| Subduction zones,where earthquakes occur,magmatic activities continue and ore deposits form are of great importance in the evolution of continental crust.Studies on the geological processes in subduction zones are the hot,important and difficult field in solid Earth sciences.In this study,we focused on two of the most important scientific issues in subduction zones,i.e.oxidation state of the mantle wedge and partial melting processes in the mantle wedge.Firstly,we discussed the oxygen fugacity?fO2?buffering techniques in high temperature and high pressure?HT-HP?experiments.The fO2 is one of the most important physicochemical parameter which could exert great influence on the nature of a geologic system.Gas-mixing furnace is a widely used apparatus for the fO2buffering experiments at 1 atm.Oxygen fugacity in the gas-mixing furnace can be modulated by changing the flux ratios of different gases poured into the furnace.The widely used gas mixtures include:CO2-CO,CO2-H2,H2-H2O systems.For a certain gas mixture system,the relation between temperature-fO2-gas mixture ratios can be calculated from basic thermodynamic data,and thus the fO2 accuracy of the gas-mixing depends on,to a large extent the accuracy of these data.We found that the old thermodynamic data used in previous studies is inaccurate and may cause failure in the buffering of fO2 in gas-mixing furnace.Therefore,we recalculate the temperature-fO2-gas mixture ratios relation for different gas mixture systems using the updated thermodynamic data.In addition,we also calculated this relation for other systems?including O2-inert gas,CO2-O2 and H2O-O2?.Our results have made up the previous defects and increased the upper limit of fO2 controlling capacity in gas-mixing furnace.In comparison to the 1 atm conditions,fO2 buffering at high pressure is full of challenges.So far,fO2 buffering technologies at high pressure include double capsule method,alloy method and fO2 buffer doped single capsule method.Different methods can only be used at certain experimental conditions.Secondly,we conducted a series of HT-HP experiments,and determined partition coefficients of the first row transition elements?FRTEs?between mantle minerals and basaltic melt.Based on the partitioning behavior of vanadium,scandium and titanium,we estimated fO2 of the mantle wedge.Whether the arc mantle is more oxidized than oceanic mantle is still a controversial issue.Studies on the iron valence state in both mantle xenolith and basalt proposed that the mantle wedge should be generally more oxidized than oceanic mantle.However,based on the ratio of multivalent vanadium to a homovalent element?Sc,Ti or Yb?,others argue that mantle wedge and oceanic mantle should be similar in fO2.We found that,the previously used partition coefficient of vanadium?DV?should be problematic,because all these D-values are obtained from one atmosphere experiments,but the mantle melts at high pressures.Therefore,we performed high pressure experiments with the piston-cylinder apparatus and determined D-values of the FRTEs at typical T-P-H2O conditions of the mantle wedge and variable fO2s.Combining our and reported D-values,we find that,in addition to fO2,temperature and phase compositions also have an important influence on DV between mantle minerals and basaltic melts.Specifically speaking,at given fO2,DV values increase with decreasing temperature;AlT in pyroxene and Cr#in spinel has a positive effect on DV for pyroxenes and spinels,respectively.What`s more,temperature-dependencies of DV in pyroxenes exceeds that of DSc and DTi.Finally,we estimated fO2s of the mantle wedge and oceanic mantle using partial melting modelling with the temperature effect on D-values considered.Our results suggests that,on average,fO2 of the mantle wedge is 0.9 log units higher than oceanic mantle.At last,we investigated melting behavior of mantle peridotite under water-saturated conditions.The upper mantle is mainly composed of peridotite,and partial melting of mantle peridotite plays key roles in the thermal and chemical evolution of the Earth.The water-saturated?or simply‘wet'?solidus of peridotite defines the initial melting temperature of the Earth's mantle under water-saturated conditions,and the second critical endpoint?SCEP?marks the end of the wet solidus.The location of the wet solidus remains an outstanding issue for over 50 years,and the position of the SCEP is hotly debated.Using a large-volume multi-anvil apparatus,we investigated the water-saturated melting behavior of a fertile peridotite?at 3-6 GPa and950-1200°C?.We successfully obtained systematic and well-preserved quenched liquids.Based on the texture and chemistry of the quenched liquid,we determined the wet solidus and proposed the SCEP at 3.8 GPa and 1025°C.The results could provide significant insights into upper mantle processes. |
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