ABSENCE OF AN ALUMINOUS PHASE IN THE UPPER PART OF THE EARTHS LOWER MANTLE

RECENT high-pressure experiments(1-3) suggest that the Earth's lower mantle is composed of MgSiO3 and CaSiO3 perovskites, (Mg, Fe)O magnesiowustite, and a minor but significant amount of aluminous phases, such as majorite garnet or an unidentified Al-rich phase. The stability of majorite garnet and the nature of any such aluminous phase, however, are controversial issues relevant to the mineralogy of the lower mantle(4-7). Here I report an experimental study of the phase transformations that occur in a pyrolite mantle composition with increasing pressure from 23 to 28 GPa (equivalent to similar to 650-770 km depth in the mantle). The results demonstrate that majorite garnet completely transforms to perovskite structures at pressures above 26 GPa (>720 km depth). Al2O3 is accommodated mainly in MgSiO3 perovskite, and no separate aluminous phase was observed at higher pressures, leading to the conclusion that the upper part of the Earth's lower mantle is composed only of two perovskites and magnesiowustite.