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【学术报告】Systematic REE isotopic studies for cosmochemistry

2019-12-10 198

报告时间:20191217日(星期二)下午 15:00-16:00

报告地点:天津大学,第16教学楼221报告厅

报告题目:Systematic REE isotopic studies for cosmochemistry

主讲嘉宾:Prof. Hiroshi Hidaka (Department of Earth and Planetary Science, Nagoya University)

嘉宾简介:Hiroshi Hidaka教授,目前任职于日本名古屋大学地球与行星科学系。1992年,毕业于日本东京大学,获得化学博士学位。1992年至1995年,任东京都立大学助理教授;1995年至2001年、2002年至2016年,分别任广岛大学副教授、教授;2016年至今,任名古屋大学教授。Hiroshi Hidaka教授长期从事地球化学和宇宙化学研究,在稀土元素同位素体系及其宇宙化学应用领域取得了众多成果,近5年在Meteoritics & Planetary ScienceGeochimica et Cosmochimica ActaPrecambrian ResearchGondwana Research等期刊发表多篇论文。1993年获得日本地球化学学会青年科学家奖,2008年获得日本放射化学学会奖,2018年获得日本地球化学学会奖。

报告摘要

The data of a series of the elemental abundances of rare earth elements (REE) in planetary materials (REE abundance pattern) have been widely used to understand the geochemical evolution processes of the materials since the pioneering work [1], because REE have several similarities of physicochemical properties due to their electron configurations of the atomic structures. Furthermore, isotopic compositions of some REE are variable, because they include radiogenic components derived from the decay systems of 138La-138Ce (t1/2=1.05×1011 yrs), 146Sm-142Nd (t1/2=6.8×107 yrs), and 147Sm-143Nd (t1/2=1.07×1011 yrs) and nucleogenic components in Sm, Gd, Dy, Er and Yb isotopes produced by neutron capture and spallation reactions caused by cosmic-ray irradiation.  Here in this talk, I would like to introduce my recent work on systematic REE isotopic analyses of meteorites and lunar samples.

Neodymium has two radiogenic isotopes, 142Nd and 143Nd, that are decayed from 146Sm and 147Sm, respectively.  147Sm-143Nd chronometry has been traditionaly applied for the determination of the formation ages of planetary materials.  On the other hand, 146Sm-142Nd chronometry, consisting of a presently extinct radionuclide 146Sm with a half life of 68 Ma, has been recently used to better understand the early differentiation processes of the solar planets [2-4].  In this study, I also performed high-precision isotopic analyses of Nd in eucrites for the development to consider the early evolutional processes of the solar planets from a chronological point of view, and to confirm of isotopic homogeneity of eucritic materials after the early differentiation events. The data set of 147Sm/144Nd-143Nd/144Nd from eight eucrites lie on a single isochron of 147Sm-143Nd isotopic systematic with a slope of 4.56 Ga.  On the other hand, their isotopic deviations of 142Nd show slightly negative to zero values relative to terrestrial standard materials (e142Nd=-0.2 to 0), and no positive values which are observed in typical non-cumulate eucrites having high Sm/Nd elemental ratios. These results are consistent with previous studies [2,3].  

Cerium has Ce has four stable isotopes with mass number 136, 138, 140 and 142.  Of the four isotopes, 138Ce isotope includes radiogenic component decayed from 138La with a half-life of 1.05×1011 years.  The Ce isotopic studies once developed for the application of 138La-138Ce chronometry [5].  138La-138Ce decay system can be one of chronometers for understanding the formation and the evolution processes of solar planetary materials. Since 138Ce isotopic excesses of eucrites correlate with their La/Ce elemental ratios, these excesses are identified to be the decay product from 138La.  However, the La/Ce elemental ratios of eucrites and diogenites show in a narrow range (138La/142Ce= 0.00317 to 0.00322), and it is difficult to make their whole rock isochron only from our data.  In this study, our data are compared with previous studies to confirm the consistency between our results and previous results [6].

Besides the chronological studies based on the decay systems of La-Ce and Sm-Nd, Sm and Gd isotopic measurements were also taken in the same samples to understand the cosmic-ray exposure histories of individual samples, because significant isotopic shifts of 149Sm-150Sm and 157Gd-158Gd are expected from the neutron capture reactions by 149Sm(n,g)150Sm and 157Gd(n,g)158Gd, respectively, in association with cosmic-ray irradiation. In particular, 149Sm and 157Gd are very sensitive to thermal neutrons. Since 149Sm, 155Gd and 157Gd have very large thermal neutron capture cross sections, their isotopic variations induced from the neutron capture reactions of 149Sm(n,g)150Sm, 155Gd(n,g)156Gd and 157Gd(n,g)158Gd can be useful indicators to understand thermalized degree of the arising neutrons [7,8].  On the other hand, 161Dy, 164Dy, 167Er and 168Yb are sensitively react with epithermal neutrons, because they have significant resonance integrals in the energy range above thermal energies (E>0.1 eV).  Recent isotopic studies suggest that the neutron energy spectrum on the lunar surface is richer in high-energy region than that proposed by previous study [9,10].  Isotopic analyses of Sm, Gd, Dy, Er and Yb in a series of lunar surface materials provide a useful informaation to reconstruct a neutron energy spectrum on the lunar surface. 

 

References

[1] Masuda, A. and Y. Matsui, Geochim. Cosmochim. Acta, 30, 239-250, 1966.

[2] Boyet, M. and R.W. Carlson, Science, 309, 576-581, 2005.

[3] Boyet M., Carlson R.W. and M. Horan, Earth Planet. Sci. Lett., 291, 172-181, 2010.

[4] Bouvier, A., Blichert-Toft J., Boyet M. and F. Albarède, Meteoritics & Planetary Science, 50, 1896-1911, 2015.

[5] Tanaka, T. and A. Masuda, Nature, 300, 515–518, 1982.

[6] Makishima, A. and A. Masuda, Chem. Geol., 106, 197-205, 1993.

[7] Russ P.G., Burnett, D.S. and G.J. Wasserburg, Earth Planet. Sci. Lett., 15, 172-186, 1972.

[8] Hidaka H., Ebihara M., and S, Yoneda, Meteoritics & Planetary Science, 35, 581-589, 2000.

[9] Albalat, E., Telouk, P. and F. Albarède, Earth Planet. Sci. Lett., 429, 147-156, 2012.

[10] Kruijer T.S., Sprung, P., Kleine, T., Leya, L. and R, Wieler, Meteoritics & Planetary Science, 48, 2597-2607, 2013.