Structure and properties of silicate magmas

The presence of magmas in the upper mantle has been reported by several geophysical observations. The properties of these magmas are more sensitive to pressure and composition than those at deeper mantle conditions. Thus, understanding the response of the structure and properties of silicate magmas to pressure and composition is significant in discussing the magma mobility and gravitational stability in the mantle, such as the lithosphere-asthenosphere boundary and the base of the upper mantle. This chapter introduces pressure- and composition-induced changes in the structure/property of magmas and their correlation to the macroscopic properties and microscopic structure, with the goal of understanding magma behavior, especially, under upper mantle conditions. High-pressure experiments on silicate melt/glass and newly acquired results are reviewed, focusing on the following four topics: (1) structure of silicate magmas; (2) density of silicate magmas; (3) viscosity of silicate magmas; and (4) elastic wave velocity of silicate glasses. Especially, pressure and compositional (the degree of polymerization) dependences of each feature are described on the basis of the results using a large-volume press.