Chromium Subgroup Isotope Analyses And Cosmochemical Applications

In Periodic table,the Chromium subgroup contains three transition metal elements,i.e.chromium(Cr),molybdenum(Mo),and tungsten(W).They are all refractory elements and have different applications in cosmochemistry.Chromium isotopes are useful for the 53Mn-53Cr decay system(half-life=3.7 Myr)and 54Cr anomaly.Molybdenum has 7 stable isotopes and can help to distinguish the mixing process of p-,s-,r-process nuclides in the early solar nebular.Tungsten isotopes are important for the 182Hf-182W system to date the early differentiation of planetesimals.In recent years,with the wide application of multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)and thermal ionization mass spectrometry(TIMS),great advances have been achieved in high-precision metal isotope analyses.High-precision isotope analyses include mass independent measurements and stable isotope measurements.The application of isotope systems of the Chromium subgroup elements in cosmochemistry is first introduced in studies of mass independent fractionations,including extinct isotope chronology,cosmogenic effects and nucleosynthetic anomalies.To specify,tungsten isotope composition of the iron meteorites can be influenced by all the three processes.Tungsten isotopes have proven to be an important tool to study the formation of planetesimals,which is the key to understand the early evolution of the Solar System.Tungsten is a widely distributed element in the Solar System,one of its isotopes,182W,is partly a decay product of 182Hf(half-life 8.9Myr).During the metal-silicate differentiation process,tungsten is siderophile and mostly sinks into the core,while hafnium is highly lithophile and completely remains in the mantle.If such differentiation happened before 182Hf was completely decayed away,the metal and silicate phase would show radiogenic variations in tungsten isotope composition,the metal is depleted in 182W,while silicate is enriched in 182W.However,the galactic cosmic rays continuously shift the isotope composition of meteorites exposed in space,leading to a neutron-capture-induced lower ?182W value than its original value inherited from 182Hf decay,which will affect the application of the 182Hf-182W system in iron meteorites.To solve this problem,a proper method is required to correct the cosmogenic effect.Recent studies showed that the platinum isotopes have relative large cross sections and can be used to correct for cosmogenic effect on ?W in iron meteorites.Similar to platinum,osmium is also siderophile and enriched in iron meteorites.We prefer to use the osmium isotopes because it is hardly affected by nucleosynthetic effects.In this study,we proposed a new method to correct cosmogenic effect on tungsten isotopes by using 192Os/189Os normalized ?190 Os.This new method is independent of iron meteorites' chemical composition.By successfully developing tungsten and osmium isotope method,we measured the tungsten and osmium isotope data and fit the?182W-?190 Os correction slope for ?D,?AB and IVB iron meteorites for the first time.We found that the result of our fitted slope is consistent with the results of our theoretical model calculation.Using this new correction method,we obtained ?182W0 data for different groups of irons.The results for ?AB,?AB,IVB are consistent with previous study using a platinum-tungsten correction method.?D irons show small discrepancy with previous study,which may reflect a potential platinum nucleosynthetic shift in ?D irons.Compared to former studies,our results support a continuous formation of different iron meteorites'parent bodies.In recent years,stable isotope studies of the Chromium subgroup elements are also making progresses and have proven to have potential applications in cosmochemistry.For example,chromium stable isotopes have been used to constrain the formation of earth-moon system.To measure the stable isotope compositions of the Chromium subgroup elements,the double spike technique is required to correct the mass fractionation in purification procedure and mass spectrometry analysis.In order to improve the analytical precision and reduce the effect from instrumental mass-fractionation,we first discussed the theory and calculation method of double spike technique.We found that mass fractionation bias on the double spike have no influence on the measurements.Then we simulated the optimal composition of the double spike for chromium and tungsten isotopes.Of all the three elements in the Chromium subgroup,chromium is the lightest and suffers the most severe interferences from isobars and polyatoms,therefore stable isotope analyses of chromium is relatively more complicated than molybdenum and tungsten.We developed a high precision method for measuring chromium stable isotopic composition by optimizing the calibrating procedure of the double spike,with various tests of isobaric interferences.We also reported chromium isotope measurements on a set of USGS geological reference materials including basalt,mica schist,diabase,peridotite and dunite.These results are fundamental to future studies of chromium stable isotope compositions of earth,moon and other planets.In summary,this thesis highlighted the importance of both mass independent and mass dependent isotope analyses of the Chromium subgroup elements in several cosmochemical evolution processes,by introducing the details of tungsten and chromium isotope analytical methods.This thesis will not only help to understand the scientific problems based on these isotope systems,but can also provide important reference for analytical methods in other isotope systems.