Neogene Mantle Dynamics of Western Mediterranean Region Constrained by Basalt Geochemistry and Residual Depth Anomalies

There is considerable interest in combining a range of geophysical, geochemical and geomorphic observations with a view to estimating the amplitude, wavelength and depth of mantle thermal anomalies on a global bases. Here, we wish to explore how forward and inverse modelling of major, trace and rare earth elements can be exploited to determine melt fraction as a function of depth for a mantle peridotitic source. Our focus is on an area that includes the Iberian Peninsula where previous work shows that long-wavelength topography is probably generated and maintained by sub-plate thermal anomalies which are manifest by negative shear-wave velocities. Geological and geomorphic studies suggest that this dynamic support is a Neogene phenomenon. 48 newly acquired Neogene basaltic samples from Spain were analyzed and combined with previously published datasets. Both major element thermobarometry and rare earth element inverse modelling are applied to estimate melt fraction as a function of depth. In this way, asthenospheric potential temperature and lithospheric thickness can be gauged. These estimates are compared with those obtained from calibrated shear-wave tomographic models. Our results show that potential temperatures and lithospheric thicknesses are 1250-1300 °C and 65-70 km, respectively. These values broadly agree with calibrated tomographic models which yield values of 1300-1350 °C and 45-70 km. We conclude that a region encompassing Iberia is dynamically supported by a combination of warm asthenosphere and thinned lithosphere. This conclusion broadly agrees with independently obtained residual depth anomalies which indicate that the Western Mediterranean region probably has moderately positive dynamic support.