Geophysical characterization of mantle melting anomalies; a crustal view

At present there is no single "unified theory" capable of explaining the variety of geological, geophysical, and geochemical observations that characterize what is generically known as hotspot magmatism. An increasing number of geophysical and geochemical observations disagree with the predictions of the conventional thermal plume model, in which excess melting is due mainly to high mantle temperatures. Other parameters such as the presence of water or the composition of the mantle source have been shown to be as important as temperature in controlling the structure and physical properties of the igneous crust. In this article we first emphasize the importance of doing proper velocity and density modeling, including comprehensive uncertainty analysis, to determine how well resolved the geophysical parameters actually are. We show that in some cases the contribution of velocity-derived lateral crustal density variations can be sufficiently significant to account for the observed gravity and topography anomalies without calling for noticeable mantle density contrasts. Next we show that the comparison of crustal geometry obtained along age-progressive volcanic tracks enables temporal variations of the hotspot-ridge distance to be estimated. Finally we use a 2-D mantle melting model to illustrate the effect of different mantle melting parameters on the resulting crustal structure. The tests made indicate that it is difficult to find a plausible combination of mantle temperature, upwelling rate, melt productivity, and thickness of the melting zone to explain either the high-velocity, underplated bodies frequently described at midplate settings or the lack of a positive crustal thickness-velocity (H-V (sub p) ) correlation found at igneous provinces originated on-ridge. We suggest that the main parameter controlling the generation of volcanic underplating is the presence of a lithospheric lid limiting the extent of the mantle melting zone, whereas the H-V (sub p) anticorrelation can be related to the presence of a major element heterogeneity, such as eclogite derived from recycled oceanic lithosphere, in the mantle source.