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

• Published in: Geochemistry, Geophysics, Geosystems
• Main Authors: Shorttle, O., Maclennan, J., Jones, S. M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: AGU and the Geochemical Society 2010
• Subjects: 05 - Petrology - Igneous; Metamorphic and Volcanic Studies; Iceland
• Online Access: http://eprints.esc.cam.ac.uk/1400/; http://eprints.esc.cam.ac.uk/1400/1/Shorttle_J_Mac_g3_2010.pdf; http://eprints.esc.cam.ac.uk/1400/2/G3_Shorttle_Maclennan.pdf; https://doi.org/10.1029/2009GC002986

The Iceland, Gal´apagos and Azores plumes have previously been identified as interacting asymmetrically with adjacent spreading centres. 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 geochemical observables as a function of plume centre location. For each plume, we find that bathymetry and crustal thickness observations can be explained using a single centre of symmetry, with these calculated centres coinciding with independently inferred plume centre locations. The existence of these centres of symmetry suggests that the flow fields and temperature structure of the three plume heads are radially symmetric. However, no centres 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 midocean 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 centres. Asymmetry in trace element and isotopic profiles develops when ridges either side of the plume centre 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 Gal´apagos in terms of an axisymmetric plume interacting with an asymmetric ridge system.