MGSIO3-FESIO3-AL2O3 IN THE EARTHS LOWER MANTLE - PEROVSKITE AND GARNET AT 1200-KM DEPTH

Natural pyroxene and garnet starting materials are used to study the effects of joint Fe and Al substitution into MgSiO3 perovskite at similar to 50 GPa. Garnet is found to coexist with perovskite in samples containing both Fe and Al to pressures occurring deep into the lower mantle (similar to 1200 km depth). The volume of the perovskite unit cell is V-0(Angstrom(3)) = 162.59 + 5.95x(FeSiO3) + 10.80x(Al2O3) with aluminum causing a significant increase in the distortion from the ideal cubic cell. On the basis of a proposed extension of the MgSiO3-Al2O3 high-pressure phase diagram toward FeSiO3, Fe is shown to partition preferentially into the garnet phase. The stability of garnet deep into the lower mantle may hinder the penetration of subducted slabs below the transition zone.