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

Abstract

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 (TiO 2 ) 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 (SiO 2 ± albite component), H 2 O, Cl, and F contents. Under the investigated conditions, TiO 2 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 TiO 2 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.