| Platinum group elements(PGE),namely Ru,Rh,Pd,Os,Ir,and Pt,have long been studied as key strategic resources and is widely used in modern life and military defense industry,and often be used as a geochemical tracer for the process of mafic-ultramafic magmatism on account of its strong sulphophile and chemical inertness.However,more and more geological evidences and experimental results showed that PGE can be mobilized under some hydrothermal environments and mainly transport with the form of PGE-Cl complexes.Nevertheless,little is known about the thermodynamic properties and mobility of PGE and its complexes at high temperature and high pressure in fluids.Therefore,clarifying thermodynamic properties and mobility of PGE and its complexes is not only helpful to understand the distribution and circulation of PGE in the deep sea from the basic theory,but also to promote the new understanding of the basic geochemical properties of PGE,and then enrich the related metallogenic theories.In this study,multi-angle scientific thought combined with multi-level experimental simulation methods at high temperature and high pressure were used to conduct innovative research,including:1)the hydrolysis method of PGE-Cl complexes was used to study the migration,differentiation and precipitation processes of PGE in hydrothermal fluids;2)the solubility method of PGE oxides was conducted to probe into the dissolution and complexation of PGE;3)the water-rock interaction between peridotite and the fluids was used to investigate the mobility and transport of PGE in natural processes.A lot of pyrohydrolysis experiments of K2Pt Cl6,K2Pd Cl6,K2Ru Cl6,K2Os Cl6,K2Ir Cl6,and K3Rh Cl6were first conducted at different temperature,pressure,and time in this study.A series of time-series experiments showed that the hydrolysis reaction of PGE-Cl complexes reaches equilibrium within 24 hours at 200°C and 100 MPa,and the hydrolysis reaction behavior is mainly controlled by the temperature,pressure,and initial concentrations of PGE-Cl complexes.The increase of temperature(200-600°C)and pressure(50-300 MPa)can promote the hydrolysis behavior of PGE-Cl complex,and the influence of temperature is more significant,while the high initial concentration of PGE-Cl complex can inhibit the hydrolysis degree.By calculating and fitting the experimental results,we obtained for the first time the cumulative hydrolysis equilibrium constant(Kh)of different PGE-Cl complexes with temperature(T,kelvin):Ln Kh=(39.87±3.192)-(48671±2096)/T(K2Pt Cl6),Ln Kh=(45.24±5.664)-(49079±3583)/T(K2Pd Cl6),Ln Kh=(42.95±4.13)–(56205±2594)/T(K2Ru Cl6),Ln Kh=(38.32±2.74)–(47560±1657)/T(K2Os Cl6),Ln Kh=(46.61±3.82)–(46455±2361)/T(K2Ir Cl6),Ln Kh=(36.26±2.90)–(40591±1804)/T(K3Rh Cl6).Combined with the Van't Hoff and Gibbs free energy equations,this study obtained?rHm?,?rSm?,?rGm?of the hydrolysis systems for PGE-Cl complexes,and parts of hydrolysis constants(?)of PGE ions and formation constants(?)of PGE-Cl complexes,such as the hydrolysis constants(?)of Pt4+and formation constants(?)of Pt Cl62-in hydrothermal fluids.On this basis,the maximum transport of PGE-Cl complexes at different temperature,p H,and chlorine content in hydrothermal fluids were gotten through the simulation calculation.Calculated results show that the maximum transport of PGE-Cl complexes increases with decreasing p H,enhances by orders of magnitude with elevated concentration of Cl in the fluids,but fluctuates with increasing temperature.The optimal migration condition of PGE-Cl complexes is relatively acid,low to moderate temperature,and high chloride concentration,or weakly alkaline,high temperature,and high chloride concentration hydrothermal fluids,reaching a maximum about tens of thousands of ppm.Comparing with the mobility of different PGE-Cl complexes in hydrothermal fluids,the sequence of the maximum migration capacity is Ru-Cl>Pt-Cl,Os-Cl>Pd-Cl>Ir-Cl,Rh-Cl,and the degree of temperature controlling is Ru-Cl>Pd-Cl>Pt-Cl>Os-Cl>Ir-Cl>Rh-Cl.Based on the multi-element mixing simulation calculation,the mobility of PGE-Cl complexes is not controlled by a single factor,but dominated by multiple factors,such as temperature,p H,and chlorine content.In addition,a lot of nanoparticles of PGE oxides with unique morphology and structure were attained by hydrolysis reaction