Thermodynamic calculation of peridotite phase relations in the system MgO-Al2O3-SiO2-Cr2O3, with some geological applications

Abstract

The system MgO-Al2O3-SiO2(MAS) comprises 88–90% of the bulk composition of an average peridotite. The MAS ternary is thus a suitable starting point for exploring peridotite phase relations in multicomponent natural systems. The basic MAS phase relations may be treated in terms of the reactions (see list of symbols etc). Extensive reversed phase equilibria data on these three reactions by Danckwerth and Newton (1978), Perkins et al. (1981), and Gasparik and Newton (1984) employing identical experimental methods in the same laboratory have been used by us to deduce the following internally consistent thermodynamic data applying the technique of linear programming:ΔH 298(1) 0 = 2536 J, ΔS 298(1) 0 =− 6.064 J/K;ΔH 298(2) 0 = 29435 J, ΔS 298(2) 0 = 8.323 J/K; andΔH 298(3) 0 =−26899 J, ΔS 298(3) 0 =−14.388 J/K.These data are also found to be consistent with results of calorimetry. Figure 2 shows the calculated phase relations based on our thermodynamic data; they are consistent with the phase equilibria experiments. Successful extension of the MAS phase relations to multicomponent peridotites pivots on the extent to which the effects of the “non-ternary” (i.e. other than MAS) components can be quantitatively handled. Particularly hazardous in this context is Cr2O3, although it barely makes up 0.2 to 0.5 wt% of such rocks. This is because Cr+3 fractionates extremely strongly into Sp. This study focuses on the peridotite phase relations in the MgO-Al2O3-SiO2-Cr2O3 (MASCr) quaternary. Thermodynamic calculations of the MASCr phase relations have been accomplished by using ΔH 298 0 and ΔS 298 0 values for the reactions (1) through (3) indicated above, in conjunction with data on thermodynamic mixing properties of The results are shown in P-T projections (Figs. 3a and b) and isobaric-isothermal sections of MASCr in a projection through the component fo onto the SiO2-Al2O3-Cr2O3 ternary (Figs. 4a and b). The most important results of this work may be summarized as follows: