ELECTRON MICROSCOPY OF HIGH-PRESSURE PHASES SYNTHESIZED FROM NATURAL GARNETS IN A DIAMOND ANVIL CELL: IMPLICATIONS FOR THE MINERALOGY OF THE LOWER MANTLE

Abstract

In order to identify the crystalline hosts for aluminium and calcium in the deep mantle, the high-pressure transformations of natural garnets (pyrope, grossular and Fe-rich garnet) have been prepared at high pressures (40 and 50 GPa) and temperatures between 1000 and 2500 K in a laser-heated diamond anvil cell (DAC). The recovered high-pressure phases have been studied by analytical transmission electron microscopy (ATEM). The host minerals for Al and Ca appear to depend on temperature and iron content:(1) At moderate temperature and without significant amounts of iron in the starting material the main host minerals for Ca and Al aluminium are CaSiO3 perovskite and a compound with an unknown structure and a (Ca,Mg)Al2SiO6 composition.(2) At higher temperature the main host minerals for Ca and Al are, respectively, CaSiO3 perovskite and a new high-pressure phase of Al2SiO5 having the V3O5 structure.(3) When the iron content is sufficiently large in the starting material (Fe/Fe + Mg # 15%), the major host mineral for Ca and Al is a compound with a hollandite-type structure and a (Ca,Mg,Fe)Al2Si2O8 composition, whatever the temperature.The solubility of Al in Mg-perovskite was also studied. It appears that this solubility strongly decreases with increasing pressure, and slightly increases with temperature. Such behaviour ofAl in the deep mantle could lead to the occurrence of a progressive phase change in the lower mantle. Mineralogical models of the deep mantle are then discussed in the light of the above results.