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RAPID MULTI-ELEMENTS AND U-PB AGE DETERMINATIONS OF ZIRCON CRYSTAL USING UV LASER ABLATION ICP-MS, AND PRELIMINARY RESULTS OF REE ANALYSIS FOR SINGLE ZIRCON CRYSTALS FROM THE PATAGONIAN ADAKITES

Y. Orihashi 1), A. Motoki 2), D. Hirata 3), T. Hirata 3), M. Haller 4), T. Ohta3), M. Schilling 5), N. Kawano 3), Y. Watanabe 1), H. Yoshida 1), and R. Anma 6)

1)Tokyo University 2) Rio de Janeiro State University 3)Tokyo Institute Technology 4) Centro Nacional Patagonico-UNPSJB 5) Chile University 6) Tsukuba University

VI Congresso Geologico Argentino, CD-ROM, \app\pdf\438.



Zircon (ZrSiO4) is characterized to be included in variable rocks, i.e., igneous rocks from mafic to felsic rocks, sedimentary and metamorphic rocks, and remain stable throughout weathering, alteration, high temperature metamorphism and anatexis processes. Thus, zircon has been extensively used in U-Pb dating due to establishment of geological ages since 1970s, and, in this case, measured mainly by thermal ionization mass spectrometry (TIMS) after chemical dissolution and dissociation. Since 1980s, new analytical techniques by secondary ion mass spectrometry (SIMS) and laser ablation ICP mass spectrometry (LA-ICP-MS) have enabled us to determine in situ trace element concentrations of zircon (as well as in situ U-Pb dating of zircon.

Several studies point out that rare earth element (REE) chemistry of zircon by SIMS and LA-ICP-MS is a powerful tool to reveal detailed petrogenesis of an evolved igneous rock and source origin of sedimentary and metamorphic rocks. However, since zircon has very low REE diffusion rate and high closure temperature (about 1,000C), zircon grains derived from an igneous rock, particularly plutonic rocks like a granite, have frequently contaminated a recycled zircon formed in the different stage and/or source. Owing to discrimination of recycled and original zircons, therefore, determinations of both trace element concentrations and U-Pb dating on the same spot are needed. Quadrapole-type mass spectrometer (QMS) in LA-ICP-MS has a potential to perform the above combination analysis, in addition, high precise U-Pb dating of zircon by LA-ICP-QMS is recently reported (There is, however, no report of the combination analysis using LA-ICP-QMS, yet.

In this presentation, we demonstrate the above analytical method and show the preliminary results of trace elements concentrations for the single zircon crystals from adakites in Cerro Pampa and Puesto Nuevo in the Patagonian province.


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