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Chromium as Cr 3+  substitutes for octahedrally coordinated Al in upper-mantle minerals, thereby reducing the activity of Al 2 O 3  in the system and hence the concentration of Al 2 O 3  in partial melts. The effect of Cr 2 O 3  on melt compositions multiply saturated with the spinel lherzolite phase assemblage has been quantified in the system CaO–MgO–Al 2 O 3 –SiO 2 –Cr 2 O 3  at 1·1 GPa as a function of 100 Cr/(Cr + Al) in the spinel (Cr# sp ). The decrease of Al 2 O 3  in the melt with increasing Cr# sp  is accompanied by increasing MgO and SiO 2 , whereas CaO remains almost constant. Consequently, the CaO/Al 2 O 3  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/Al 2 O 3  ratios. The concentration of Cr 2 O 3  in the melt remains low even at high Cr# sp , which means that the strong effect of Cr 2 O 3  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 2 O 3  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.

The Effect of Cr 2 O 3 on the Partial Melting of Spinel Lherzolite in the System CaO–MgO–Al 2 O 3 –SiO 2 –Cr 2 O 3 at 1·1 GPa