Seismic evidence for an Iceland thermo-chemical plume in the Earth's lowermost mantle

We constrain the geographic extent, geometry and velocity structure of the seismic anomaly near the Earth's core-mantle boundary (CMB) beneath Iceland, based on travel time and three-dimensional waveform modeling of the seismic data sampling the lowermost mantle beneath Iceland. Our analysis suggests a mushroom-shaped low velocity anomaly situated in the lowermost mantle beneath Iceland surrounded by a high velocity province. The best fitting mushroom-shaped model is 600 km high and has a stem with a radius of 350 km in the lowermost 250 km of the mantle and a cap with increasing radii from 550 km at 250 km above the CMB to 650 km at 600 km above the CMB. The shear velocity structure varies from 0% at the top to −3% at 250 km above the CMB and to −6% at the CMB. These inferred seismic features, in combination with the previous evidence of existence of ultra-low velocity zones at the base of the mantle beneath the region, suggest that Iceland represents a thermo-chemical plume generated by interaction of downwelling and a localized chemical anomaly at the base of the mantle.