Control of the symmetry of plume-ridge interaction by spreading ridge geometry

[1] The Iceland, Galapagos, and Azores plumes have previously been identified as interacting asymmetrically with adjacent spreading centers. We present evidence that the flow fields in these plume heads are radially symmetric, but the geometry of the mid-ocean ridge systems imparts an asymmetric compositional structure on outflowing plume material. First, we quantify the degree of symmetry in geophysical and geo-chemical observables as a function of plume center location. For each plume, we find that bathymetry and crustal thickness observations can be explained using a single center of symmetry, with these calculated centers coinciding with independently inferred plume center locations. The existence of these centers of symmetry suggests that the flow fields and temperature structure of the three plume heads are radially symmetric. However, no centers of symmetry can be found for the incompatible trace element and isotopic observations. To explain this, we develop a simple kinematic model to predict the effect of mid-ocean ridge geometry on the chemical composition of outflowing plume material. The model assumes radially symmetric outflow from a compositionally heterogeneous plume source, consisting of a depleted mantle component and enriched blebs. These blebs progressively melt out during flow through the melting regions under spreading centers. Asymmetry in trace element and isotopic profiles develops when ridges on either side of the plume center receive material that has been variably depleted according to the length of flow path under the ridge. This model can successfully explain compositional asymmetry around Iceland and Galapagos in terms of an axisymmetric plume interacting with an asymmetric ridge system.