Comprehensive geochemical analyses of small amounts (< 100 mg) of extraterrestrial samples for the analytical competition related to the sample return mission MUSES-C



We have participated in the analytical competition related to the sample-return mission, MUSES-C of the ISAS, to be launched in May 2003 and returned in summer 2007. In this competition, we have determined major and trace element abundances and isotopes (as many as possible), both for two powdered samples provided by the ISAS and for chondrules separated from Allende meteorite as an additional demonstration of our high-spatial resolution analytical capabilities for fragment samples. 100 mg of each of the competition samples, 1C and 2C, were split into two fractions of approximately 30 and 70 mg for the determinations of elemental abundances and the isotope analyses, respectively. The bulk concentrations of 55 elements in each 30 mg sample were analyzed by quadruple-type ICP-MS and sector-type ICP-MS with analytical uncertainties better than 10 percent (1 sigma). B, Pb, Li, Rb, Sr, Sm and Nd were successively separated from the remaining 70 mg of each of the samples, using a novel integrated, multi-ion exchange column chemistry approach, and then the isotopes of these elements were successfully determined by TIMS with analytical errors similar to those for analyses of terrestrial samples. The Sr, Nd, Re and Os isotopic compositions were also analyzed using the remaining aliquots of the sample solutions for trace element analyses by ICP-MS. The results obtained in this study indicate that 1C and 2C samples are probably ordinary chondrite and the Allende meteorite, respectively. However, extreme W, Ta and Nb enrichments in the 1C were probably caused by contamination during sample preparation. Furthermore, both of the samples were extensively contaminated by B from the borosilicate glass container in which the samples were delivered. Five chondrules were separated from Allende meteorite and then sliced into three pieces using a dicing saw. The center slices were used to petrographically determine the mineral phases, compositionally map these phases by SEM-EDX, and obtain quantitative major and trace element compositions (30 elements) by SEM-EDX and ion microprobe. The two outer slices, for which sample masses were less than 1 mg, were chemically treated to obtain the bulk trace elements and isotope compositions of the chondrules. For the chondrule slices, 24 trace elements and Sr and Nd isotopic compositions were determined with extremely small analytical uncertainties by ICP-MS and TIMS (for Sr and Nd, respectively). Based on the analytical experience, including this competition, we believe that all of the analyses carried out could have been completed within 50 days if we could have concentrated on this project alone during that 50-day period. In this report, we describe the analytical techniques employed for the powdered samples and the chondrules, and we present all of the results obtained during the approximately 3-month period of the analytical competition.

資料番号: SA0200006000



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