Constitution of the mantle. 2. Petrological models of transition zone based on FMS phase diagram

Abstract

Abstract A detailed picture is presented of phase transformations in the mantle transition zone, based upon the study of phase relations and topologies in the FeOMgOSiO2 (FMS) system. The (P - T)x diagram for the composition close to pyrolite (SiO2 = 40 mol.% and Fe/(Fe + Mg) = 0.12) is used in modelling the structure of the mantle transition zone, to connect phase transitions in minerals to the observed seismic discontinuities. The mineralogical nature of the first seismic discontinuity, at a depth of about 400 km, is associated either with a univariant transformation Px + α + γ → α + β + Px or with a sharp divariant transformation α + Px → α + β + Px → β + Px, which should be closer to 420 km. For the fixed composition close to pyrolite at a depth of about 650 km neither univariant nor invariant equilibria are observed in the FMS system. It has been found that, in the FMS system, two extremely narrow (1–2 km wide) stability fields of divariant mineral assemblages γ + Pv + St and γ + Pv + Mw, separated by a trivariant zone γ + Pv (about 5 km wide) might be responsible for the nature of the 650 km discontinuity. Because of the negative slope of the phase boundaries, the second seismic discontinuity in the cold subducting slab is deeper than in the surrounding mantle.