Developments And Applications Of Laser Microanalytical Methods With High Spatial Resolution And Rapid Solution Analytical Methods For Ti Isotopes

Titanium（Ti）is a transition and high field strength element with five stable isotopes(⁴⁶Ti,⁴⁷Ti,⁴⁸Ti,⁴⁹Ti and ⁵⁰Ti),which occurs mainly in rutile（TiO₂）,ilmenite（FeTiO₃）,titanite（CaTiSiO₅）in geological samples.The Ti stable isotope fractionation is significant in terrestrial and extraterrestrial rocks.The co-ordination state of Ti between Fe-Ti oxides and silicate melts is one of major controls on Ti isotope fractionation during magmatic processes.Ti isotope fractionation during Fe-Ti oxide crystallization enriches the heavy Ti isotopes in the more evolved magmas,and the magnitude of fractionation can vary significantly between different types of magmas.For ten years,the development of analytical method,the determination of geological reservoir,the clarification of fractionation mechanism and the exploration of geological application are focus of Ti isotopes.The analytical method for Ti stable isotopes with high precision,as one of the non-traditional stable isotopes,is a key of the geological application.Nowadays,Ti isotope analytical method consists of the solution method and the in situ method.Solution method with the advantages of high analytical precision and accuracy is suitable for a wide range of rocks and minerals.However,the sample pretreatments are tedious and time-consuming due to the digestion with acids and the purification with multi-columns separation for solution Ti isotope analysis.Other,the solution analysis provides only the bulk Ti isotopic ratios in geological samples.Laser in situ analysis with the high spatial resolution and the fast analytical speed is one of important geochemical tools and used to obtain the isotope compositions of tiny crystals and geological samples with complex structures.Compared to the solution method,laser in situ analysis for Ti stable isotopes in Ti-rich minerals is seriously insufficient and limited due to the lack of suitable reference materials.In this study,the main contents and conclusions are as following.1.A rapid NH₄HF₂ digestion was developed for Ti-rich minerals such as refractory rutiles.Complete sample digestion is a prerequisite to achieve the accurate and precise results of element concentrations and isotope compositions in geological samples.The digestion time is 72 h for dissolving refractory Ti-rich minerals such as rutiles and ilmenites using the common acid digestion.In this study,a rapid ammonium bifluoride（NH₄HF₂）digestion was developed to completely digest large amounts of rutile,ilmenite,titanite and perovskite within 4 h.The different amounts（2-200 mg）and different sizes（31-150μm）of these Ti-rich minerals were not affect the digestion results.The NH₄HF₂digestion provides identical major Ti and thirty-six trace element contents to the HNO₃-HF closed-vessel acid digestion for Ti-rich samples（except pure rutile）.These element concentrations（Sc,Ti,V,Cr,Zr,Hf,Nb,Ta and U）obtained by NH₄HF₂ digestion were consistent with the values measured in a pressed powder pellet by laser ablation-inductively coupled plasma-mass spectrometry（LA-ICP-MS）for the rutile.The insoluble residue was not observed for NH₄HF₂ digestion.Hence,the proposed NH₄HF₂digestion is effective in completely dissolving Ti-rich materials and suitable for the determinations of element contents and isotope ratios.2.One-column separation was established for solution Ti isotope analysis in Ti-rich minerals.The solution Ti isotope analysis is time-consuming and reagent-consuming due to the two/three-columns ion exchange procedures.In this study,one-column separation for Ti isotope analysis was established by LN Spec resin.Herein,Ti isotopic ratio was measured by multi-collector inductively coupled plasma mass spectrometry（MC-ICP-MS）with standard-sample bracketing（SSB）method.Mo element is not affected for accurate Ti isotope analysis due to extremely low Mo contents in Ti-rich minerals,although it is difficult to remove thoroughly Mo element from Ti solution using one-column separation with LN Spec resin.Finally,these samples including three rutiles,two ilmenites,three titanites and three perovskites were determined by SSB method and the double spiked method,respectively,the identical results affirmed the accuracy of our separation procedure.The one-column separation is suitable for the sample with low Mo/Ti ratio,and it has some advantages such as saving time and reducing reagent rather than two/three-columns separation.3.In situ Ti isotope analysis in rutiles with high