High-precision Measurement Of Mg Isotope Compositions And Their Application For Study Of Extraterrestrial Materials

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.