JpGU-AGU Joint Meeting 2017

Presentation information

[JJ] Oral

P (Space and Planetary Sciences) » P-PS Planetary Sciences

[P-PS10] [JJ] Formation and evolution of planetary materials in the solar system

Tue. May 23, 2017 3:30 PM - 5:00 PM 105 (International Conference Hall 1F)

convener:Tomohiro Usui(Earth-Life Science Institute, Tokyo Institute of Technology ), Masaaki Miyahara(Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University), Akira Yamaguchi(National Institute of Polar Research), Yoko Kebukawa(Faculty of Engineering, Yokohama National University), Chairperson:Yoko Kebukawa(Faculty of Engineering, Yokohama National University)

4:15 PM - 4:30 PM

[PPS10-28] High precision Mg isotopic measurement of chondrules from ordinary chondrite meteorite using MC-ICPMS

Akinobu Hayakawa1, Kohei Fukuda1, Tsuyoshi Iizuka1, *Hajime Hiyagon1 (1.Graduate School of Science, The University of Tokyo)

Keywords:Mg isotopes, ICPMS, chondrule, CAI, early solar system

Aluminum-26, a short lived-nuclide with a half life of 0.73 My, has been widely used for discussing relative ages of planetary materials. Based on precise measurements of Al-Mg isotopes in Calucium-aluminum-rich inclusions (CAIs, the oldest solids of the solar system), the initial 26Al/27Al ratio at the birth of the solar system has been determined to be 5.25 x10-5 [1,2]. It has also been argued that chondrules, a major component of chondritic meteorites, formed at 150 to 400 My after CAI formation [3]. Recently, however, precise U-Pb dating suggested that some chondrules might have ages as old as CAIs [4]. Furthermore, recent Al-Mg isotopic measurements of angrites (achondrites) of known U-Pb ages gave a much lower value of 1.33 x10-5 for the initial 26Al/27Al ratio of the solar system [5]. These conflicting data suggest a possibility of heterogeneous distribution of 26Al in the early solar system.
Distribution of 26Al in the early solar system has crucial importance chronologically (i.e., justification of the Al-Mg chronometer) and also in view of an important heat source for understanding evolution of planets (e.g., their differentiation processes). In order to discuss its distribution, precise measurements of Al-Mg isotopes for various components (CAIs, chondrules, etc.) in various types of chondrites. Among them, there have been very few analyses for chondrules, and they are limited to those in carbonaceous chondrites [e.g., 6,7]. In order to better understand spacial distribution of 26Al in the early solar system, we have developed a high precision Mg isotope analysis technique using MC-ICPMS. Using this technique, we have analyzed three CV CAIs to examine if our analysis give a canonical 26Al/27Al ratio consistent with previous works. We have also applied the technique to chondrules in an ordinary (LL) chondrite and compared the results with those of carbonaceous chondrite (CC) chondrules and discussed distribution of 26Al in the early solar system.
The analyzed samples were two CAIs from NWA 3118 (CV3), a CAI from Allende (CV3) and 6 chondrules from NWA 7936 (LL 3.15). We have prepared a Mg isotope standard DSM-3 (pure solution of terrestrial Mg [8]), and all the Mg isotope results (excess 26Mg) were expressed as µ26Mg* (i.e., ppm deviation from the result of DSM-3). The results for two terrestrial standards, BCR-2 and JB-2, gave µ26Mg* values of -5.9 ±11.2 and 2.3 ±20.0, respectively, i.e., good precision and accuracy comparable to those by other laboratories [e.g., [9]]. If we apply a single isochron for the data of three CAIs, we obtain (26Al/27Al)o = (5.08 ±0.84) x10-5 from the slope and µ26Mg* = -25 ±103 ppm from the y-intercept, which are consistent with previous studies [1,2].
Five out of 6 LL chondrules show Al/Mg ratios (0.091-1.04), similar to the solar composition (~0.10). If we assume homogeneous (26Al/27Al)o =5.23 x10-5 and homogeneous stable Mg isotopic composition in the early solar system, chondritic material with solar Al/Mg ratios must have µ26Mg* values of ~0 ppm. However, the present results for the 5 chondrules with solar-like Al/Mg show variable µ26Mg*, and some of them show negative values beyond the error limit (2 σ). The results for LL chondrules tend to show µ26Mg* values even lower than those for CC chondrules. This suggests heterogeneous distribution of 26Al in the early solar system.
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