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 e ffects of joint Fe and Al substitution into MgSiO3 perovskite at simil ar to 50 GPa. Garnet is found to coexist with perovskite in samples co ntaining both Fe and Al to pressures occurring deep into the lower man tle (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 alu minum causing a significant increase in the distortion from the ideal cubic cell. On the basis of a proposed extension of the MgSiO3-Al2O3 h igh-pressure phase diagram toward FeSiO3, Fe is shown to partition pre ferentially into the garnet phase. The stability of garnet deep into t he lower mantle may hinder the penetration of subducted slabs below th e transition zone.