The effect of titanium on the silica content and on mineral-liquid partitioning of mantle-equilibrated melts

High pressure, high temperature phase equilibria experiments were performed to investigate the effect of TiO2 on mineral-liquid equilibria fur near natural olivine + orthopyroxene-saturated mafic liquids. At 1.2 GPa and 1360 degreesC, and at 2.8 GPa and 1530 degreesC, near-natural picritic basaltic liquids with TiO2 concentrations ranging from 0.4 to 22 wt.% were forced to be in equilibrium with olivine (ol) + orthopyroxene (opx) by adding appropriate quantities of San Carlos olivine with increasing TiO2. Titanium enrichment of the ol + opx saturated mafic liquids results in nearly pressure-independent linear decreases in SiO2 amounting to 0.60 +/- 0.04 and 0.56 +/- 0.05 wt.% SiO2 per wt.% TiO2 at 1.2 and 2.8 GPa, respectively. Olivine-liquid Fe-Mg K-D decreases at 1.2 GPa from 0.33 at 0.4 wt.% TiO2 down to 0.22 at 19 wt.% TiO2. A similar trend, shifted to slightly higher K-D values, is observed at 2.8 Gpa. With increasing TiO2, the activity coefficient of SiO2 in the liquid increases owing to association of networking-modifying cations with Ti4+. Consequently, a smaller mole fraction of SiO2, X-SiO2(Liq), is required to maintain the silica activity imposed by the coexistence of olivine and orthopyroxene. Strong association between Ti and Fe in the liquid reduces a(FeO)(Liq) and gamma (Liq)(FeO), thereby causing olivine-liquid Fe-Mg K-D to decrease. These effects are expected to result in small decreases in SiO2 and very small increases in FeO in TiO2-enriched mantle-derived sources, such as those in OIB source regions derived from small degrees of melting of peridotite at high pressure. Stronger effects on the compositions of low-degree partial melts of more titanium-enriched rocks, such as pyroxenites, may he expected. Solubility of TiO2 in olivine under magmatic conditions and at modest (1.2-2.8 GPa) pressure is near 0.2 wt.%. For TiO2, partitioning between olivine and liquid is Henrian over the range of compositions studied, indicating constant activity coefficients: for liquids between 0-20 wt.% TiO2 and for olivine between 0-0.23 wt.%. Orthopyroxene/liquid partitioning of TiO2 is variable, presumably owing to complex interactions between Al and Ti in orthopyroxene. Olivine/liquid partitioning of Al2O3 is nearly constant, but partitioning of Al2O3 between orthopyroxene and liquid is variable, owing to the aforementioned interactions between Al and Ti. Olivine/liquid and orthopyroxene/liquid partitioning of CaO diminish with increasing TiO2, suggesting stabilization of CaO in silicate liquids by formation of CaO-TiO2 complexes. At 1.2 GPa, saturation in aluminous armalcolite is found for liquids at 19.2 wt.% TiO2, in contrast, at 2.8 GPa, Ti-rich oxide saturation is not encountered for Liquids with as much as 21.8 wt.% TiO2. The large TiO2 enrichment in the liquid required for oxide saturation in the presence of olivine + orthopyroxene at these pressures suggests that near-solidus liquids generated by melting of Ti-rich oxide-bearing peridotite would have greater than or equal to 20 wt.% TiO2. The general absence of such Ti-rich primitive terrestrial lavas either precludes magmas from such source assemblages or indicates that they are strongly diluted by other magmas. Our results also suggest that armalcolite may be a significant phase in the Ti-enriched portions of the lunar mantle. Copyright (C) 2000 Elsevier Science Ltd.