【作者】 王桂琴；

【导师】 林杨挺； 梁细荣；

【作者基本信息】 中国科学院研究生院（广州地球化学研究所） ， 地球化学， 2006， 博士

【摘要】 本文主要包括三个方面的内容,即（1）Mg同位素组成高精度测定实验方法的建立;（2）陨石Mg同位素组成的测定及其天体化学意义,（3）陨石的全岩化学组成分析和宁强碳质球粒陨石的分类研究。 1、Mg同位素组成高精度测定实验方法的建立 本项研究建立了适用于陨石和其他类似岩石样品Mg同位素组成高精度测定的实验方法,δ²⁶Mg和δ²⁵Mg的分析精度（2σ）分别好于0.14‰和0.08‰。除陨石全岩和熔壳样品用常规的酸溶法溶样外,球粒陨石中的单个集合体（如富Ca,Al包体、球粒、蠕虫状富橄榄石集合体）等微小样品用微量溶样器溶解。样品溶液中Mg与其它元素的分离采用AG50W-X12阳离子树脂。纯化后的Mg溶液用多接受电感耦合等离子质谱仪（MC-ICPMS）精确测定其同位素组成。 （1）Mg的化学分离:通过一系列条件试验,确定了分离Mg的最佳流程和条件。条件试验的内容包括采用HCl,HNO₃和有机溶剂作为淋洗液的实验和对比,以及淋洗液的酸浓度及树脂量和树脂高度对Mg的提纯的影响等。试验过程中连续分段接取淋出液（2-5ml）,并用ICP-MS测定Mg和其它干扰元素的含量。最后对确定的流程进行重复性实验,结果证实具有好的重现性,并且Mg的回收率>98%。 （2）Mg同位素的MC-ICPMS测定:通过对国际标样SRM980、国家标样GSB和实验室标样IGGCAS三种标准的测定,确定仪器的Mg同位素分馏系数β=0.5177,接近同位素平衡分馏的理论值。对实验室高纯水,硝酸中的Mg同位素干扰进行了测定,其对Mg同位素比值的影响约为0.1‰,并通过扣除本底和标准-样品-标准夹层测量技术,将其影响减少到可忽略程度。Mg分离流程对Ni的分离效果不理想,为此采用不同比例的Mg/Ni混合溶液进行模拟实验,MC-ICPMS测定表明Mg/Ni比<1:2时,溶液中的Ni不会对δ²⁶Mg和δ²⁵Mg测定值产生影响,它们的变化小于长期外部精度0.14‰和0.08‰。此外,研究了标准溶液与样品溶液的Mg浓度差异对Mg同位素比值测定可能产生的影响。分析结果表明,如果标准溶液中Mg的浓度为1ppm,则样品溶液中Mg的浓度控制在0.3-3ppm的范围应为最佳。 通过对相同样品的化学分离重复实验,以及Mg溶液的仪器重复测定,证明更多还原

