The nature and cause of mantle heterogeneity

The right to search for truth implies also a duty. One must not conceal any part of what one has discovered to be true. O verview The lithosphere clearly controls the location of volcanism. The nature and volume of the volcan-ism and the presence of 'melting anomalies' or 'hotspots,' however, reflect the intrinsic chemical and petrologic heterogeneity of the upper mantle. Melting anomalies-shallow regions of ridges, volcanic chains, flood basalts, radial dike swarms-and continental breakup are frequently attributed to the impingement of deep mantle thermal plumes on the base of the lithosphere. The heat required for volcanism in the plume hypothesis is from the core; plumes from the deep mantle create upper mantle heterogeneity. This violates the dictum of good Earth science: never go for a deep complex explanation if a shallow simple one will do. Mantle fertility and melting point variations, ponding, focusing and edge effects, i.e. plate tec-tonic and near-surface phenomena, may control the volumes and rates of magmatism. The magnitude of magmatism may reflect the fertility and homologous temperature, not the absolute temperature , of the asthenosphere. The chemical and isotopic heterogeneity of the mantle is, in part, due to recycling and, in part, due to igneous processes internal to the mantle. The fertility and fertility heterogeneity of the upper mantle are due to subduction of young plates, aseismic ridges and seamount chains, and to delami-nation of the lower continental crust, as discussed in Part II. TI1ese heterogeneities eventually warm up past the melting point of eclogite and become buoyant low-seismic-velocity diapirs that undergo further adiabatic decompression melting as they encounter thin or spreading regions of the lithosphere. The heat required for the melting of cold subducted and delaminated material is extracted from the essentially infinite heat reservoir of the mantle, not the core. Melting in the upper mantle does not require an instability of a deep thermal boundary layer or high absolute temperatures. Melts from fertile regions of the mantle, recycled oceanic crust and subducted seamounts, can pond beneath the lithosphere, particularly beneath basins and suture zones, with locally thin, weak or young lithosphere, or they can erupt. The stress state of the lithophere can control whether there is underplating and sill intrusion, or eruption and dike intrusion. Absolute mantle temperature has little to do with this. The characteristic scale lengths-150 km to 600 km-of variations in bathymetry and magma chemistry, and the variable productivity of …