Searching for the signal of the Iceland plume: Seismic observations of mantle discontinuities

Abstract:

The presence of a mantle plume beneath Iceland has long been hypothesised to explain its high volumes of crustal volcanism. Practical constraints in seismic tomography mean that thin slow velocity anomalies representative of a plume signature are difficult to image. However it is possible to infer the presence of temperature anomalies at depth from the effect they have on phase transitions in surrounding mantle material. Here, we use P to S seismic wave conversions at mantle discontinuities to search for the signal of a mantle plume beneath Iceland. We employ a large data set from a wide range of seismic stations across the North Atlantic region and a dense network in Iceland, including more than 100 University of Cambridge run stations. Data are used to create over 6000 receiver functions which are converted from time to depth including 3D corrections for variations in crustal thickness and upper mantle velocity heterogeneities. The global transition zone discontinuities at depths of 410 and 660km are thought to be caused by phase changes in the olivine component of mantle rocks. We find that both the 410 and 660 discontinuities are depressed under Iceland compared to normal depths in the surrounding region. The opposite signs of the Clapeyron slopes describing the olivine phase transitions predict anti-correlation of discontinuity topography, thus observations of correlated discontinuities are generally dismissed as an artefact due to under corrected upper mantle velocity variations. We suggest instead that the correlated topography we observe is caused by a garnet (as opposed to olivine) phase transition at 660 described by a positive Clapeyron slope, such that depression of the 660 is representative of a hot anomaly at depth. Observations of additional discontinuities in the upper mantle as well as observations of a deep ~1000km discontinuity also have the potential to shed light on the presence of a mantle plume at depth.