The Development Of Flux-free Fusion Sample Preparation Technology And Precise Determination Of Magnesium Isotopic Compositions By Precipitation Technology

Analytical Geochemistry is an important part of modern geochemistry,and it is an interdisciplinary subject of analytical chemistry and earth science.The subject provides elements or isotopic composition information in diverse geological samples for geologist,by using analytical chemistry technology.The development of modern earth science has been promoted by the development of analytical geochemistry.Quadrupole/multi-collector inductively coupled plasma mass spectrometry(Q/MC-ICP-MS)is a powerful tool for determination of major and trace element concentrations and isotopic ratios of geological samples,due to its low detection limit,high sensitivity,wide linearity range,rapid analysis,simultaneous multielement determination,and isotopic analysis.In this study,we present a new method for the dissolution of silicate rocks by flux-free fusion using a boron nitride vessel in preparation for analysis of bulk rock trace elements and Sr-Nd-Hf isotopic analysis;this method is especially applicable to felsic intrusive rocks.The synthesized silicate glass is suitable for in situ analysis of trace elements and lead isotopes by LA-Q/MC-ICP-MS.Herein,we report an ultrafine grinding method to synthesize homogeneous sulfide reference materials in the form of nano-particulate pressed sulfide powder tablets(PSPTs).Finally,this study present a chemical protocol for the separation of Mg that is particularly adapted to diverse igneous rock samples,especially for high-K and low-Mg rocks.This protocol is based on a combination of three procedures:K element is first removed from the samples using precipitation procedure,followed by the separation of Fe and Ca using 2 ml AG50W-X12 cation exchange resin,finally removed Al,Ti,Na,and Fe elements using 0.5 ml AG50W-X12 cation exchange resin.Mg standard solution and rock reference materials have been developed during Mg isotopic analysis.1.Knowledge of the chemical composition of geological samples is a prerequisite for understanding their characteristics,provides important information regarding crustal and mantle evolution and aids the quantification of other geochemical processes.Bulk dissolution inductively coupled plasma-mass spectrometry(ICP-MS)has been widely used for trace elements determination on geological samples environmental,material and archaeological science studies.The sample preparation method is critical for ICP-MS analysis of bulk rock powdered samples.Commonly used preparation techniques include the use of(a)open and closed vessel acid digestionl;(b)microwave dissolution;(c)alkali fusion.Closed vessel high pressure acid digestion offers a much better alternative although the dissolution time is long.Open vessel acid digestion is successful for basaltic samples although inadequate for felsic rocks where accessory minerals such as zircon are often not dissolved completely.Microwave digestion method is not widely applied in the total digestion of geological samples.Whilst basalts can be successfully digested,with felsic rocks there are often undissolved residues containing accessory minerals.This may be due to the short digestion times with a microwave.Alkali fusion does allow complete decomposition of felsic rocks but the high dissolved solids content is not suitable for an ICP-MS.High purity reagent is essential and it can be difficult to determine very low levels of trace elements such as those in ultramafic rocks.In this study,we present a new method for the dissolution of silicate rocks by flux-free fusion using boron nitride vessel in preparation for bulk rock trace elements that is especially applicable to felsic intrusive rocks.The method was evaluated by analyzing five intrusive rocks(GSP-2,GSP-1,G-2,GSR-1 and QLO-1)and five extrusive rocks(AGV-2,BCR-2,BHVO-2,GSR-3 and RGM-2).The optimum condition for the fusion of intrusive rocks was determined to be heating at 1600 ? for 1 min,which is effective to break up strong Si-O bonds.These fused glasses were dissolved using HF/HNO3 acids in closed screw-top Savillex vials on a hotplate without requiring high-pressure PTFE digestion bomb.Zr and Hf were completely recovered from granodiorite GSP-2 reference material in 6 hours at 150 ?,which was 8 times faster than that using conventional high-pressure PTFE digestion bomb at 190 ?.Precision(RSD%,Is)for elemental determinations in nearly 92%of the samples analyzed was better than 4%.Replicate analyses(n=3)of reference materials indicated that most of the results agreed with the reference values,within a relative error of<10%.Additionally,the flux-free fusion kept all the same in solution and was valid for volatile elements.2.Second,we established a rapid acid digestion method in capped Savillex vials without a high-pressure PTFE bomb after the flux-free fusion technique in order to determine the Hf-Sr-Nd isotopic ratios in geological samples using multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS).Conditions of 1600? for 1 min and 1400? for 1 min were employed for fusing intrusive rocks and extrusive rocks,respectively.The rapid acid digestion technique conducted for 6 h was 16-28 times faster and superior in terms of the digestion time compared with the previous high-pressure PTFE bomb method.The results obtained for the procedural blanks for Hf-Sr-Nd in this study were identical to those produced using