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 models, focusing on three buoyancy sources: temperature, melt depletion, and melt retention. The width, W, to which a plume spreads along a ridge axis, depends on plume volume flux, Q, full spreading rate, U, buoyancy number, B, and ambientrplume viscosity contrast g . When all melting effects are considered, our numerical Ž .1r2Ž .0.04 results are best parameterized by Ws2.37 QrU Bg . Thermal buoyancy is first-order in controlling along-axis plume spreading while latent heat loss due to melting, and depletion and retention buoyancy forces contribute second-order Ž . effects. We propose two end-member models for the Iceland plume beneath the Mid-Atlantic Ridge MAR . The first has a broad plume source with temperature anomaly DT of 758C, radius, a, of 300 km, and Q of 1.2=10 kmrmy. The second p is of a narrower and hotter plume source with DT of 1708C, a radius of 60 km, and Q of 2.1=10 kmrmy. The broad p plume source predicts successfully the observed seismic crustal thickness, topographic, and gravity anomalies along the MAR, but predicts an along-axis geochemical plume width substantially broader than that suggested by the observed Srr Sr anomaly. The narrow plume source model predicts successfully the total excess crustal production rate along the Ž 5 3 . 87 86 MAR 2.5=10 km rmy and a geochemical width consistent with that of the Srr Sr anomaly, but it requires substantial along-axis melt transport to explain the observed along-axis variations in crustal thickness, bathymetry, and gravity. Calculations suggest that lateral plume dispersion may be radially symmetric rather than channelled along the ridge axis and that the topographic swell, which is elongated along the Reykjanes Ridge, may be due to rapid off-axis subsidence associated with lithospheric cooling superimposed on a broader hotspot swell. The two plume source models predict seismic P-wave velocity reductions of 0.5–2% in the center of the plume, producing travel time delays of 0.2–1.2 s. Predicted P-wave delay times for the narrow plume source model are more consistent with recent seismic observations beneath Iceland, suggesting that this model may be more representative of the Iceland plume.