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

Abstract High pressure, high temperature phase equilibria experiments were performed to investigate the effect of TiO 2 on mineral-liquid equilibria for near natural olivine + orthopyroxene-saturated mafic liquids. At 1.2 GPa and 1360°C, and at 2.8 GPa and 1530°C, near-natural picritic basaltic liquids with TiO 2 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 TiO 2 . Titanium enrichment of the ol + opx saturated mafic liquids results in nearly pressure-independent linear decreases in SiO 2 amounting to 0.60 ± 0.04 and 0.56 ± 0.05 wt.% SiO 2 per wt.% TiO 2 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.% TiO 2 down to 0.22 at 19 wt.% TiO 2 . A similar trend, shifted to slightly higher K D values, is observed at 2.8 GPa. With increasing TiO 2 , the activity coefficient of SiO 2 in the liquid increases owing to association of networking-modifying cations with Ti 4+ . Consequently, a smaller mole fraction of SiO 2 , X SiO 2 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 γ FeO Liq , thereby causing olivine-liquid Fe-Mg K D to decrease. These effects are expected to result in small decreases in SiO 2 and very small increases in FeO in TiO 2 -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 be expected. Solubility of TiO 2 in olivine under magmatic conditions and at modest (1.2–2.8 GPa) pressure is near 0.2 wt.%. For TiO 2 , partitioning between olivine and liquid is Henrian over the range of compositions studied, indicating constant activity coefficients for liquids between 0–20 wt.% TiO 2 and for olivine between 0–0.23 wt.%. Orthopyroxene/liquid partitioning of TiO 2 is variable, presumably owing to complex interactions between Al and Ti in orthopyroxene. Olivine/liquid partitioning of Al 2 O 3 is nearly constant, but partitioning of Al 2 O 3 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 TiO 2 , suggesting stabilization of CaO in silicate liquids by formation of CaO-TiO 2 complexes. At 1.2 GPa, saturation in aluminous armalcolite is found for liquids at 19.2 wt.% TiO 2 , in contrast, at 2.8 GPa, Ti-rich oxide saturation is not encountered for liquids with as much as 21.8 wt.% TiO 2 . The large TiO 2 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 ≥20 wt.% TiO 2 . 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.