the high-pressure PTFE bomb method and capped Teflon Savillex vials on a hotplate,but lower than those obtained by flux fusion.Replicate analyses of international certified reference materials(CRMs)indicated that the 176Hf/177Hf,87Sr/86 Sr,and 143 Nd/144Nd isotopic ratios agreed well with previously published data acquired using MC-ICP-MS or TIMS.The external reproducibility(2SD,n = 5)for 10 CRMs was ± 0.000030 for 87Sr/86Sr,± 0.000030 for 143Nd/144Nd,and ± 0.000018 for 176Hf/177Hf.3.Fused glass disks are generally more homogenous than pressed powder pellets and are therefore preferable for laser ablation analysis.A new glass-making method was developed to allow the in situ analysis of trace elements and lead isotopes in rock samples.The apparatus used to prepare homogeneous glass samples from silicate rock powder consisted of a small(450 mm3)boron nitride vessel and a high-temperature furnace.The concentrations of trace elements have been measured by laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)and lead isotopic compositions by femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry(fsLA-MC-ICP-MS).The method was evaluated by analyzing glass disks fused from six geochemical reference materials(spanning the compositional range basalt-andesite-rhyolite).The measured 34 trace element contents match the reference values to within 15%,and the mean Pb isotopic ratios measured by fsLA-MC-ICP-MS agree with those obtained by solution nebulizer MC-ICP-MS within 2s uncertainty intervals.Using seven Faraday cups operating in static mode we were able to obtain 207Pb/206Pb and 208Pb/206Pb results with a factor of 2-3 improved precisions to previous single-collector sector-field ICPMS and LA-MC-ICP-MS.The accuracies of 207Pb/206Pb and 208Pb/206Pb values obtained by fsLA-MC-ICP-MS lie between-0.073%and+0.023%,which is well determined for geological research.Measured 208Pb/204Pb,207Pb/204Pb and 206pb/204Pb isotope ratios for the USGS reference RGM-2(20 ?g g-1 total Pb)and BCR-2(11 ?g g-1 total Pb)agree within 0.029%and 0.055%respectively of the preferred values.For GSR-3(4.27 ?g g-1 total Pb),the accuracies of the 208Pb/204Pb,207 Pb/204Pb and 206pb/204Pb ratio obtained by fsLA-MC-ICP-MS lie between-0.064%and-0.004%with the accepted external precision of<0.15%(1RSD),which are improved over previous LA-MC-ICP-MS data in terms of quantification limits.We also provide precise Pb isotopic data for the reference materials GSR-2,GSR-3,QLO-1,and RGM-2 obtained by solution nebulizer MC-ICP-MS.Our method is simpler,cheaper and faster to determinate trace element and Pb isotopic ratios in geological samples,while retaining similar levels of accuracy and precision.4.Controlling the accuracy and precision of sulfur and lead isotopic compositions in natural sulfide minerals requires the use of well-characterized reference materials with matrices similar to those of the unknown samples being analyzed.However,it is often a challenge to prepare natural or synthetic reference materials with homogeneous isotope compositions on a micro-scale.Herein,we report a novel method to synthesize homogeneous sulfide reference materials in the form of nano-particulate pressed sulfide powder tablets(PSPTs).Non-matrix-matched external calibration was used to obtain accurate and precise lead isotopic ratios,while mass bias was corrected using a standard-sample bracketing method and a thallium standard solution doping process.In situ analysis of lead isotopic ratios of PSPTs using laser ablation multi-collector inductively coupled plasma mass spectrometry(LA-MC-ICP-MS)yielded good external precisions of<0.026%for 207,208Pb/206Pb and<0.057%for 206,207,208aPb/204Pb(2RSD).Meanwhile,the standard deviations of results afforded by LA-MC-ICP-MS were better than 0.41‰ for 34S/32S(2SD).The in situ analytical results of PSPTs were consistent with the values obtained using solution nebulizer MC-ICP-MS,thus demonstrating the reliability and robustness of our analytical protocol.This method can also be used to synthesize different material matrices with homogeneous sulfur and lead isotopes to correct mass bias when using standard-sample bracketing for in situ analysis.5.Finally,this study presented a chemical protocol for the separation of Mg that was particularly adapted to diverse igneous rock samples,especially for high-K and low-Mg rocks.This protocol was based on a combination of three procedures:K element was first removed from the samples using precipitation procedure,followed by the separation of Fe and Ca using an 2 ml AG50W-X12 cation exchange resin,finally removed Al,Ti,Na,and Fe elements using 0.5 ml AG50W-X12 cation exchange resin.Effect of acidity and concentration mismatch,and matrix effect were rigorously evaluated using Nu Plasma II MC-ICP-MS with wet plasma mode.The Mg isotopic ratios of the GSB and Alfa Mg standard solutions are-2.05± 0.05‰(2s,n = 106)and-3.91 ± 0.04 ‰(2s,n = 51),respectively,which are served as in-house Mg standard in State Key Laboratory of Continental Dynamics.The average?26Mg of one mono-elemental Mg standard GSB-1,made by adopting GSB with matrix elements,agreed well with the recommended value after precipitation and column chemistry.The long-term reproducibility,assessed by repeated measurements of Mg standard solution and reference materials,is 0.06‰.The robustness of our method was further assessed by replicated analyses of fifteen rock standards and olivine with large variations in MgO and K2O contents.