Development Of Tungsten Isotope Analysis Method And Application To Crust-mantle Tungsten Cycling

Isotope geochemistry helps to trace the source and geological processes.The variations in tungsten(W)isotope composition include mass-dependent fractionation and mass-independent fractionation(isotope anomaly),in which W isotope anomalies can restrict the processes of early differentiation,late accretion,and core-mantle interaction.Stable W isotopes can trace W cycles in different geochemical reservoirs.This study mainly studies the mass-independent and mass-dependent fractionation processes of W isotopes.The methods of W isotope anomaly include MC-ICP-MS and N-TIMS analyses.In this study,N-TIMS analysis was developed and compared with MC-ICP-MS analysis to research the differences in W isotope anomalies measured by different mass spectrometers.In this study,the effects of oxides were effectively deducted by real-time online oxygen isotope composition.Meanwhile,by optimizing the cup configurations,the single measurement time of the sample was shortened to less than 4 hours,and the test efficiency was significantly improved.The external reproducibility of mass-independent W isotopic analyses was usually better than 5 ppm,which can be applied to discern small variations in W isotopic compositions in silicate samples.The results of MC-ICP-MS and N-TIMS were consistent within the error range.In addition,the origin and influence of the 183W mass-independent effect on the W isotope analysis were discussed.It was found that the effect of 183W correction was more obvious for MC-ICP-MS than N-TIMS,which may reflect the difference in sample loading between MC-ICP-MS and N-TIMS.W isotope anomalies are common in Archean mantle-derived samples.This study explores the relationship between W isotope anomalies and the evolution of time and region by analyzing the W isotope compositions in rocks from 3.54 Ga to 3.27 Ga in the Kaapvaal Craton of South Africa and 3.8 Ga to 2.5 Ga in the North China Craton.The results showed that the 3.8 Ga sample from the North China Craton had significant 182W positive anomalies,while the young sample after 3.4 Ga did not have 182W positive anomalies.No 182W anomalies were found in 3.54 Ga to 3.27 Ga samples from the Kaapvaal Craton in South Africa.Combined with previous studies on ancient mantle-derived rocks,there are temporal and regional differences in the W isotope compositions in the Archean mantle,which may indicate temporal and regional differences in mantle convection or plate tectonics.Due to plate tectonics and regional mantle convection,the heterogeneous mixing rates of different ancient mantles are also inconsistent,resulting in differences in W isotopic compositions in different ancient mantles.However,the mantle gradually homogenized,and most mantle-derived rocks that formed afterward had W isotopic compositions consistent with the modern mantle.With the development of mass spectrometry,stable W isotopes can also be used as a new potential isotopic indicator to study the W cycle during different processes(high and low temperatures)and in different reservoirs on Earth.In this study,the method for stable W isotopic analyses using the 180W-183W double-spike technique and MC-ICP-MS and the interference factors affecting the measurement of the W isotope were analyzed systematically and comprehensively.The δ186/184W values were not significantly affected when the final W solution had Hf/W ≤3 × 104,Ta/W ≤1,Os/W≤0.06,Ce/W≤0.0075,Nd/W≤3.5,and Sm/W≤5.The intermediate measurement precision of both the standard solutions(NIST SRM 3163 and Alfa Aesar W)and the geological reference materials(NOD-A-1)was better than ±0.024‰(2SD).We also obtained a precision of 0.026‰ for a minimum sample loading mass of 5 ng,showing the accurate analyses of samples with low W contents.Replicated measurements of geological reference materials(AGV-2,BCR-2,BHVO-2,GSP-2,RGM-1,SDC-1,NOD-A-1,and NOD-P-1)yielded δ186/184W values ranging from 0.017‰ to 0.144‰.The W isotopic compositions of international standards of two tungsten ore minerals(scheelite and wolframite)were reported for the first time.The W isotopic compositions of scheelites was consistent with the mantle value within the error range and was systematically 0.04‰ heavier than that of wolframites.The W isotope difference in ore minerals may reflect the difference in the crystal structure.Scheelite with tetrahedral coordination was more likely to bind heavy W isotopes than wolframite with octahedral coordination.The wolframite and scheelite samples from the Xingluokeng deposit in Fujian Province showed similar characteristics to the international standards,and the average value of scheelite samples was also 0.04‰heavier than the W isotopic compositions of wolframite samples.In addition,the average values of scheelite and wolframite samples fall within the corresponding ranges of mineral standards,indicating that the W isotopic compositions of wolframite and scheelite itself may be relatively homogeneous and can be used as an end member of the W isotopic composition in geochemical cycle processes.The variation in the stable W isotope composition in different lithology rocks and tungsten ore minerals makes W isotopes a potential geochemical index for studying the W cycles in geological reservoirs.