ARE HYDROUS PHASES MORE COMPRESSIBLE? IMPLICATIONS FOR HIGH-VELOCITY ZONES IN THE DEEP MANTLE

Abstract

Raman studies of both anhydrous and hydrous α, β and γ phases of Mg2 SiO4 provide us with evidence that some hydrous phases are less compressible than their anhydrous counterparts. This is contrary to the generally accepted view that all hydrous phases are more compressible than corresponding anhydrous phases. It is suggested that the inclusion of these hydrous phases, together with hydroxyl-clinohumite and phase D (MgSi2 H2 O6 ), in some mantle material may be responsible for the high velocities observed in horizontal slabs lying above the 650-km discontinuity and in front of subducted slabs in the lower mantle.