Reversal of the orthoferrosilite - high-P clinoferrosilite transition, a phase diagram for FeSiO3 and implications for the mineralogy of the Earth's upper mantle
Woodland, Alan B.; Angel, Ross J.
European Journal of Mineralogy Volume 9 Number 2 (1997), p. 245 - 254
published: Jun 26, 1997
manuscript accepted: Nov 13, 1996
manuscript received: Apr 18, 1996
ArtNo. ESP147050902002, Price: 29.00 €
Abstract 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 multi-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 ΔV298K = -0.90(4) cm3 mol-1 from X-ray diffraction, we obtain ΔS298K = -4.11 (21) J mol-1 K-1 and ΔH298K = -76(138) J mol-1 for the OFS→HCFS transformation. The OFS-HCFS-LCFS (low-P clinoferrosilite) triple point is located at 820(10)°C and 4.9(2) GPa from the intersection of the OFS-HCFS boundary with the LCFS-OFS boundary previously reversed by Lindsley (1965). The topology of the FeSiO3 phase diagram is, therefore, the same as that for MgSiO3, but with the orthorhombic to high-P monoclinic and low-P monoclinic to high-P monoclinic structural transformations occurring at lower pressures. When combined with previous work on the MgSiO3 end-member, our results suggest that high-P clinopyroxene is a stable phase in a pyrolite mantle below ~ 300 km. However, its occurrence is limited by the progressive dissolution of pyroxene into garnet at higher pressures. High-P C2/c clinopyroxene should be an even more important phase in the deeper portions of subduction zones where a cooler thermal regime will stabilise this phase at shallower depths. The orthorhombic - high-P monoclinic transition in Ca-poor pyroxene, producing a ~ 3 % density increase, provides a reasonable petrologic explanation for the "X seismic discontinuity" (Revenaugh & Jordan, 1991), which lies at a depth of- 300 km.