REVERSAL OF THE ORTHOFERROSILITE-HIGH-P CLINOFERROSILITE TRANSITION, A PHASE-DIAGRAM FOR FESIO3 AND IMPLICATIONS FOR THE MINERALOGY OF THE EARTHS UPPER-MANTLE

The reconstructive phase transition between orthoferrosilite (OFS) and high-pressure C2/c clinoferrosilite (HCFS) has been reversed for the FeSiO3 composition over the pressure range of 5.0 to 7.5 GPa in a mult i-anvil press. The position of the boundary is best fit by: P (GPa) = 0.00457(11) T (K) -0.084(153) From this result and Delta V-298K = -0.9 0(4) cm(3) mol(-1) from X-ray diffraction, we obtain Delta S-298K = -4 .11(21) J mol(-1) K-1 and Delta H-298K = -76(138) J mol(-1) for the OF S-->HCFS transformation. The OFS-HCFS-LCFS (low-P clinoferrosilite) tr iple point is located at 820(10)degrees C and 4.9(2) GPa from the inte rsection of the OFS-HCFS boundary with the LCFS-OFS boundary previousl y reversed by Lindsley (1965). The topology of the FeSiO3 phase diagra m is, therefore, the same as that for MgSiO3, but with the orthorhombi c to high-P monoclinic and low-P monoclinic to high-P monoclinic struc tural transformations occurring at lower pressures. When combined with previous work on the MgSiO3 end-member, our results that high-P clino pyroxene is a stable phase in a pyrolite mantle below approximate to 3 00 km. However, its occurrence is limited by the progressive dissoluti on of pyroxene into garnet at higher pressures. High-P C2/c clinopyrox ene should be an even more important phase in the deeper portions of s ubduction zones where a cooler thermal regime will stabilise this phas e at shallower depths. The orthorhombic - high-P monoclinic transition in Ca-poor pyroxene, producing a approximate to 3% density increase, provides a reasonable petrologic explanation for the ''X seismic disco ntinuity'' (Revenaugh & Jordan, 1991), which lies at a depth of approx imate to 300 km.