演題概要

1P-21（1A-O1-1530）

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天体衝突により形成した溶融脈を含むNWA 722隕石の岩石学的研究と希ガス同位体分析

(¹東大院総合文化・ ²東大先端研・ ³早大院創造理工)
^o山本蒼邦¹・ 日比谷由紀²・ 角野浩史²・ Fagan, Timothy³

Northwest Africa 722 (NWA 722) is a meteorite classified as an ordinary chondrite and characterized by an impact melt vein and a host domain with relict chondrules and matrix. In this study, mineral textures, major element compositions, and noble gas compositions of NWA 722 (classified as L-chondrite) are analyzed and compared with three other ordinary chondrites, Valera (L5), Aba Panu (L3) and Bjurbole (L/LL4). The main goals of the project on NWA 722 are to determine: (1) the relative timing of impact vein formation vs. parent body metamorphism; (2) noble gas characteristics of the impact vein vs. host domain.
EPMA analyses show that olivine grains from the impact melt vein and host lithology of NWA 722 have similar compositions near Fa₁₈. Fe-Ni-rich metal grains from the host domain have compositions which correspond to either kamacite or taenite, whereas Fe-Ni-metal compositions in the melt vein are intermediate between kamacite and taenite. With regard to noble gas, the cosmic ray exposure ages (T₂₁) estimated from cosmogenic ²¹Ne concentrations of the host and vein are 1.45±0.090 Ma and 2.19±0.099 Ma, respectively, showing that the vein contains more cosmogenic ²¹Ne.