Effect of Cr on the liquidus phase relations in the system Mg2SiO4-CaAl2Si2O8-CaMgSi2O6-SiO2 in air at 1 atm.

Abstract

The effects of the addition of a small amount of MgCr2O4, (0.3 wt.%) to the system Mg2SiO4 (forsterite)-CaAl2Si2O8(anorthite)-CaMgSi2O6(diopside)-SiO2 at 1 atm have been studied. The system is an important model to the crystallization and generation of tholeiitic basalt. The primary field of spinel is enlarged compared with that in the Cr-free system at the expense of anorthite primary field, and as a result forsterite and anorthite do not coexist at the liquidus temperatures. Two invariant points have been confirmed : one at about 1298°C shows liquid coexisting with forsterite, protoenstatite, diopside, and spinel, and the other at the temperature below 1265°C shows the assemblage of spinel, protoenstatite, diopside, tridymite, and liquid. The first mineral assemblage corresponds to that of spinel lherzolite. In the Cr-free system this assemblage is not stable at 1 atm and an invariant point having plagioclase lherzolite mineral assemblage is present. It is generally said that the transformation from spinel lherzolite to plagioclase lherzolite takes place at about 8 kbar at 1000°C. The results obtained in the present study indicate that the presence of small amount of Cr in magma significantly reduces this transformation pressure. Another important aspect of the result is that the position of the invariant point is possibly moved by the change of bulk Cr content in the system. If the liquid generated by the partial melting of the spinel lherzolite mantle has the composition of this point, Cr content of the mantle would give significant effect to the composition of magma.