【Abstract】 This dissertation mainly includes three parts as follows （1） method of high-precision measurement of Mg isotope compositions;（2） determination of Mg isotopes of the meteorites and their cosmochemical significance;（3） Analysis of bulk chemical composition and the classification of the Ningqiang carbonaceous chondrite.1. Establishing experiment method for high-precision measurement of Mg isotope compositions.This study develops a new method for high-precision measurement of Mg isotope, which is suitable for extraterrestrial materials and earth rocks with similar composition. Analyzing precisions of δ²⁶Mg and δ²⁵Mg are better than 0.14%o and 0.08%., respectively. A normal acid-dissolving methods is applied to dissolve bulk samples of the meteorites and the crust samples in the meteorites, meanwhile a mini-dissolution device is used to dissolve samples with small volume such as, Ca-, Al-rich inclusions （CAIs）, chondrules and amoeboid olivine aggregates （AOAs）. Mg of sample solution is chemically separated from other elements via AG50W-X12 resin, and then the purified Mg solutions are accurately measured by MC-ICPMS for their Mg isotope composition.（1） Purification of element Mg: The best procedure for Mg purification has been determined by a series experiments. These experiments included to have used HCl, HNO₃ with different acidity and organic solution as eluents, as well as adjusted different resin amount and height in columns, investigating for them how to effect Mg purification. 2-5 ml eluate was collected one by one, then contents of Mg and interfering elements （e.g., Ni, Cr） in each eluate were analyzed by ICP-MS. Finally, the best procedure of Mg separation was determined. According to the procedure, multiple repeated experiments showed rather goodrepeatability, and the recovery of Mg is better than 98%.（2） High-precision measurement of Mg isotopes by MC-ICPMS: Three standards, i.e., SRM980 （NIST）, IGCAS and GSB are chosen and analyzed in order to determine the Mg isotope fractionation factor （ 3 ） during MC-ICPMS instrument measurement. Present study obtained 3 value of 0.5177, which is close to the theoretic value of Mg isotopic equilibrium. Mg-isotope ratios of air, ultra-pure water, and 0.3M HNO3 acid were also analyzed, the results showed them no important interference for Mg isotope measurement of a sample solution. Contributions of the interference from such as ultra-pure water were only less than 0.1 %o when 24Mg/24Mg and 26Mg/24Mg ratios were measured. Adopting auto-subtraction blank and bracketing technique can eliminate these interferences. On the other hand, present Mg- purified procedure can not separate Mg from Ni well, therefore a simulating experiment was carried out in order to evaluate whether different Mg/Ni ratios may result in variation of Mg isotope composition measured by MC-ICPMS. The simulating results revealed that measured 26Mg and 25Mg values were not changed when Ni/ Mg ratio in solution was less than 2:1. Furthermore, our experiment results showed that 0.3-3 ppm concentration range for Mg solution of a sample would be best to fit lppm Mg standard solution.In summary, through many repeated-experiments of Mg chemical separation and MC-ICPMS measurement for a same sample, the measured results in present study are rather stable. Reproducibility of 826Mg and 825Mg values is better than 0.14%o and 0.08%o, respectively.2. Mg-isotope compositions of the meteorites and cosmochemistry significance.（1） Mg isotope compositions of different chemistry group （e.g., carbonaceous chondrite, Allende, NQ, ordinary chondrite, JL;Mars meteorite, and GRV 99027, et al.） were measured. These meteorites plot a small field, the values of 526Mg and 525Mg are （0.06±0.03）%o and （0.01±0.02）%o, （average, ±2ct）, hinting that Mg isotope compositions of the supernova material in the Solar System are very uniform.（2） the feldspar ingredient in the Mars meteorite GRV 99027 was analyzed, its Mg isotope ratios are 5 26MgDSM3 = （—0.21+0.02）%0 and 8 25MgDSM = （—O.15±O.O2）%o, （±2a）, respectively. Relative to bulk rock, the feldspar ingredient shows more enriched light-Mgisotope, indicating that mass fractionation of Mg isotopes occurred during igneous crystallization fractionation on the Mars.（3） The Al and Mg isotope compositions of CAIs and AOAs in Allende and Ningqiang meteorite were measured. Excess 26Mg from in situ decay of the short-lived 26A1 nuclide （0.74 Myr half life） has been recognized. Both CAIs and AOAs have a similar initial 26A1/27A1 ratio （5.54xl0⁵ and 5.42xlO’5）. Allende chondrules have unclear excess 26Mg, upper limit of their initial 26A1/27A1 ratios is 4.84xlO’5. These data probably imply that the CAIs, AOAs and chondrule were contemporaneously generated. On the other hand, it should be noticed that enrichment of heavy Mg isotope has not been found in CAIs and AOA, even some of the CAIs, AOAs and chondrules exhibit more enriched light Mg-isotope relative to bulk meteorite. Therefore the CAIs have not experienced high temperature evaporation during formation.（4） Some of the chondrules in present study show weak enrichment of heavy Mg isotope, hinting that these chondrules took place differentiation with different degree when they underwent high temperature melting. Meanwhile, crusts of the chondrites show no mass fractionation of Mg isotope, indicating that forming condition of crusts from the chondrites was distinct from that of the chondrules.3. Bulk chemical composition of several meteorites and classification of Ningqiang carbonaceous chondrite.Bulk chemical compositions of Ningqiang, Jilin, and Allende meteorites have been measured. According to new data, we discuss the classification of Ningqiang carbonaceous chondrite, which is a long debated topic. This study compares the REE, lithophile elements and siderophile elements of Ningqiang meteorite with CV and CK group meteorites, classifies the Ningqiang to be ungroup carbonaceous chondrites.更多还原