The role of TiO₂ phases during melting of subduction-modified crust: Implications for deep mantle melting

Partitioning of Nb, Ta, Hf and Zr between rutile, its high-pressure polymorph TiO₂(II) and silicate melt has been experimentally determined at 2 GPa/1200 °C, 6 GPa/1600 °C, 8 GPa/1800 °C and 10 GPa/1900 °C. Results show that characteristic depletion of Nb and Ta in partial melts due to the presence of rutile in solid residues (for example during melting of subducting oceanic crust) is strongly dependent on depth of partial melting. With increasing pressure, changes in melt structure result in marked reduction in D_min/melt for Nb (14.8, 5.4, 2.5, and 2.4 with increasing P/T) and Ta (28.0, 17.0, 6.9, and 5.5). A strong pressure effect is also noted in D_min/melt for Zr (2.1, 0.6, 0.9, and 1.2) and Hf (4.1, 0.9, 1.3, and 1.3), although for these elements the rutile to TiO₂(II) transition also influences partitioning behaviour. Results have important implications for melting of oceanic crust in Earth's deep mantle. Ancient subduction-modified crust cannot be a direct source for ocean-island basalts (OIB) unless depth of melting is greater than 300 km, or degree of partial melting is much higher than suggested on the basis of previous trace element modeling work (and sufficient to remove TiO₂ phases from solid residues). Likewise, the absence of strong depletion of Nb and Ta in OIB also provides constraints on degree of partial melting vs depth of partial melting for models where melting of ancient crust acts as an indirect source for OIB by metasomatic interaction with the mantle. The controlling influence of melt structure on partitioning behaviour of high-field strength elements (HFSE) implies that relative enrichment of Nb and Ta and reduction in Zr/Nb in high-pressure partial melts should occur even when TiO₂ phases are not present in solid residues. As such, depth of partial melting may be as important a factor as mineral and melt chemistry and degree of partial melting in constraining the composition of partial melts from Earth's deep interior.