Layered lithospheric mantle in the central Baltic Shield from surface waves and xenolith analysis

Information on the structure of the upper mantle comes from two main sources. Regional seismic studies provide indirect information on large portions of the lithosphere, and mantle xenoliths provide direct information about the composition and physical properties of the small regions sampled by kimberlites and other magmas. Fundamental mode Rayleigh wave arrival times at seismic stations of the SVEKALAPKO seismic experiment, with periods between 10.5 and 190 s, were inverted to obtain a regional average shear-wave velocity model in the central Baltic Shield to a depth of 300 km. This model is very well constrained except for the crust and immediately below the Moho. Calculated velocities are approximately 4% faster than in standard Earth models for the upper mantle down to 250-km depth. A low velocity zone that could define the base of the lithosphere is absent. We compared our seismically derived shear-wave velocities to models derived from the compositions of lherzolite and harzburgite xenoliths in Finnish kimberlites, sampled in regions where the geotherm is well constrained. The velocities are similar for depths between 160 and 300 km. For depths shallower than 160 km, our seismically derived velocities are slower than those from the petrologic models, and they have a positive gradient with depth in contrast with the negative gradient predicted for homogeneous material in this depth interval. Our data are best explained by a chemical layering of the lithospheric mantle: A layer with abnormally low velocities in the upper part of the lithosphere apparently grades downwards into more normal peridotitic compositions. Possible candidates for the slow composition of the shallower mantle are metasomatized peridotites, or ultramafic cumulates or restites.