Gravity compensation in the mantle beneath the neovolcanic zone of Iceland

An enormous component of isostatic compensation(～?430±40mgal) is provided by subcrustal material beneath the neovolcanic zone of Iceland. Previously published values for the component of gravity contributed by anomalous mantle material beneath this region range from ?250 to ?140 mgal. These values are only a partial indication of the magnitude of the compensation mechanism. If one takes into account constraints on crustal thickness from seismic refraction studies and compares Iceland to less active tectonic regions near by, one obtains a mantle gravity effect of approximately?430±40mgal, which for a simple slab model leads to a vertically integrated mass deficit per unit surface area of 10⁶ g/cm². The effects of thermal expansion, solid-solid phase transitions and partial fusion can provide significant contributions to the total mass deficiency; however, none of these mechanisms alone seems sufficient to account for the entire anomaly.The relation of this mass anomaly to the evolution of the Iceland-Faeroe ridge is considered by examining two extreme end-members of a suite of possible evolutionary models for this region. The first of these is the case where, in time, the 10⁶ g/cm² mass deficit will be resorbed into the mantle with the effects of chemical segregation playing a minor role. The second case, which is preferred, involves a significant redistribution of material from the mantle such that basaltic melt segregated from high levels in the mantle accumulates and crystallizes in a zone at the base of the crust. In this latter model, if the Iceland-Faeroe ridge is considered to have evolved under a steady rate of magma production over the last 30–40 × 10⁶ years, then underplating of the crust may account for its increasing thickness as it matures from 8–10 km in the neovolcanic zone to a value of approximately 32 km for the Iceland-Faeroe ridge. If we assume a 10% increase in density upon crystallization, thickening of the crust by 22 km through underplating by magma accumulation leads to an increase in mass per unit surface area of 0.6 × 10⁶ g/cm², and accounts for approximately 60% of the total mass difference in the mantle between Iceland and the Iceland-Faeroe ridge.