The system enstatite-pyrope at high pressures and temperatures and the mineralogy of the earth's mantle

Abstract

In a diamond-anvil pressure cell coupled with laser heating, the system enstatite (MgSiO 3)-pyrope (3 MgSiO 3 · Al 2O 3) has been studied in the pressure region between about 100 and 300 kbar at about 1000°C using glass starting materials. The high-pressure phase behavior of the intermediate compositions of the system contrasts greatly with that of the two end-members. Differences between MgSiO 3 and 95% MgSiO 3 · 5% Al 2O 3 are especially remarkable. The phase assemblages β-Mg 2SiO 4 + stishovite and γ-Mg 2SiO 4 (spinel) + stishovite displayed by MgSiO 3 were not observed in 95% MgSiO 3 · 5% Al 2O 3, and the garnet phase, which was observed in 95% MgSiO 3 · 5% Al 2O 3 at high pressure, was not detected in MgSiO 3. These results suggest that the high-pressure phase transformations found in pure MgSiO 3 would be inhibited under mantle conditions by the presence even of small amounts of Al 2O 3 (≳4% by weight). On the other hand, pyrope displays a wide stability field, finally transforming at 240–250 kbar directly to an ilmenite-type modification of the same stoichiometry. The two-phase region, within which orthopyroxene and garnet solid solutions coexist, is very broad. The structure of the earth's mantle is discussed in terms of the phase transformations to be expected in a simple mixture of 90% MgSiO 3 · 10% Al 2O 3 and Mg 2SiO 4. The seismic discontinuity at a depth of 400 km in the earth's mantle is probably due entirely to the olivine → β-phase transition in Mg 2SiO 4, with the progressive solution of pyroxene in garnet (displayed in 90% MgSiO 3 · 10% Al 2O 3) occurring at shallower depths. The inferred discontinuity at 650 km is due to the combination of the phase changes spinel → perovskite + rocksalt in Mg 2SiO 4 and garnet → ilmenite in 90% MgSiO 3 · 10% Al 2O 3. The 650-km discontinuity is thus characterized by an increase in the primary coordination of silicon from 4 to 6. A further discontinuity in the density and seismic wave velocities at greater depth associated with the ilmenite-perovskite phase transformation in 90% MgSiO 3 · 10% Al 2O 3 is expected.