The effect of Cr₂O₃ on the partial melting of spinel lherzolite in the system CaO-MgO-Al₂O₃-SiO ₂-Cr₂O₃ at 1.1 GPa

Chromium as Cr³⁺ substitutes for octahedrally coordinated Al in upper-mantle minerals, thereby reducing the activity of Al ₂O₃ in the system and hence the concentration of Al₂O₃ in partial melts. The effect of Cr₂O₃ on melt compositions multiply saturated with the spinel lherzolite phase assemblage has been quantified in the system CaO-MgO-Al₂O₃-SiO ₂-Cr₂O₃ at 1·1 GPa as a function of 100 Cr/(Cr + Al) in the spinel (Cr#_sp). The decrease of Al₂O₃ in the melt with increasing Cr# _sp is accompanied by increasing MgO and SiO ₂, whereas CaO remains almost constant. Consequently, the CaO/Al₂O₃ ratio of the melt increases with Cr#_sp, and the melt becomes richer in normative diopside, hypersthene and quartz. The effect may explain certain mantle melts with unusually high CaO/Al₂O₃ ratios. The concentration of Cr₂O₃ in the melt remains low even at high Cr# _sp, which means that the strong effect of Cr ₂O₃ on partial melting equilibria is not readily apparent from its concentration in the melt itself. The existence of a highly refractory major component such as Cr ₂O₃ nullifies simplified conclusions from the 'inverse approach' in the experimental study of basalt petrogenesis, as there is insufficient information in the composition of the partial melt to reconstruct the conditions of melting.