Japan Geoscience Union Meeting 2016

Presentation information


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

[P-PS12] Formation and evolution of planetary materials in the solar system

Tue. May 24, 2016 10:45 AM - 12:15 PM 104 (1F)

Convener:*Masaaki Miyahara(Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University), Akira Yamaguchi(National Institute of Polar Research), Tomohiro Usui(Department of Earth and Planetary Sciences,Tokyo Institute of Technology), Yoko Kebukawa(Faculty of Engineering, Yokohama National University), Wataru Fujiya(Ibaraki University, College of Science), Yusuke Seto(Graduate School of Science, Kobe University), Shoichi Itoh(Graduate school of Science, Kyoto University), Chair:Tomohiro Usui(Department of Earth and Planetary Sciences,Tokyo Institute of Technology)

11:00 AM - 11:15 AM

[PPS12-08] Os isotope compositions and fractionation of siderophile elements in metal phases from CB chondrites

*Nao Nakanishi1, Tetsuya Yokoyama1, Tomohiro Usui1, Hikaru Iwamori1,2 (1.Department of Earth and Planetary Sciences Tokyo Institute of Technology, Japan, 2.Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, Japan)

Keywords:Chondrite, Metal phase, CB chondrite, Osmium isotopes, Highly siderophile elements, In-situ analysis

Metal is one of the main components of chondritic meteorites and a significant reservoir of Fe along with silicates and sulfides. Metal plays a key role in physicochemical processes that fractionate siderophile elements from lithophile elements in the early solar system, generating variable chemical reservoirs before the onset of planetesimal formation. Highly siderophile elements (HSEs: Re, Os, Ir, Ru, Pt and Pd) have a great affinity for Fe-metal relative to silicate; HSEs are very refractory and exist as gas only at high temperature. Therefore, HSEs in metals in a variety of meteorites can provide an important clue for understanding of high temperature processes in the nebula. Specifically, the 187Re–187Os isotope system yields chronological information regarding the fractionation of HSEs. Numerous studies have conducted comprehensive analyses of HSE abundances in chondritic metals utilizing laser ablation ICP-MS (LA-ICP-MS) [e.g., 1–2]. However, these studies scarcely include in-situ Os isotope data due to analytical difficulties. We have developed a technique for in-situ measurement of Os isotopes in metal grains using a micro-milling system coupled with N-TIMS. Our previous study reported that individual CB metals have 187Os/188Os ratios close to the bulk CI chondrite value with limited variation [3]. This study is a follow up of our previous investigation that places emphasis on spot analyses of HSEs and other siderophile elements in CB metals where 187Os/188Os ratios have been obtained. We utilize fs-LA-ICP-MS for conducting precise HSEs analysis in metal samples. By integrating overall measurements, we discuss the origin of metal grains in different types of CB chondrites.
We examined multiple metal grains in three CB chondrites: Bencubbin (CBa), Gujba (CBa), and Isheyevo (CBb). The details for Os isotope analysis using a micro milling system and N-TIMS are described in [3]. The concentrations of P, S, Cr, Fe, Co, and Ni in analytical spots adjacent to the sampling pits for Os isotope analysis were determined by EPMA (JEOL-JXA-8530F). The concentrations of HSEs in analytical spots adjacent to the sampling pits were analyzed with fs-LA-ICP-MS (IFRIT, Cyber Laser).
Our Re–Os isotope data are mostly plotted on the 4.567 Ga Re–Os reference line. Nearly homogeneous 187Os/188Os ratios in CB metals indicate that fractionation of Re and Os was minuscule during metal formation at ~4.57Ga. Because Re and Os are ultra-refractory elements with similar 50% condensation temperatures (Re: 1821 K, Os: 1812 K), the limited Re/Os variation may suggest simultaneous condensation of Re and Os from the nebular gas during metal formation. The limited Os isotopic variation suggests that the redistribution of Re and Os during metal formation associated with planetary collision was not significant as are the cases of solidification of liquid metal. A positive correlation of Re/Os ratios calculated from the 187Os/188Os ratios and Os/Ir for CBa metal grains suggests that the condensation of CBa metal grains occurred at an equilibrium condition in a cooling gas until the condensation temperature of Ir (~1600 K). Unlike ultra-refractory HSEs, Pd/Fe and Ni/Fe ratios in CBa and CBb metals exhibit a strong positive correlation. This positive correlation cannot be explained by nebular condensation but condensation in extremely high gas pressure (107 x solar nebula).
References: [1] Jacquet E. et al. (2013) Meteorit. Planet. Sci. 48, 1981–1999. [2] Campbell A. et al. (2002) GCA 66, 647–660. [3] Nakanishi N. et al. (2013) LPSC, abstract #2407.