Experimental Study On The Geochemical Behaviors Of Vanadium And Zirconium In Subduction Zones

Oxygen fugacity and mass migration are the frontier research topics of subduction zone processes.The geochemical behaviors of variable variable-valence vanadium(V)and high field strength element zirconium(Zr)are important indicators of redox state and material migration in subduction zones,respectively.The study of their geochemical behaviors is of great significance for understanding the process of subduction zones.In this paper,the partition behavior of vanadium during partial melting of subduction plate eclogite and the solubility of Zr in the mantle minerals were studied by high temperature and high pressure experiments.We investigated the geochemical behavior of vanadium(V)during the partial melting of eclogite,and evaluated the significance of V migration during the partial melting of eclogite by using the partition coefficient of V between clinopyroxene and melt.The relationship among oxygen fugacity,temperature and system composition was used to estimate the oxygen fugacity of natural rock samples which contained clinopyroxene.We also measured the solubility of zirconium in the mantle minerals by experimental study,so that we could better understand the genesis of zircon in peridotite that occurred in orogenic belts.Besides,the zirconium solubility study also provided scientific evidence for melt/fluid migration in subduction zones.This study includes three parts as following.Firstly,we took advantage of the piston cylinder apparatus to model the distribution of V during the partial melting of water-bearing eclogite and then estimated the oxygen fugacity of the mantle wedge.Vanadium(V)is a common variable element,and its distribution between mineral and silicate melt is mainly controlled by oxygen fugacity.In recent years,the relationship between the distribution behavior of V and oxygen fugacity has been widely used to reveal the redox state of the mantle.During plate subduction,whether V can migrate is a key point for understanding the geochemical behavior of V in subduction zones.What is more,it is also an important prerequisite for discussing the oxygen fugacity of the mantle wedge in subduction zones.Therefore,we designed the following experiments:the initial material was the average mid-ocean ridge basalt that contained 5.0wt%H₂O;the pressure was 2.5 GPa;the temperature was 900-1125?,and the sample capsule was a gold-palladium-iron alloy.We measured the distribution coefficient of vanadium between garnet,clinopyroxene,rutile and coexisting silicate melt,and calculated the experimental oxygen fugacity(?FMQ)that was between-5.24 and+0.74.The experimental results showed that(1)V is a compatible element in garnet,clinopyroxene and rutile,and D_V^rut>D_V^cpx>D_V^grt.(2)The distribution coefficient of V is also a function of oxygen fugacity,temperature and melt polymerization degree.It is negatively correlated with oxygen fugacity and temperature,and positively correlated with melt polymerization degree.The simulation results of partial melting of eclogite show that the metasomatism between mantle wedge and melt resulting from partial melting of eclogite does not significantly change the V content in the mantle.Therefore,we believe that V should not migrate during the eclogite partial melting process,and the partition coefficient of V between mantle minerals and melts can be used to estimate the oxygen fugacity in the mantle.Secondly,clinopyroxene,an important reservoir of trace elements in the mantle,plays a vital role in distributing many trace elements in the processes of partial melting and crystallization of the mantle.Therefore,we used the piston cylinder to simulate the distribution of V between clinopyroxene and andesitic melts under the conditions of 0.5GPa,1000?1100°C,NNO and HM oxygen fugacity.The experimental results showed that the distribution coefficient of V between clinopyroxene and the melt is controlled by oxygen fugacity,temperature and system composition.Combined with previous data,the quantitative relationship between the partition coefficient of V between clinopyroxene and melt and oxygen fugacity,temperature and system composition were fitted:log D_V^cpx/melt=-4.19(±0.33)+0.94(±0.15)?D_Al+5730(±480)/T-0.24(±0.01)??FMQ(n=71,r²=0.92).This equation can be used to estimate the oxygen fugacity of natural rock samples that contained clinopyroxene.Finally,we measured the solubility of Zr in the mantle minerals.In recent years,the chemical migration of zirconium in subduction zones and the origin of zircon in peridotite in orogenic belts have attracted extensive attention.The zirconium-bearing melt/fluid metasomatized the local mantle peridotite,when Zr content reached saturation,a great mount of zircon crystallized.Therefore,the study of the solubility of zirconium in the mantle minerals is of great significance to understand the origin of zircon in peridotite in orogenic belt.In this study,by using a piston cylinder apparatus and a large-volume six-sided anvil apparatus,we performed high-temperature and high-pressure experiments to determine the solubility of Zr in the mantle minerals.The experimental conditions as follows:2.0-6.0 GPa and 1050-1200°C;the starting material is the mixture of peridotite and basanite in different proportions,and 1.5-5.0wt%zircon(Zr Si O₄)is added,water content is 5.0-10wt%.The experimental results showed that the solubility of Zr in the mantle minerals as follows:S_Zr^ol(3.68±0.27 ppm)<S_Zr^opx(4.67±0.23 ppm)<S_Zr^cpx(28-1403 ppm)?S_Zr^grt(294-2222 ppm).The solubility of Zr in amphibole and oxide are S_Zr^amp:189-546 ppm and S_Zr^oxide:263-630 ppm,respectively.The solubility of Zr in olivine and orthopyroxene are very low,and the effects of temperature,pressure and composition are negligible.The significant variations of the solubility of Zr in clinopyroxene and garnet depend on their respective mineral composition and P-T conditions.The solubilities of Zr in clinopyroxene and garnet both increase with temperature,but decrease with pressure in clinopyroxene;the solubility of Zr also increases with Al₂O₃ in clinopyroxene and increases with X_Alm(molar fraction of almandine)in garnet.Following important conclusions can be drawn from these results.(1)This study reported the solubility data of Zr in the mantle minerals.(2)In the mantle peridotite,the solubilities of Zr in olivine and orthopyroxene are the lowest,followed by clinopyroxene,and the solubilities of Zr in garnet is the highest.Therefore,when the Zr-bearing melt metasomatism with the mantle wedge,Zircon can be easily obtained in the dunite and harzburgite.Garnet is the main reservoir of Zr in the mantle peridotite.(3)The solubility of Zr in garnet has an excellent correlation with X_Alm and T expressed as log S_Zr^grt=6.87(±0.36)+0.023(±0.003)X_Alm-5990(±490)/T(n=12,r²=0.96),which provides potential zircon saturation thermometers for garnet-bearing rocks(garnet peridotite and eclogite,etc.).