ridge (MAR). The first has a broad plum e source with temperature anomaly Delta T-p of 75 degrees C, radius, a , of 300 km, and Q of 1.2x10(7) km(3)/my. The second is of a narrower and hotter plume source with Delta T-p of 170 degrees C, a radius of 6 0 km, and Q of 2.1x10(6) km(3)/my. The broad plume source predicts suc cessfully the observed seismic crustal thickness, topographic, and gra vity anomalies along the MAR, but predicts an along-axis geochemical p lume width substantially broader than that suggested by the observed S r-87/Sr-86 anomaly. The narrow plume source model predicts successfull y the total excess crustal production rate along the MAR (2.5x10(5) km (3)/my) and a geochemical width consistent with that of the Sr-87/Sr-8 6 anomaly, but it requires substantial along-axis melt transport to ex plain the observed along-axis variations in crustal thickness, bathyme try, and gravity. Calculations suggest that lateral plume dispersion m ay be radially symmetric rather than channelled along the ridge axis a nd that the topographic swell, which is elongated along the Reykjanes Ridge, may be due to rapid off-axis subsidence associated with lithosp heric cooling superimposed on a broader hotspot swell. The two plume s ource models predict seismic P-wave velocity reductions of 0.5-2% in t he center of the plume, producing travel time delays of 0.2-1.2 s. Pre dicted P-wave delay times for the narrow plume source model are more c onsistent with recent seismic observations beneath Iceland, suggesting that this model may be more representative of the Iceland plume.