Unconventional Analysis Methods For Carbon And Oxygen Isotopic Compositions Of Silicate And Carbonate Minerals

Using the CO₂ laser fluorination system?LAF?and isotope ratio mass spectrometry equipped with Multiprep^TM and Dual Inlet systems of the State Key Laboratory of Isotope Geochemistry in the Guangzhou Institute of Geochemistry,Chinese Academy of Science,this dissertation aimed at establishing the analytical methods for measuring triple oxygen isotopic compositions of silicate minerals and for automatically determining the carbon and oxygen isotopic compositions of each individual mineral in coexisting carbonates and acid-resistant carbonates.This dissertation analyzed a series of silicate minerals by using the LAF-DI-IRMS,and obtained high-precision?¹⁷O_VSMOW,?¹⁸O_VSMOW and?¹⁷O values.The precisions of?¹⁷O_VSMOW,?¹⁸O_VSMOW and?¹⁷O values are±0.05‰?1??,±0.07‰?1??and±0.03‰?1??,respectively,which are comparable to those published internationally.In the?¹⁷O_VSMOW-?¹⁸O_VSMOW diagram,the analysis results of these silicate minerals form a single mass-dependent line,?¹⁷O_VSMOW=0.525?±0.006?×?¹⁸O_VSMOW-0.160?±0.05?.Its slope is identical to that of the Terrestrial Fractionation Line?TFL?,which verifies the credibility of our analytical method for triple oxygen isotopic compositions of silicate minerals in this dissertation.Because of the lack of silicate mineral references with certified?¹⁷O_VSMOW,this dissertation proposes a new method for calibrating?¹⁷O_VSMOW of reference gas used in IRMS,which is based on the?¹⁷O of air O₂ and the known?¹⁸O_VSMOW of the reference gas.Using this calibration method,the analysis results of mineral references are consistent with the previous studies within analytical errors,confirming the validity of this calibration method.In addition,a large amount of NF₃ was observed in the newly purified BrF₅,which could result in abnormally high?¹⁷O_VSMOW due to the isobaric interference from NF⁺.Fortunately,the amount of NF₃in the newly purified BrF₅ will be gradually eliminated along with the increasing numbers of analysis until the isobaric interference of NF⁺disappears.Hence,the newly purified BrF₅ could be used for measuring?¹⁸O_VSMOW value of silicate minerals before the amount of NF₃ is low enough to accuratly analyze?¹⁷O_VSMOW of silicate minerals by using the LAF-IRMS.Carbon and oxygen isotopic compositions of individual minerals in coexisting calcite and dolomite could be measured by the selective acid extraction technique based on the different reaction rates of calcite and dolomite with phosphoric acid.The isotopic compositions of calcite and dolomite in mixtures were automatically analyzed by the Multiprep^TM-DI-IRMS in this dissertation.First of all,calcite and dolomite were reacted with phosphoric acid under different reaction conditions using the Multiprep^TM-DI-IRMS,to study the relationships between isotopic compositions and CO₂ yield,and reaction time and temperature.This dissertation observes that the 75-80?m calcite is completely reacted with phosphoric acid in 5 min at 50oC and in 45 min at 25oC,respectively;however,only⁸%CO₂ yield was obtained from the 75-80?m dolomite by reacting for 45 min at 50oC.Secondly,according to the known reaction rates of calcite and dolomite,five artificial mixtures with varied calcite/dolomite ratios were analyzed under different experimental conditions.Based on the analysis results of artificial mixtures,the reaction conditions for analyzing isotopic composition of calcite in the mixtures were optimized as follows.Samples should have grain sized of 75-80?m;and samples with calcite content of>50%,30-50%and<30%should be reacted with phosphoric acid for 10-15 min at 50oC,6 min at 50oC,and 45 min at 25oC,respectively.Using these optimized reaction conditions,the?¹³C_VPDBPDB and?¹⁸O_VPDB of the calcite in the mixtures are more accurate and precise than the previous studies.In order to resample the CO₂ liberated from dolomite in the mixtures after analysis of calcite,the script file that controls the Multiprep^TM-DI-IRMS actions was recoded and the software configuration was also modified.With the technique of resampling headspace bottles,the CO₂ liberated from residual dolomite in the mixtures could be automatically collected and analyzed for their isotopic compositions.The analysis results of the dolomite in the mixtures indicate that the accuracy and precision of the dolomite?¹³C_VPDBPDB and?