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.

Genesis of Intermediate and Silicic Arc Magmas Constrained by Nb/Ta Fractionation

“…Although there is an increase in D Nb /D Ta with decreasing temperature and melt H 2 O concentration, D Ta is always larger than D Nb for each experiment (Figure 3a). Our data show a good agreement with previous studies on rutile‐liquid partitioning (Figure 3b), and this conclusion holds across a temperature range of 650–1900°C, a pressure range of 1 atm–10 GPa, and a liquid compositional range of aqueous fluid, supercritical fluid, and dry melt to hydrous melt (Bromiley & Redfern, 2008; Chen et al., 2018; Green & Adam, 2003; Green & Pearson, 1987; Holycross & Cottrell, 2020; Horng & Hess, 2000; Jenner et al., 1993; Kessel et al., 2005; Klemme et al., 2002, 2005; Rustioni et al., 2019; Schmidt et al., 2004; X. L. Xiong, 2006; Xiong et al., 2005, 2011). The stronger compatibility of Ta relative to Nb in rutile should be ascribed to the crystal chemistry and the slight difference in properties between Nb 5+ and Ta 5+ .…”

“…D Nb /D Ta shows a negative dependence on melt H 2 O concentration and temperature and is below 1 (Figure 3a), which is consistent with X. Xiong et al (2011) and Chen et al (2018). The slight negative correlation between D Nb /D Ta and H 2 O may result from Nb and Ta species in melts.…”

“…(2011) and Chen et al. (2018). The slight negative correlation between D Nb /D Ta and H 2 O may result from Nb and Ta species in melts.…”

“…Melts were quenched into smooth and transparent glasses at low H 2 O concentrations (<15 wt.%; Figure 1b), whereas the quenched glasses were variably vesiculated at high H 2 O concentrations (>15 wt.%; Figure 1c). The vesiculated glass pieces resulted from H 2 O exsolution during thermal quenching rather than the immiscibility of two phases during experiments (Chen et al., 2018; Hayden & Manning, 2011). The H 2 O added to capsules in our experiments was believed to completely dissolve in melts due to the absence of large void spaces (∼10 um), which indicate the coexistence of aqueous fluid and hydrous melt (Mibe et al., 2011).…”

High‐Pressure Fluid‐Rock Interaction and Mass Transfer During Exhumation of Deeply Subducted Rocks: Insights From an Eclogite‐Vein System in the Ultrahigh‐Pressure Terrane of the Dabie Shan, China

“…mobilization of Ti-Nb-Ta and Zr-Hf during fluid-rock interaction from several HP and UHP terranes (e.g., Beinlich et al, 2010;Gao et al, 2007;Guo et al, 2012;John et al, 2008;Li et al, 2013;Rubatto & Hermann, 2003;Taetz et al, 2016;Zhang et al, 2008). Rutile has a very low solubility in pure H 2 O (Audétat & Keppler, 2005;Tropper & Manning, 2005), but the addition of F and Na-Al-Si components in aqueous fluids can significantly 10.1029/2019GC008521 Geochemistry, Geophysics, Geosystems enhance the solubility of rutile such that it facilitates the mobilization of Ti-Nb-Ta due to the formation of various complexes (Antignano & Manning, 2008;Chen et al, 2018;Hayden & Manning, 2011;Manning et al, 2008;Rapp et al, 2010;Tanis et al, 2016). As discussed above, the active HP fluid in the quartzgarnet vein was rich in Na-Al-Si components.…”