The extraction of magma from the crust and mantle

The ability of a melt to separate from the residual crystals depends principally on the viscosity of the melt. Low-viscosity magmas like carbonatites can separate from regions as thick as 100 km in a few million years even when the melt fraction is as small as 0.1%, whereas granitic magmas will undergo only limited separation, even when the melt fraction is 10% and is water-saturated. Estimates of the total amount of melt generated by adiabatic upwelling beneath ridge axes agree well with estimates from depleted peridotites dredged from the oceans, but not with those from studies of incompatible trace elements. Both the trace element and the isotope observations can, however, be explained if incompatible elements are extracted from large volumes of the mantle which do not undergo extensive melting. Such extractions can easily be achieved by the separation of a carbonatitic or a hydrous magma. Nephelinitic and komatiitic magmas may also be able to move at small melt fractions, but are probably less important than are volatile-rich magmas as transporting fluids.