A tilted plume originating from the lower mantle
Iceland, one of the most thoroughly investigated hotspots, is generally accepted to be the manifestation of an upwelling mantle plume. Yet whether the plume originates from the lower mantle or from a convective instability at a thermal boundary layer between the upper and lower mantle near 660 km depth remains unconstrained. Tomographic inversions of body wave delay times show that low seismic velocities extend to at least 400 km depth beneath central Iceland, but cannot resolve structure at greater depth. A new approach to address the question of the depth of origin of mantle plumes is to map the mantle discontinuities near 410 and 660 km depth, features that have been identified with the mantle mineral phase transitions. The depths to the 410- and 660-km discontinuities respectively increase and decrease with increasing temperature, and thus provide information on lateral temperature variations and associated mantle circulation patterns. Using compressional-to-shear wave conversions at the 410- and 660-km discontinuities, we found that the transition zone between the two discontinuities is 20 km thinner than in the average Earth beneath central and southern Iceland but is of normal thickness beneath surrounding areas, a result indicative of a hot and narrow plume originating from the lower mantle.
Shown are the locations of the piercing points of the ray paths of P-to-S converted phases at the 660-km discontinuity (crosses) and the P660s ray paths (color lines) from the central patch (grey squares) to the ICEMELT seismic station and the GSN station BORG (dots) on Iceland. The Icelandic coast and the Reykjanes and Kolbeinsey ridges are represented by regional bathymetry at 1-km contours.
Map view of differences between the observed P660s and P410s differential times and the value predicted for the iasp91 average earth model. Red and yellow colors indicate significantly smaller differential times (thinner mantle transition zone) than in iasp91, while blue colors denote normal or somewhat greater differential times. The cross marks the center of the mapped area of thinned transition zone. The circle, square, and triangle represent the locus of thickest crust and the center of low P and S velocities in the uppermost mantle, respectively. The offset is evidence for a tilted pluem conduit in the upper mantle, the results of either northward flow of the Icelandic asthenosphere or southward flow of the upper part of the lower mantle
Shen, Y., S. C. Solomon, I. Th. Bjarnason, G. Nol et, W. J. Morgan, R. M. Allen, K. Vogfjord, S. Jakobsdottir, R. Stefansso n, B. R. Julian, and G. R. Foulger, Seismic evidence for a tilted mantle plume and north-south mantle flow beneath Iceland, Earth Planet. Sci. Lett., 197, 261-272, 2002. (pdf)
Y. Shen, S. C. Solomon, I. Th. Bjarnason, and C. J. Wolfe, Seismic evidence for a lower-mantle origin of the Iceland mantle plume, Nature, 395, 62-65, 1998. (pdf)
Shen, Y., S.C. Solomon, I. Th. Bjarnason, and G .M. Purdy, Hot mantle transition zone beneath Iceland and the adjacent Mi d-Atlantic Ridge inferred from P-to-S conversions at the 41 0- and 660-km discontinuities, Geophys. Res. Lett., 23, 352 7-3530, 1996. (pdf)