Seismic Imaging of Mantle Transition Zone Suggests Hot Deep Plume Underneath Iceland-Mid-Atlantic Ridge Region

Iceland has long been a focal point of geophysical research due to its potential association with a mantle plume. While earlier studies generally supported a plume originating from the core-mantle boundary with minimal lateral displacement during ascent, recent high-resolution tomographic images have indicated the presence of a curved, ascending mantle plume beneath Iceland. Consequently, the detailed source region and morphology of the Iceland plume remain debated. In this study, we provide new constraints on the Iceland mantle plume by examining the structure of the mantle transition zone (MTZ) using SS precursor imaging. We collected a large SS precursor dataset from 1976 to 2023 and adopted a recently proposed multi-dimensional reconstruction method to enhance the weak SS precursor phases for improved probing of the MTZ.

Our seismic observations reveal substantial thinning (~230 km) of the MTZ beneath Iceland compared to the regional average of 238 km and the global average of 242 km. This thinning is characterized by a slight depression of the 410 km discontinuity (~5 km) and a pronounced uplift of the 660 km discontinuity (~12 km). Temperature anomalies estimated using Clapeyron slopes suggest respective perturbations of +50 K and +300 K at the 410 and 660 discontinuities beneath Iceland. The former estimate is significantly lower compared to the reported thermal anomalies at major hotspots, e.g., ΔT410 ≈ +200 K west of Hawaii. This large temperature contrast suggests that either strong thermal heterogeneities exist across the MTZ or an alternative mechanism is required to explain the thinning of the MTZ beneath Iceland. We suggest that the mildly depressed 410 may be partly attributed to the influence of water during the ascent of the mantle plume. The presence of water effectively reduces pressures for the phase transition from olivine to wadsleyite, causing upward displacement of the 410 km discontinuities. This result suggests that variations in water distribution and content play a critical role in the structural anomalies observed in the MTZ beneath Iceland. The observed thinning of the MTZ supports the existence of a deep mantle plume, potentially supplying material to the shallow hotspots near the Iceland-Mid-Atlantic Ridge.