Effect of chromium on the silicate systems

The effect of Cr on the phase equilibrium diagram of the silicate system has been studied in air atm by adding a small amount of Mg&lt；Cr_2&gt；&lt；O_4&gt；(0.3wt.%)to the system&lt；Mg_2&gt；&lt；SiO_4&gt；(Forsterite)-&lt；CaAl_2&gt；&lt；Si_2&gt；&lt；O_8&gt；(Anorthite)-CaMg&lt；Si_2&gt；&lt；O_6&gt；(Diopside)-&lt；SiO_2&gt；，which is an important petrologic model to the formation of basaltic magma.The spinel primary field is enlarged compared with that of the Cr-free system at the expense of the anorthite field，and as a result forsterite and anorthite do no coexist at the liquidus temperature.Two invariant points have been confirmed：one shows liquid coexisiting with forsterite，protoenstatite，diopside and spinel，and the other shows the assemblage of anorthite，protoenstatite，diopside，silica mineral and liquid.The mineral assemblage at the first invariant point correponds to that of spinel lherzolite。In the Cr-free system this assemblage is not stable at1atm and an invariant point having plagioclase lherzolite mineral assemblage is present。It is generally said that the transformation from spinel lherzolite to plagioclase lherzolite is about8kbar at 1000°C.However，the result obtained in the present study indicates that the presence of small amount of Cr in magma significantly reduces this transformation pressure。Another important aspect of the result obtained is that the position of the invariant point is possibly moved by the cange of bulk Cr content in the system，because the spinel primary field becomes large with increasing Cr content in the system。Therefore，if the liquid generated by the partial melting of the spinel lherzolite mantle has the composition of this point，the Cr content of the mantle would give significant effect to the composition of magma thus formed。

The effect of Cr on the phase equilibrium diagram of the silicate system has been studied in air at 1 atm by adding a small amount of Mg <Cr_2> <O_4> (0.3 wt.%) to the system <Mg_2> <SiO_4> (forsterite)- <CaAl_2> <Si_2> <O_8> (anorthite)-CaMg <Si_2> <O_6> (diopside) - <SiO_2> , which is an important petrologic model to the formation of basaltic magma. The spinel primary field is enlarged compared with that of the Cr-free system at the expense of the anorthite field, and as a result forsterite and anorthite do no coexist at the liquidus temperature. Two invariant points have been confirmed: one shows liquid coexisiting with forsterite, protoenstatite, diopside and spinel, and the other shows the assemblage of anorthite, protoenstatite, diopside, silica mineral and liquid. The mineral assemblage at the first invariant point correponds 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 is about 8 kbar at 1000゜C. However, the result obtained in the present study indicates that the presence of small amount of Cr in magma significantly reduces this transformation pressure. Another important aspect of the result obtained is that the position of the invariant point is possibly moved by the cange of bulk Cr content in the system, because the spinel primary field becomes large with increasing Cr content in the system. Therefore, if the liquid generated by the partial melting of the spinel lherzolite mantle has the composition of this point, the Cr content of the mantle would give significant effect to the composition of magma thus formed.