THE FORSTERITE–DIOPSIDE–SILICA SYSTEM AT 7 GPA AND ITS SIGNIFICANCE IN THE MELTING BEHAVIOR OF THE MANTLE

Investigation of the liquidus phase relations in the forsterite–diopside–enstatite system has been made at 7 GPa and variable temperatures under anhydrous condition using a Walker-type multi-anvil high pressure apparatus. A total of 16 compositions were synthesized to facilitate study of this system. Location of the piercing point on the forsterite–diopside bounding join has been established at Fo₃₉Di₆₁ (all compositions are in wt.%) and 1980 ± 20 °C. Electron microprobe analyses of the phases indicate that forsterite_ss can incorporate about 1 to 1.5 wt.% monticellite (CaMgSiO₄) in solid solution. In the diopside–enstatite join, a temperature minimum occurs at Di₆₆En₃₄ and 1975 ± 30 °C, whereas the peritectic point, where orthopyroxene_ss reacts with liquid to form clinopyroxene_ss, occurs at En₆₅Di₃₅ and 2010 ± 20 °C. The maximum amount of MgSiO₃ that can be incorporated in diopside_ss has been measured to be about 82 wt.%. Because of the increase in the solid solution proportion of MgSiO₃ in diopside_ss compared to that at 2 and 5.1 GPa, the peritectic point in the diopside–enstatite bounding join also shifts further toward the enstatite apex. In the ternary system, the invariant point has been located at Fo₅₄Di₃₅Q₁₁ and 2010 ± 30 °C, where orthopyroxene_ss + forsterite_ss + liquid ↔ clinopyroxene_ss + forsterite_ss + liquid occurs. Our present study confirms that the primary phase field of forsterite shrinks and that of clinopyroxene expands further in comparison to that observed at 2 and 5.1 GPa. Preliminary experimental results from composition Fo₈₀Di₁₂Qtz₈ with 4 wt.% Al₂O₃ indicate that garnet (Pyrope₉₈Grossularite₂) and forsterite (with 0.31 wt.% monticellite) are the liquidus phases, followed by clinopyroxene (Al₂O₃ contents between 3.4 to 2.7 wt.%). At 1970 °C, the average composition of the liquids obtained through electron microprobe analysis was as follows: SiO₂ 49.81, MgO 34.72, CaO 7.29, and Al₂O₃ 6.75 wt.%, which is somewhat similar to a komatiitic melt. When projected onto the forsterite–diopside–enstatite plane this liquid composition plots very close to our cotectic line. Taking model mantle composition of a garnet peridotite, a shift in the compositional path of the residual mantle materials has been determined after extraction of the volume percent of the melt using the Lever rule.