SPINEL SILICATE EQUILIBRIA IN THE SYSTEM MGO-FEO-FE2O3-AL2O3-CR2O3-SIO2

We have investigated the distribution of Mg, Fe, Al, and Cr among olivine, orthopyroxene, spinel, and liquid in the system MgO-FeO-Fe2O3-Al2O3-Cr2O3-SiO2 for temperatures from 1309-degrees to 1471-degrees-C under an atmosphere of constant CO2/H2 = 10/1 at a total pressure of 1 atm. Crystal-liquid and crystal-crystal distribution coefficients defined (with Fe and Mg as examples) in the form K(D)c-liq = (Fe2+/Mg)crystal/(Fe2+/Mg)liquid, and K(D)c-c = (Fe2+/Mg)crystal 1/(Fe2+/Mg)crystal 2, were calculated from the experimental data. The crystal-liquid coefficients for the silicate minerals are relatively constant [K(D)sp-liq(Fe2+/Mg) =0.33; K(D)opx-liq(Fe2+/Mg) = 0.29]. However, the crystal-liquid coefficient for spinel is more variable [K(D)sp-liq(Fe2+/Mg) = 1.2 to 1.71. Chromium is strongly partitioned into spinel relative to iq coexisting liquid, olivine, and orthopyroxene (D(CR)sp-liq = 80 to 170, D(Cr)sp-ol = 100 to 230, and D(Cr)sp-opx = 60 to 120), and Al is enriched in the liquid and the spinel relative to olivine and orthopyroxene. Values of (Al/Cr)opx/(Al/Cr)sp range from 4 to 8. The experimental results also show that the Al/Cr ratio in spinel increases with decreasing activity of SiO2 in the melts.