¹⁸O_VPDB values are worse than those of the calcite.When calcite content in mixtures increases,the dolomite?¹³C_VPDBPDB and?¹⁸O_VPDB values gradually shift from their reference values and the standard deviations become worse.Nevertheless,the accuracy of dolomite?¹³C_VPDBPDB value is better than previous studies.As the rates for acid-resistant carbonate minerals to react with phosphoric acid are generally very slow,at least a few hours are needed for them to be completely digested.As a result,current commercial automated devices for analyzing isotopic compositions of carbonate are not suitable to analyze these carbonate minerals.With our technique of resampling headspace bottles mentioned above,an automatic procedure is developed in this dissertation to measure carbon and oxygen isotopic compositions of acid-resistant carbonates.The automatic procedure can normally run on Multiprep^TM-DI-IRMS and improve the analysis efficiency of acid-resistant carbonates.However,the sample vials need to be placed in air for a long time to make sample completely digested,during which the storage effect of vials can change the isotopic compositions of CO₂ in vials and influence the accuracy and precision of the analysis results.Improvement in vial sealability could diminish the storage effect.In this respect,two T-shape septum,T₁ and T₂,are designed for more contact area between vial and septum.The commercial PTFE-face silicon rubber septum,T₁ silicon rubber septum,T₁ butyl septum and T₂ silicon rubber septum are tested for their availability.Unexpectedly,even though T₁ and T₂ septum have more contact area than PTFE-faced silicon rubber septum,the accuracy and precision in isotopic compositions by using T₁silicon rubber septum,T₁ butyl septum and T₂ silicon rubber septum are even worse than those using PTFE-face silicon rubber septum.T₂ septum's structure make it easy to slip away from the top of headspace bottle,resulting in air leakage,and hence loss of CO₂ in the vial or large isotope fractionation.Inert PTFE film could prevent possible interaction between CO₂ gas and septum.However,the current manufacturing process is not able to make T-shape septum coated with PTFE film.Thus,the septum materials,silicon rubber or butyl,may interact with CO₂ gas via absorption/desorption or isotope exchange,which will result in poor accuracy and precision for?¹³C_VPDB and?¹⁸O_VPDB.The complete septum without pinhole has better sealability,which is conducive to the preservation of CO₂ isotopic compositions.In order to ensure septum without pinhole before sampling,this dissertation designed and manufactured vacuum screw cappers for V-bottom and U-bottom vials.The vacuum screw capper is used to evacuate the vial instead of single-hole needle puncturing septum,and tighten the vial in vacuum after evacuation finished.Like this,there will be no pinhole in PTFE-faced silicon rubber septum before collecting CO₂ gas in vials.The precision of analysis results using the septum without pinhole is better than that using septum with pinhole in the same storage time,but both accuracies are similar.It is shown that the integrity of septum could improve the sealability of vials,but the improvement is not enough to completely eliminate the storage effect.A relationship between?¹³C_VPDB,?¹⁸O_VPDB and CO₂ yield of the acid-resistant carbonates is found when vials are placed for a long time.The relationship indicate that the inward or outward diffusion of CO₂ caused by gas pressure difference between inside and outside of vials is a possible mechanism for the storage effect.When gas pressure inside the vial is greater than that outside the vial,CO₂diffuses from inside to outside,which will result in loss of light isotopes in the vial and the isotopic composition of the remaining CO₂ become heavier.As a result,sample size of acid-resistant carbonates could not be increased in this dissertation to suppress the influence of storage effect on analysis results.In summary,currently common headspace bottles cannot be used to preserve isotopic composition information of CO₂for a long time,so they cannot be applied to the automatic procedure designed for acid-resistant carbonates.In order to improve the accuracy and precision of carbon and oxygen isotopic compositions of acid-resistant carbonates,it is necessary to abandon the headspace bottle and redesign the sample reaction container with higher air tightness when using the automatic procedure.