TiO2 Solubility and Nb and Ta Partitioning in Rutile-Silica-Rich Supercritical Fluid Systems: Implications for Subduction Zone Processes

Abstract

To understand Ti, Nb, and Ta mobility in supercritical fluids and Nb/Ta fractionation in subduction zones, we conducted piston cylinder experiments to determine rutile (TiO2) solubility and Nb and Ta partition coefficients (Dru/sf Nb and Dru/sf Ta) in rutile (ru)-silica-rich supercritical fluid (sf) systems at 1.5–2.5 GPa and 920–1150 °C over variable fluid chemistries including solute (SiO2 ± albite component), H2O, Cl, and F contents. Under the investigated conditions, TiO2 solubility in the supercritical fluids varies from 761 ± 107 to 9795 ± 448 ppm; Dru/sf Nb and Dru/sf Ta vary from 12 ± 1 to 208 ± 30 and 34 ± 5 to 2464 ± 140, respectively. Higher solute, Cl, and F contents in the systems and higher temperatures result in higher TiO2 solubilities and lower Dru/sf Nb and Dru/sf Ta, and thus, fluid chemistry and temperature exert main controls on Ti, Nb, and Ta mobility. In all cases, Dru/sf Nb/Dru/sf Ta < 0.70, suggesting that supercritical fluids released from subducting slabs are higher in Nb/Ta relative to their protoliths. Therefore, such fluids are expected to result in higher Nb/Ta ratios in mantle wedges and arc magmas. Although Ti, Nb, and Ta could be mobile in supercritical fluids, Nb/Ta ratios in primitive arc basalts compared to those in mid-ocean ridge basalts show that only ~10% of arc basalts are possibly disturbed by solute-rich supercritical fluids during their generation. Therefore, if the fluids released from subducting slabs are dominantly supercritical, then most of them must be stable only in narrow P-T spaces and hence too short-lived to ascend very far into mantle wedges.