spatial resolution was developed by femtosecond laser multi-collector inductively coupled plasma mass spectrometry（fs-LA-MC-ICP-MS）.Crystallization of Fe-Ti oxides can induce significant Ti isotope fractionation during magmatic differentiation.As such,rutile,a common Fe-Ti oxide present in igneous,metamorphic and sedimentary rocks,can serve as a tracer of the geological processes.Generally,rutiles in geological samples have tiny or complex structures,but the reliable laser in situ analytical method with high-spatial resolution is lacking for Ti isotope analysis.In this study,a new method with high spatial resolution was developed to determine Ti stable isotopes in rutiles using fs-LA-MC-ICP-MS by optimizing cone combinations,laser ablation frequencies and plasma conditions.A high sensitivity cone combination（standard sample cone+X skimmer cone）,a signal smoothing device and a wet plasma condition were conducted simultaneously,the spatial resolution at horizontal-and vertical-axis can be promoted to 10μm and≈4μm,respectively,meanwhile ensuring accurateδ⁴⁹Ti results within the internal precision（2SE）of 0.10‰.The long-term measurements of a rutile USA75 by fs-LA-MC-ICP-MS showed a good reproducibility（2SD）ofδ⁴⁹Ti（±0.11‰）.Measurements of 9 natural rutile crystals by both two solution methods and fs-LA-MC-ICP-MS provided identicalδ⁴⁹Ti values,confirming the robustness of the fs-LA-MC-ICP-MS.A significant variation ofδ⁴⁹Ti up to≈2.9‰among all rutiles was observed,indicating that Ti isotopes in rutiles could be a useful geochemical tracer.Four rutile crystals（i.e.,USA75,BRA12,SCO2and BRA6）with the good reproducibility(<±0.13‰inδ⁴⁹Ti)can serve as reference materials for in situ Ti isotopic ratios measurement.4.High precision Ti isotope analysis in Ti-rich minerals was developed by nanosecond laser（ns-LA）MC-ICP-MS,and matrix effects among different Ti-rich materials were evaluated.Nanosecond and femtosecond laser are commonly used for in situ analysis,but the reported reproducibility obtained by ns-LA-MC-ICP-MS is poor（±0.5‰,2SD）for in situ Ti isotope analysis.In this study,high precision Ti isotope analysis was conducted by ns-LA-MC-ICP-MS with SSB method,where matrix-matched references used as the analytical external standard.The internal precision（0.08‰,2SE）and the long-term reproducibility（≈±0.15‰,2SD）obtained from ns-LA-MC-ICP-MS are consistent with the results of fs-LA-MC-ICP-MS.The matrix effect induced by fs-LA-MC-ICP-MS was weaker than ns-LA-MC-ICP-MS.The accurate result of Ti metal was obtained by fs-LA-MC-ICP-MS under wet plasma condition with a rutile as external standard.The matrix effects among Ti-rich minerals were serious even if by fs-LA-MC-ICP-MS under wet plasma condition.Nowadays,the matrix-matched reference material is one of keys for high precision Ti isotope analysis by LA-MC-ICP-MS.Herein,ilmenite GER16 and titanite MAD12 were recommended as potential reference materials for in situ Ti isotope ananlysis.5.High-quality reference materials of ilmenite,titanite and perovskite were provided for Ti isotope microanalysis.Ilmenite,titanite and perovskite are common Ti-rich mineral phases crystallized during magmatic and metamorphic processes.However,in situ Ti isotope analysis is limited due to the lack of reference materials.Here,eight ilmenites,thirteen titanites and seven perovskites were analyzed to obtained Ti isotope compositions using fs-LA-MC-ICP-MS under a wet plasma condition,where a matrix-matched standard-sample-bracketing（SSB）protocol was applied to correct mass bias.The internal precision（2SE）of individual measurements forδ⁴⁹Ti was related to Ti signal intensity,not mineral type.Under the wet plasma setting,the signal-dependent Ti isotope fractionation was not observed at ⁴⁹Ti signal intensity of 0.3 to 3.7 V.Twelve Ti-rich minerals measured by fs-LA-MC-ICP-MS protocol and two independent solution protocols provided consistentδ⁴⁹Ti _OL-Ti values(i.e.,the per mil difference of ⁴⁹Ti/⁴⁷Ti ratio relative to the OL-Ti standard),confirming the accuracy of the proposed fs-LA-MC-ICP-MS.Theδ⁴⁹Ti _OL-Ti values of these 12 Ti-rich minerals range from-0.46‰to±2.12‰,which ensures the suitability as reference materials.Four ilmenites（i.e.,SIN1,CZE2,PAK122 and RUS10）,three titanites（i.e.,PAK8,AUS10,NAM12 and MAD8）,and three perovskites（i.e.,ITA4,RUS5 and RUS2）have relatively homogenous Ti isotope compositions between individual chips with the long-term reproducibilities（2SD）of±0.13‰to±0.17‰onδ⁴⁹Ti _OL-Ti.These materials can serve as potential Ti isotope reference materials to be shared with other labs for in situ Ti isotope analysis.