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 1:45 PM - 3:15 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:Yoko Kebukawa(Faculty of Engineering, Yokohama National University)

2:15 PM - 2:30 PM

[PPS12-15] Hydrogen isotopic compositions and Raman spectra of organic particles in NWA 801 CR2 chondrite.

*Minako Hashiguchi1, Sachio Kobayashi2, Hisayoshi Yurimoto2 (1.Japan Aerospace Exploration Agency, 2.Hokkaido University)

Keywords:Carbonaceous chondrite, Organic materials, Raman spectrum, Hydrogen isotopic composition

Carbonaceous chondrites contain up to 3-4% organic C, which mostly occur as insoluble organic matter (IOM) [1]. Significant deuterium (D)-rich organic globules have been identified from IOM or matrix [e.g. 2, 3]. It is widely accepted that the D-rich organic globules were formed in extreme cold environment in molecular clouds or outer protosolar nebula and record formation and evolution histories of the extraterrestrial organic materials [e.g. 2, 3].
Our previous study identified D-rich organic globules in matrix of Northwest Africa (NWA) 801 CR2 chondrite and revealed their various H isotopic compositions and morphologies [3]. In the matrix of NWA 801, organic particles without large D-enrichment (less D-rich) also occur [3]. Previous studies reported release of D-enrichments of IOM during aqueous alteration and thermal metamorphism [e.g. 4], thus, H isotopic compositions of organic materials are important tracer to understanding their evolution histories. Furthermore, Raman spectra are also useful parameter for metamorphism grade of organic materials [e.g. 5]. In this study, we investigate H isotopic compositions and Raman spectra of the D-rich and less D-rich organic materials in the NWA 801 to understand their evolution histories.
A polished thin section of NWA 801, covered with a 30 nm thick carbon film [3], was used in this study. In situ quantitative H isotope ratio imaging was performed on matrix regions of the thin section using HokuDai isotope microscope system. The selection criterion for distinguishing D-rich materials that one of their D/H ratios is 2 sigma away from the 3 sigma the distribution of the surrounding matrix. Elemental ratio of H/C for the organic materials were measured using secondary ion intensity of Hand C by SIMS analysis, assuming that average H/C ratio of less D-rich organic materials is corresponding to that of bulk CR2 IOM reported from previous study [4]. Identification and observation of organic materials were performed using FE-SEM-EDS. Raman experiments were performed by Ranishaw Invia Reflex Raman microscope at the Open facility of Hokkaido University. An excitation wavelength is 532 nm (Nd: YVO4) with spot size of ~1 µm. The laser power on the sample was less than ~300 µW.
[Results and Discussion]
Less D-rich organic particles in NWA 801 occur as sub-micron-sized globules with various morphology that can be classified as follows: ring globule, globule aggregate, round globule, and irregular-shaped globule, that are similar to those of D-rich organic globules in NWA 801 [3]. Although molecular structure of IOM is modified and their H/C ratio is decreased by thermal metamorphism and aqueous alteration [e.g. 4], D-rich and less D-rich organic globules in NWA 801 showed similar H/C ratio, mostly <1.5. Our results imply that the less D-rich organic globules in NWA 801 are not attributed to release of D-enrichment from D-rich organic globules by metamorphism and/or alteration.
Raman spectra obtained from D-rich organic globules, less D-rich organic globule, and matrix showed "D-band" (at ~1400 cm-1) and "G-band" (at ~1550 cm-1). D- and G-band feature of the Raman spectra indicate more metamorphosed feature of D-rich organic globules than that of less D-rich globules and matrix, based on previous results of chondritic IOM [5]. Our results suggest that the D-rich organic globules in NWA 801 would have experienced thermal metamorphism individually, probably in the early solar nebula.
[Acknowledgement] We thank Open facility of Hokkaido University for their support of this Raman analysis.
[1] Gilmour (2003) In Meteorites,Comets and Planets p.269. [2] Busemann et al. (2006) Science 312, 727. [3] Hashiguchi et al. (2013) GCA 122, 306. [4] Alexander et al. (2007) GCA 71, 4380. [5] Busemann et al. (2007) MAPS 42, 1387.