Phase relations in the transition zone

Abstract

Phase relations in the systems MgO‐Al 2 O 3 ‐SiO 2 and CaO‐MgO‐SiO 2 have been investigated experimentally at 166–226 kbar pressures and 1350°–2500°C temperatures with a split sphere anvil apparatus (USSA 2000). MgSiO 3 ilmenite is stable to lower pressures (175 kbar) than found previously. CaSiO 3 perovskite forms at P (bar)=15 T (°C)+151,000. Three new high‐pressure phases are reported: a phase with the diopsidic composition (the CM phase), a high‐pressure polymorph of Al 2 O 3 , and superphase B with the tentative composition Mg 10 Si 3 O 14 (OH, F) 4 . The invariant point for coexisting majorite (Mg 4 Si 4 O 12 ), perovskite (MgSiO 3 ), beta phase (Mg 2 SiO 4 ), and liquid has been located at 224 kbar, 2430°C, with the eutectic melt composition on the enstatite‐forsterite join at 40–44 wt% of forsterite. The melting temperature of MgSiO 3 perovskite at 225 kbar is predicted to be 2600°C. The new data made possible the calculation of a temperature‐pressure phase diagram for the system CaO‐MgO‐Al 2 O 3 ‐SiO 2 (CMAS) in the pressure range 0–280 kbar. The mineralogy of the Earth's mantle and the possible mechanisms for producing large seismic velocity gradients in the transition zone are discussed on the basis of the derived phase relations.