of PGE-Cl complexes.Based on the multi-layer nested experimental method,this study conducted an experimental study on the solubility of PGE oxides such as Pt O2,Pd O,Ru O2,Os O2,Ir O2,and Rh2O3.The experimental conditions were set as the saturated vapor pressure,temperature(150-350°C),concentration of HCl(0.001-0.1 mol/L)and Na Cl(0.04-4mol/L)in the solutions.Time-series experiments showed that the dissolution reaction of Pt O2reaches equilibrium within 7 days at 150°C and saturated vapor pressure,which is much longer than the equilibrium time of hydrolysis reaction for PGE-Cl complexes because of the heterogeneous phase reaction of dissolution.Controlled by the temperature,p H,and chlorine content in hydrothermal fluids,the dissolution of PGE oxides could be greatly promoted by increasing chlorine concentration and decreasing p H,which are conducive to the stable existence of PGE-Cl complex in the fluids.And the dissolution reaction of PGE oxides is also affected by elevated temperature,but less significantly.According to the thermodynamic calculation and Van't Hoff equation,PGE could be existed at the form PGE-Cl complex ion with six coordination in hydrothermal fluids after dissolution reaction.And the dissolution constant(Ks)of different PGE-Cl complexes with temperature(T,kelvin)was obtained:Pt O2:Ln Ks=-(46.78±4.08)+(41406±2185)/T,Pd O:Ln Ks=-(20.95±6.81)+(20097±3466)/T,Ru O2:Ln Ks=-(57.19±9.93)+(42966±5103)/T,Os O2:Ln Ks=-(49.05±4.51)+(43234±2350)/T,Ir O2:Ln Ks=-(37.67±5.28)+(31318±2800)/T,Rh2O3:Ln Ks=-(14.40±4.86)+(21588±2575)/T.At the same time,we also gained other thermodynamic parameters,such as?rHm?,?rSm?,?rGm?of the dissolution systems,parts of hydrolysis constants(?)of PGE ions,and parts of formation constants(?)of PGE-Cl complexes in hydrothermal fluids.Base on the simulation calculation for the dissolution constants of PGE oxides,the transport of PGE-Cl complexes mainly controlled by the temperature,p H,and chlorine content in hydrothermal fluids,and transport in large quantities at relatively acid and high chloride concentration fluids,reaching a maximum about tens of thousands of ppm.Comparing with the dissolution constants of PGE oxides,this study displayed that the sequence of the transport capacity of PGE-Cl complexes is Pt-Cl>Os-Cl>Ru-Cl>Ir-Cl and the degree of temperature controlling for stability of PGE-Cl complexes is Ru-Cl>Pt-Cl>Os-Cl>Ir-Cl>Rh-Cl in hydrothermal fluids.Therefore,the complexes,such as Pt-Cl and Os-Cl,is easy to be transported in hydrothermal fluids because of high stability,but some complexes,such as Ir-Cl and Ru-Cl,is difficult to be transported in hydrothermal fluids on account of poor stability or significant influence of temperature.Experimental studies on the metasomatism of mantle peridotite by chlorine-bearing fluids with the pressure of saturated vapor pressure-200 MPa,temperature of 200-650°C,HCl concentration of 0.001-0.091 mol/L,Na Cl concentration of 0.005-0.1 mol/L were carried out to further verify the mobility of PGE at natural samples in this study.The result showed that PGE can transport in the chlorine-bearing fluids,and its mobility can be promoted by elevated temperature.In addition,the transport of PGE in the chlorine-bearing fluids is different,the mobility of Pt,Pd,and Os is greater than that of Ru,Rh,and Ir.In a word,the consistent results of PGE-Cl complex hydrolysis experiments and PGE oxide dissolution experiments showed that PGE could be mobilized and exist stably in the chlorine-bearing fluids,and mainly controlled by temperature,Cl concentration and p H,among which Cl concentration plays a leading role.The PGE can transport in large quantities in relatively acid and high chloride concentration fluids,reaching a maximum about tens of thousands of ppm.At the same time,the water-rock reaction experiments between mantle peridotite and chlorine-bearing fluids further confirmed that PGE also has good mobility in natural samples and the migration ability difference between PGE.Therefore,the mobility and controlling factors of PGE in the chlorine-bearing fluids can be used to probe into the processes of transport,deposition,and mineralization of PGE in hydrothermal fluids and provide a possible explanation for the rare PGE deposits in the crust. |
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