| Carbon（C）isotopes are widely used as important indicators to infer the formation conditions of carbonate rocks,including the mineral precipitation temperature,the source of carbon and the nature and source of the fluids involved（such as ocean,atmosphere or hydrothermal fluid）.Carbon isotope information in carbonate is of great significance for understanding the diagenetic and metamorphic history of formation and inferring their unique paleoenvironmental conditions.Non-traditional magnesium（Mg）isotopic composition is considered as an effective indicator to infer mineral formation temperature,mineral growth rate and identify weathering mechanism.Magnesium is abundant in seawater,silicate rocks and many carbonate minerals.The magnesium isotope fractionation between these phases is sufficient to indicate complex geological processes.Especially,the high concentration,uniform chemical and isotopic distribution of magnesium in modern seawater,as well as its existence in biological and abiotic carbonates,make magnesium isotopes an excellent tool for studying the oceanographic and biogeochemical conditions recorded in rocks.In addition,magnesium isotopes have good application prospects in the study of the formation and diagenetic transformation of carbonate minerals.High precision analysis of bulk carbon and magnesium isotopes can provide a powerful means for geochemical research.However,in many cases,the bulk analysis method leads to the averaging of potential isotope information,due to the limitation of sampling technology,thus neglecting many microscale changes in carbon and magnesium isotopes.Therefore,in situ carbon and magnesium isotope analysis in microscale has attracted attention.Its high precision and high resolution analysis has broad application prospects in the research fields of chemical stratigraphy,biochemical sedimentary deposits,paleoceanography and paleoclimatology.At present,the in situ analysis technology for carbon and magnesium isotopic composition of carbonate is still in its infancy,and the key factors that may affect the accuracy and precision,such as isotopic fractionation behavior,matrix effect and isobaric interference,need to be studied in detail.Therefore,this study focused on these key issues to carry out high-precision in-situ micro analysis of carbonate carbon and magnesium isotopes,and developed carbon and magnesium isotopes of carbonate minerals（calcite,dolomite,magnesite,siderite）using LA-MC-ICP-MS.In addition,in view of the lack of homogeneous standard of C and Mg isotopes of carbonate minerals,we also carried out the study on the preparation of carbonate solid standard samples by powder pressing and sintering under normal pressure.In the study of carbon isotope compositions determine by fs-LA-MC-ICP-MS,spectral interferences of doubly charged ions(24Mg2+for 12C+,26Mg2+for 13C+)and polyatomic species(12CH+for 13C+)were evaluated.CH+interference mainly comes from experimental gases（Ar and He）,and superimposed on the high mass side of 13C mass peak.The interference can be negligible at low sample and carrier gas flow rates.Mg2+interference mainly comes from the sample itself（e.g.,dolomite and magnesite）.26Mg2+and 24Mg2+interferences are superimposed on the low-quality side of 13C and12C peaks.Signal acquisition can be carried out on the high mass side of carbon mass peak to avoid interference,or can be deducted by the interference reduction formula.The associated internal precision of in situ C isotope analysis correlates with intensities,and is better than 0.20‰（2SE）when the measured 12C ion signal is>12.5 volts.Carbonate samples DOL-8,MGS-1 and SD-5 show relatively uniform bulk and in situ carbon isotope compositions with precisions better than 0.24‰and 0.45‰（2SD）,and are adopted as in-house standards.Laser ablation analyses of various calcite,dolomite,magnesite and siderite withδ13C in the range of-6.28‰to 0.32‰agree excellently with IRMS determined values using the standard-sample bracketing（SSB）calibration,with precision of 0.37-0.68‰（2SD）.Matrix effects have been investigated and determined to be insignificant for the same carbonate matrix with variable chemical compositions when adopting the femtosecond laser.Instrumental carbon isotope fractionation observed among different carbonate matrices is non-negligible（e.g.,up to4.29‰between calcite and magnesite）using fs-LA-MC-ICP-MS,and an external multi-reference calibration can be applied for non-matrix matched analyses.Double charged Mg interference is more significant adopting ns-LA-Nu plasma II MC-ICP-MS to analyze C isotopes at low resolution mode.Thus,the Mg2+interference correction is adopted.The corrected in-situδ13C values are also well consistent with the IRMS measurements,and the analytical uncertainty ranges from 0.33‰to 1.27‰.Matrix effects of carbonates with distinct chemical and physical properties are the main factors affecting the accurate and precise determination of Mg isotopes by laser ablation multi-collector inductively coupled plasma mass spectrometry（LA-MC-ICP-MS）.This study investigates Mg isotope microanalysis of magnesite,dolomite and siderite under wet and dry conditions.The results demonstrated that the main isobaric interference of 48Ca2+on 24Mg+in carbonate with high Ca/Mg ratios（up to 6.79）can be significantly suppressed and downhole isotopic fractionation can be reduced with the addition of water vapor before the ablation cell（e.g.,wet condition）.MGS-3,DOL-8,DOL-9 and SD-5 show homogeneous in situ Mg isotope compositions for the investigated pieces（>1cm2）with intermediate precision better than 0.14‰and 0.15‰forδ26Mg andδ25Mg,and are adopted as in-house standards.The obtained in situδ26Mg andδ25Mg values of matrix-matched samples were consistent with those determined by solution nebulization（SN）-MC-ICP-MS with uncertainties of 0.16-0.27‰and 0.12-0.25‰（2SD）under dry condition and 0.15-0.18‰and 0.06-0.10‰（2SD）under wet condition.This further indicates that wet-laser analyses improved the analytical precision of Mg isotope determinations by 1.1 to 2.5 times.Inaccurate Mg isotope data were obtained under dry condition when carbonate samples were measured against a physical property and/or chemical composition different standard,such deviation can be largely improved in wet-laser analyses.The wide application of LA-MC-ICP-MS analysis method for carbonate C and Mg isotopes is limited by the lack of suitable carbonate reference materials with matrix matched.Here,we propose a new method to prepare carbonate reference materials,which can greatly improve the strength and hardness of carbonate pressed powder pellet by sintering under normal pressure without changing the mineral phase.The results show that smaller particle size and higher sintering temperature help to enhance hardness and density of powder pellet.The Aladdin Ca CO3 powder particle size is200nm-1μm.The Vickers hardness range of ALD powder pellet sintered at 500℃has the best hardness（40.16 HV）,and the repeatability ofδ13C composition is less than0.22‰（2SD）,which is twice better than that of natural calcite.The GBW07217dolomite sintered powder pellet with particle size less than 2μm and sintering at 400℃shows the best repeatability ofδ13C,δ26Mg andδ25Mg,which is 0.48‰,0.15‰and0.07‰（2SD）,respectively.Noteworthy,there is no C and Mg isotopic fractionation between the powder pellet（calcite sintered at 500℃and dolomite at 400℃）and mineral crystals.To sum up,the powder particle size for preparing sintered carbonate solid standard is less than 2μm,and the sintering temperature of 500℃for calcite and400℃for dolomite is recommended.In conclusion,the key issues such as interference of double charged ions and polyatoms,isotope fractionation behavior in the laser ablation process,and matrix effect between carbonates with different compositions in the carbonate C and Mg isotopes LA-MC-ICP-MS analysis are studied in detail in this work,and effective correction methods are proposed.We successfully established a high-precision in-situ analysis method for a serious of carbonate C and Mg isotopes and proposed a preferable preparation condition for developing carbonate powder standard samples,which effectively filled the technical gap in preparing carbonate C and Mg isotope solid standard samples for in situ analysis,and laid a good foundation for the widely used of carbonate C and Mg isotope analysis by LA-MC-ICP-MS. |
