On the cooling of ascending andesitic magma

Abstract

In ascending through the lithosphere, andesitic magma probably cools by about 300 K. Since the ascent velocity and dynamics of ascent are unknown, several different phenomenological cooling models are considered to account for this temperature decrease. These results are compared to the phase relations of andesitic magma in order to estimate an ascent velocity which can be used to investigate a dynamic model of ascent. The cooling models approximate an initially crystal-free magma ascending by elastic crack propagation (plate model), viscous blobs (spherical model), and by flow up a pipe. It is shown that heat transfer is predominantly by convection and conduction, and that an adiabatic ascent is unlikely. The calculated cooling curves have the general shape of the geotherm and are concave to the liquidus and solidus of the magma. Hence the magma is likely to become superheated over much of its ascent which precludes crystal fractionation, and the petrology of the lavas seems to support this. For a viscous sphere of magma the ascent velocity must be greater than about 10 -7 ms -1 , but for a crack of the same volume, because of its large surface area, ascent is at least 10 -3 ms -1 . Because of the paucity of mantle xenoliths in andesitic lavas, this latter ascent velocity seems unlikely.