The Hydrolysis And Mobility Of HFSE-F Complexes In Hydrothermal Conditions

High-Field-Strength Elements?HFSEs?are the elements with high valences,small radius and high field strength of ions,including Ti,Nb,Ta,Zr,Hf,Sn???and so on.Generally,they were unable to mobile in hydrothermal fluids,resulting into the depletion character of HFSEs in arc magma region.However,with the discovery of more geological evidences,such as the rutile in ultra-high-pressure quarz veins,veined rutile and titanite during contact metamorphic processes,daughter minerals like rutile,titanite and baddeleyite in metamorphic fluid inclusions,it has been gradually questioned the fluid-imbolibilty of the HFSEs with many evidences.In order to explore the conditions of the mobility of HFSEs in hydrothermal fluids,experimental geochemists have carried out a large number minerals solubility experiments.The experimental results indicated that the halogens,especially fluorine,significantly improved the solubility of HFSEs minerals in hydrothermal fluids.It is still unclear about the solubility of HFSEs because of the different experimental techniques and conditions in different labs.In order to acquire the accurate mobility of HFSEs in varied conditions,we have set up a HFSEs mobility research method on the basis of complex hydrolysis regularity of high-temperature and high-pressure experiments and the thermodynamic simulation.Compared with the previous mineral solubility method,the hydrolysis method have distinct advantages with simple reaction system,short balance time,simple product analysis and convenient data expansion.On the basis of hydrolysis method,we have carried out the hydrosis experiments of 6 HFSE-F compex including K₂TiF₆,K₂NbF₇,K₂TaF₇,?NH₄?₂ZrF₆,K₂HfF₆ and K₂SnF₆.Furthermore,we discussed the mobility and influence factors of 6 HFSEs,including Ti,Nb,Ta,Zr,Hf and Sn???,in F-rich hydrothermal fluids.Since Ti-rich minerals like rutile contain a lot of HFSEs,they usually control the migration of the HFSEs in the system in hydrothermal fluids.The rutile solubility experiments indicated that Ti has an extremely high mobility in F-rich fluids.The results of the hydrolysis experiments indicated that the hydrolysis rate of K₂TiF₆ in HPHT conditions increased with temperature and decreased with the initial concentration.On the other hand,the pressure and cation types had little effect on the hydrolysis of K₂TiF₆.Through the linear fitting of the experimental results,for the first time we acquired the accumulated hydrolysis constant expression of K₂TiF₆,lnK_Ti=-10806/T+9.1856.In the hydrolysis reaction of K₂TiF₆,the?^?₈)=+89.8±3.8 kJ/mol and?^?₈)=+76.4±6.7 J/?mol·K?.Based on the thermodynamic data above,we acquired the maximal migration capacity of Ti-F complex at varied F contents,temperatures and pH in the system by thermodynamic simulation.The results show that maximal migration capacity of Ti-F complex increased with F content and decreased with temperature.In addition,the Ti-F complex has the higher mobility in neutral or weak-acid fluids.The precipitate analysis and in-situ observation indicated the growth of the hydrothermal TiO₂ minerals was controlled by the nucleation,aggregation,attachment and Oswald ripening successively.We have carried out the hydrolysis experiments of K₂NbF₇ and K₂TaF₇ on the basis of previous studies.The results indicated the hydrolysis behaviors of K₂NbF₇ and K₂TaF₇ were similar to the K₂TiF₆.The hydrolysis rate of K₂NbF₇ and K₂TaF₇ was increased with the temperature and decreased with the initial concentration.Through the linear fitting of the experimental results,we acquired the accumulated hydrolysis constant expression of K₂NbF₇ with lnK_Nb=-6575.3/T-7.8736 and of K₂TaF₇ with lnK_Ta=-11968/T+7.8633.In the hydrolysis reaction of K₂NbF₇,?^?₈)=+54.7±8.2kJ/mol and?^?₈)=-65.5±12.7 J/?mol·K?,and of K₂TaF₇,?^?₈)=+99.5±5.7 kJ/mol and?^?₈)=+65.4±8.7 J/?mol·K?respectively.Based on the thermodynamic data above,the maximal migration capacity of Nb-F and Ta-F complex at varied F contents,temperatures and pH in the system were calculated by thermodynamic simulation.In the neutral or weak-acid fluids,the maximal migration capacity of Nb-F and Ta-F complex can be up to thousands of ppm if the F content in the system was more than3000 ppm.In the low F concentration environment?c_F<0.2 mol/L?,the mobility of Nb-F complex is significantly greater than Ta-F complex.While in the high F concentration environment?c_F>0.5 mol/L?,there is a similar molarity between Nb-F and Ta-F complex.Our simulation results are similar with the minerals solubility experimental results in previous studies with c_F>0.1 mol/L in the system.The Nb/Ta ratio is the function of temperature on the basis of accumulated hydrolysis constants.The Nb/Ta ratio function is Nb/Ta=0.48691×0)^{-5392.7+15.7369},indicating that Nb/Ta ratio is temperature dependent.The ultra significant fractionation of Nb/Ta may be vital for the formation of Nb/Ta ore and the local decoupling of Nb/Ta.It is still highly debated on the solubility of zircon and baddeleyite among different researchers.Besides,there is still no related study about the mobility of Hf in the hydrothermal fluids.According to the hydrolysis experiments,we acquired the hydrolysis behavior information of?NH₄?₂ZrF₆ and K₂HfF₆ in HPHT conditions for the first time.The results indicated the hydrolysis behavior of?NH₄?₂ZrF₆ and K₂HfF₆ are similar to K₂TiF₆.The hydrolysis rate of?NH₄?₂ZrF₆ and K₂HfF₆ was increased with the temperature and decreased with the initial concentration.According to the linear fitting of the experimental results,we acquired the accumulated hydrolysis constant expression of?NH₄?₂ZrF₆ with lnK_Zr=-19884/T+14.493 and of K₂HfF₆ with lnK_Hf=-19890/T+11.525.In the hydrolysis reaction of?NH₄?₂ZrF₆,?^?₈)=+165.3±14.0kJ/mol and?^?₈)=+120.5±24.8 J/?mol·K?;and for K₂HfF₆,?^?₈)=+165.4±19.3kJ/mol and?^?₈)=+95.8±30.5 J/?mol·K?.Based on the thermodynamic data above,we acquired the maximal migration capacity of Zr-F and Hf-F complex at varied F contents,temperatures and pH in the system by thermodynamic simulation.In the neutral or weak-acid fluids,the maximal migration capacity of Zr-F and Hf-F complex reached thousands of ppm if the F content in the system was more than 2000 ppm.The mobility of Hf is always twice as high as Zr under any circumstances.The Zr/Hf ratio in F-rich fluids is calculated to be 0.026 based on the accumulated hydrolysis constants,which is not effected by the temperature and F content in the fluid system.It demonstrates F-rich fluids will be relatively enriched in Hf and depleted in Zr.Therefore,the existence of F-rich fluids may be an interpretation of the extremely low Zr/Hf ratio in pegmatitic zircons.It is widely considered that Sn???-Cl complex are the main species in the tin ore-forming fluids.Recently,a study shows that Sn???-Cl complex has the mobility of the same order of magnitude to the Sn???-Cl complex.However,the mobility of Sn???-F complex is rarely reported.In this study,we have carried out the hydrolysis experiments of K₂SnF₆ at HPHT conditions.Our experimental results revised the previous data in high-temperature region.Furthermore,we reacquired the accumulated hydrolysis constant expression of K₂SnF₆ with lnK_Sn=-10615/T+12.058.In the hydrolysis reaction of K₂SnF₆,the?^?₈)=+88.2±4.5 kJ/mol and the?^?₈)=+100.3±6.9 J/?mol·K?.Based on the thermodynamic data above,we acquired the maximal migration capacity of Sn???-F complex at varied F contents,temperatures and pH in the system by thermodynamic simulation.The simulation results indicate Sn???-F complex can only migrate effectively in neutral and weak-acidic fluids.The mobility of Sn in low-temperature?<400°C?fluids will reach a thousand of ppm only if the F content in the system was more than 7000 ppm,which is slightly lower than the mobilily of Sn???-Cl complex in low oxygen fugacity but is in the same number order of Sn???-Cl and Sn???-Cl complex in high oxygen fugacity.Because the tin ore-forming fluid is characteristic of low acidity,Cl-rich and low oxygen fugacity,the mobility and stability of the Sn???-F complex display no superiority over the Sn-Cl complex.Therefore,Sn-Cl might be still the main species in the tin ore-forming fluids.The presence of Sn-F complex may have little impact on the formation of tin ore.With the cpmparison of the hydrolysed behaviors of 6 complexes above,we found that different HFSE-F complexes have similar hydrolysis behavior in HPHT fluids.The hydrolysis rate is increased with temperature and decreased with initial concentration.The simulation results indicate Ti,Nb,Ta,Zr,Hf and Sn???have a great mobility at high F content,low temperature and neutral or weak-acidic fluids.In a simple system,the order of the maximal migration capacity is Hf-F>Zr-F>Nb-F>Ta-F>Ti-F>Sn???-F at low F content?c _F=2000 ppm?and is Hf-F>Ta-F>Nb-F?Zr-F>Sn???-F>Ti-F at high F content?c _F=10000 ppm?.In a complex system,however,the mobility of HFSEs in F-rich fluids is controlled by many factors.Among all,the stability of the HFSE-F complex is the major influence factor.However,the envionment variables,such as the elemental abundance,temperature,F content and pH can significantly influence the mobility of HFSEs as well.More specific regulation remains to be continued in future research.