Abstract
Liquidus phase relations on the join forsterite (Mg2SiO4)-anorthite (CaAI2Si2O8)-silica (SiO2) with 0.3 wt% MgCr2O4 were determined in air at 1 atm by ordinary quenching method. Spinel primary field is considerably expanded compared with that in the Cr-free forsterite-anorthite-silica join at the expence of anorthite primary field. As a result, phase assemblages at the points at which 4 phases coexist are completely changed : the assemblages forsterite+spinel+anorth. ite+liquid (piercing point), forsterite+ protoenstatite+ anorthite +liquid (peritectic point), and protoenstatite+anorthite+tridymite+liquid (eutectic point) on the Cr-free join disappear and the assemblages f orsterite +spinel+protoenstatite+liquid and protoenstatite+cristobalite+spinel+ liquid (both are piercing point) are present. However, the phase boundaries between the fields of forsterite and protoenstatite and those of protoenstatite and cristobalite are not affected by the addition of chromium. On the basis of the present data and those of the join forsterite-anorthite-diopside with 0.3% MgCr2O4 (Onuma and Tohara, 1983), phase relations of the system forsterite-anorthite-diopside-silica with 0.3% MgCr2O4 are inferred and its petrologic significance is briefly discussed.
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