グローバルCOE地球惑星科学 - 変動地球惑星学の統合教育研究拠点

グローバルＣＯＥ地球惑星科学
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アーメッド エル ゴレシー 教授
バイロイト大学バイエルン地球化学研究所
2012 年 6 月 1 日(金) 15:00 - 16:00
地球科学系研究棟 503 号
宮原 正明 COE 助教
内線#6687/ E-mail:[email protected]
Mineralogical, Chemical and Isotopic Study of the Earliest
Solarcondensates in Unequilibrated Enstatite Chondrites From EL-3
Fragments MS-17 and MS-177 of Almahata Sitta TC3 Asteroid.
Introduction: Origin of primitive EH-3 and EL-3 chondrites is controversially debated: (1) accreted solar condensate fragments with
distinct REE patterns of oldhamite (CaS) and isotopic signatures of source solar reservoirs [1], or (2) produced by shock melting to
“impact melt breccia” of preexisting proto-asteroids [2, 3]. Uncovering the formational mechanisms requires meticulous petrography;
LA-ICPMS of CaS, NanoSIMS study of pertinent mineral constituents, especially the refractory condensates graphite + sinoite
(Si2N2O), their intergrowth relationship and isotopic inventory. We investigated this assemblage, its abundance in matrices and
chondrules in the EL-3 chondrites MS-17 and MS-177 of Almahata Sitta TC3 asteroid. Special care was devoted to the nature of
carbon species: if graphite, any of its diverse types, or poorly graphitized carbon (PGC), their intergrowth relationships with sinoite and
CaS. In MS-17 graphite is confined to the FeNi nodules among chondrules. It occurs as feathery spray or chaplets. In bothe
morphologies idiomorphic oldhamite (CaS) crystals and idiomorphicf sinoite or its fragments are encapsulated between the graphite
crystallites Thus indicating their formation prior to graphite. We analyzed by NanoSIMS CAMECA 50 L the C- and N-isotopic
compositions of petrographically characterized graphite and PGC and N-isotopic compositions of diverse sinoite in MS-17,
respectively. REE abundances in CaS were studied by LA-ICPMS. Both EL-3s contain FeNi nodules in matrices. The FeNi nodules
depict in MS-17 oldhamite encapsulated in a spray of idiomorphic sinoite needles (or stubby prisms in MS-177) and graphite.
Graphite feathers or chaplets enclosing both euhedral CaS and sinoite prisms occur in FeNi nodules in MS-17. Silicate matrix in
MS-177 contains in contrast both idiomorphic graphite books and aggregates of PGC heterogeneously distributed. Texture is
suggestive of repeated condensation events including plastering of FeNi melts on preexisting sinoite in nodules.
Results: CaS in MS-17 depicts the mainstream flat REE pattern of EL-CaS with negative Eu anomaly. δ13C of graphite in MS-17
ranges from -33.5 to -26.5‰, whereas δ13C in MS-177 is much heavier, as it varies from +11.9 to +25.3‰. δ15N in graphite in
MS-17 ranges from +20.1 to +24.7‰. N isotopes in MS-177 are much heavier: δ15N varies between +42.2 to +87.6‰. Although
originated from the same TC3 asteroid Almahata Sitta both MS-17 and MS-177 must have emerged from different nebular sources
with different C- and N-isotopic signatures. N is spatially heterogeneously distributed in graphite manifesting condensation from a
solar source with heterogeneous abundance and isotopic composition. CaS, sinoite and graphite could never have in situ exsolved
from a metal shock induced melt. Abundance of sinoite in the metal nodules is >14 vol. % by far much higher than the maximum
solubility of N in Fe melt (0.34 wt. %).
Discussion: (1) Results indicate repeated sinoite-graphite condensation followed by plastering with metal melt droplets in the solar
source region. (2) Hence texture and isotopic compositions negate impact melting claimed by [2 and 3]: (3) No shock-induced
high-pressure inventory was encountered. (2) Sinoite is idiomorphic crystalline and not amorphized. The mineral is nonresistant to
dynamic shock-wave propagation. It amorphizes upon shock compression (3) PGC negates thermal event following impact melting
and“igneous activity”. (4) C- and N- isotopic integrity of carbon wouldn’t survive impact melting. (5) A condensation sequence
scheme CaS→Si2N2O→graphite in MS-17 nodules was deduced.
主
催 ： 東北大学 グローバルＣＯＥプログラム
『変動地球惑星学の統合教育研究拠点』
拠点リーダー 大谷 栄治
連絡先: 宮城県仙台市青葉区荒巻字青葉
東北大学大学院理学研究科 地学棟 404 号
GＣＯＥ地球惑星科学事務室 苫米地 由布
TEL/FAX 022(795)6668