Solubility of tin in (Cl, F)-bearing aqueous fluids at 700 °C, 140 MPa: A LA-ICP-MS study on synthetic fluid inclusions

Synthetic fluid inclusions in quartz were grown from cassiterite-saturated fluids in cold-seal pressure vessels at 700°C/140MPa/fO2∼NNO and subsequently analyzed by laser ablation-ICP-MS. Most inclusions were synthesized using a new technique that allows entrapment of fluid that had no immediate contact to the capsule walls, such that potential disequilibrium effects due to alloying could be avoided. Measured Sn solubilities increase with increasing ligand concentration in the fluid, ranging from 100 to 800 ppm in NaCl-bearing fluids (5–35 wt% NaCl), from 70 to 2000 ppm in HF-bearing fluids (0.5–3.2 m HF), and from 0.8 to 11 wt% in HCl-bearing fluids (0.5–4.4 m HCl). Two runs performed with the in-situ cracking method after 1 week of pre-equilibration demonstrate that the speed of hydrogen diffusion through the capsule wall relative to that of fluid inclusion formation is a critical factor in fO2fO2-dependent solubility studies. Graphical evaluation of the solubility data suggests that Sn may have been dissolved as Sn(OH)Cl in the NaCl-bearing fluids, as Sn(OH)Cl and SnCl2 in the HCl-bearing fluids, and as SnF2 in the HF-bearing fluids. Experiments with NaF-bearing fluids produced an additional melt phase with an approximate composition of 53 wt% SiO2, 25 wt% H2O, 14 wt% NaF and 8 wt% SnO, which caused the composition of the coexisting fluid to be buffered at 0.5 wt% NaF and 150 ppm Sn. Fluorine-rich, peralkaline melts may therefore serve as important transport media for Sn in the final crystallization stages of tin granites. Based on the available cassiterite-solubility data in fluids and melts, DSnfluid/melt in natural granite systems is estimated to be in the order of 0.1–4 (depending on their aluminosity), suggesting that Sn is not easily mobilized by magmatic-hydrothermal fluids. This interpretation is in accordance with the high degrees of Sn-enrichment commonly observed in highly fractionated melt inclusions. DSnfluid/melt is primarily controlled by the HCl concentration in the fluid, which in turn is a function of the aluminum saturation index of the magma. Compared to HCl, the effect of fluorine on DSnfluid/melt is subordinate.