Abstract

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 forsteritess 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 orthopyroxeness reacts with liquid to form clinopyroxeness, occurs at En65Di35 and 2010 ± 20 °C. The maximum amount of MgSiO3 that can be incorporated in diopsidess has been measured to be about 82 wt.%. Because of the increase in the solid solution proportion of MgSiO3 in diopsidess 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 orthopyroxeness + forsteritess + liquid ↔ clinopyroxeness + forsteritess + 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.

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