A model for rutile saturation in silicate melts with applications to eclogite partial melting in subduction zones and mantle plumes

This experimental study examines the solubility of rutile in silicate melts and presents a model for rutile saturation as a function of temperature, pressure and melt composition. Rutile saturation experiments were carried out in the system SiO2-TiO2-Al2O3-MgO-CaO-Na2O-K2O at 1 bar to 35 kbar and 1150 to 1450 °C on model rhyodacite (∼ 69 wt.% SiO2) and haplobasalt (∼ 54 wt.% SiO2) base melt compositions. At rutile saturation, the concentration of TiO2 in the model rhyodacite base melt increases at 1 bar from 3.27 ± 0.03 wt.% at 1150 °C to 13.88 ± 0.04 wt.% at 1450 °C. At 1350 °C it decreases from 8.89 ± 0.08 wt.% at 1 bar to 2.06 ± 0.13 wt.% at 35 kbar. Rutile solubility is significantly higher in the haplobasalt base melt at a given pressure and temperature. The concentration of TiO2 in rutile-saturated haplobasalt increases at 1 bar from 20.9 ± 0.3 wt.% at 1300 °C to 39.0 ± 0.3 wt.% at 1450 °C. At 1350 °C it decreases from 25.8 ± 0.3 wt.% at 1 bar to 15.67 ± 0.16 wt.% at 15 kbar. Results from these experiments were combined with data from the literature to formulate a model for rutile saturation in silicate melts. Application of this model to partial melting of MORB-type eclogite indicates that beneath volcanic arcs low-degree, hydrous partial melts of rutile-bearing subducted oceanic crust contain only ∼ 600 ppm TiO2. Therefore, rutile will remain as a residual phase in the eclogite and the amount of TiO2 that will be transferred to the mantle wedge will be small because so little TiO2 is dissolved in the melt. Partial melting of recycled oceanic crust in an upwelling mantle plume can exhaust rutile from the residual solid at moderate degrees of partial melting (∼ 22%). The retention of rutile in subducted oceanic lithosphere during dehydration and/or partial melting, combined with exhaustion of rutile during partial melting of eclogite in mantle plumes suggests that HFSE enrichments in recycled crust that were established during subduction may be detectible in OIB.