JaLCDOI 10.18926/ESR/13957
フルテキストURL ESR_1_27.pdf
著者 岡野 修| 西谷 七代| 加々美 寛雄|
抄録 Rb-Sr isotopic study as well as petrologic investigations have been done on sixteen individul chondrule samples separated from the Allegan (H5) chondrite. The Rb-Sr chondrule isochron does not define a very precise age (4.42±0.17 Ga) and 87Sr/86Sr initial ratio (0.6997±0.0014) due to the very small variation in Rb/Sr ratio for the chondrules, whereas the model ages give a much more speciffic value (4.42±0.02 Ga). These ages may probably be indicative of the time of primary process (es) (e.g. chondrule formation) rather than secondary (metamorphic reheating) or tertiary (shock heating) processes in and/or on the H-chodrite parent body (or bodies). Chemical compositions of oliveines (Fa=17.5mol%, mean deviation (MD)=1.3%) and orthopyoxenes (Fs=15.4mol%, MD=3.2%) are moderately homogenized among the chondrules, while certains chemically uneguilibrated features are also distinctly preserved in this chondrite. Homogeneity of the mineral compositions, therefore, could not have been caused by thermal metamorphism but were probably inherited from processes prior to aggregation of the components. The overall implications of the chondrule isochron and model ages in relation to the petrological features and in comparison with Rb-Sr data of other chondrites of different chemical groups and petrologic types are discussed.
キーワード chondrule age Rb-Sr isochron Allegan H- chondrite meteorite metamorphism olivine pyroxene
出版物タイトル Okayama University Earth Science Report
発行日 1994-09-20
1巻
1号
開始ページ 27
終了ページ 36
ISSN 1340-7414
言語 English
著作権者 岡山大学地球科学研究報告編集委員会
論文のバージョン publisher
NAID 120002310261
JaLCDOI 10.18926/ESR/13949
タイトル(別表記) Rb-Sr age of K-rich LL-chondrite Yamato-74442
フルテキストURL ESR_2_91.pdf
著者 西谷 七代| 岡野 修| 加々美 寛雄| 中村 昇|
抄録 Yamato-74442, a brecciated LL-group chondrite, consists of fragments with igeous texture, chondrules, mineral fragments, and host. The fragments involve K-rich fragments as well as normal ones. Typically, both fragments consist mainly of euhedral porphyritic olivines and a few orthopyroxenes in a glassy grundmass. EPMA analyses for these fragments indicate that olivines and orthoptroxenes have a range of Fe/(Fe+Mg) ratio of 27.19~31.33 and 20.10~26.91 mol%, respectively. There is no significant difference in these rations between K-rich (Na2O/K2O<1) and normal framents. The K2O contents of groundmass for K-rich fragments range from 0.51 to 4.89 wt% and that of the normal fragments is 0.38~090 wt%. K2O contents of groundmass vary from fragment to fragment but are rather homegeneous in a single fragment. Rb-Sr isotopic determination on eighteen K-rich fragments (1.5 to 9.5mg in weight) gave an internal isochron age of 4.514±0.022 (2σ) Ga (λ87Rb=1.42×10-11y-1) and initial 87Sr/86Sr ratio of 0.7016±0.0024. This age is somewhat old compared with a whole rock isochron age of LL chondrites (4.493±0.18 Ga; Minster and Allegre, 1981). From a comoarison of the present result with others so far reported, it is inferred that the alkali differentiation as observed in Yamato-74442 probably occurred prior to or during a process of accumulation from LL chondrite parent material in the early solar nebula. 
キーワード LL-chondrite Rb-Sr age Sr isotope K-rich fragment
出版物タイトル Okayama University Earth Science Report
発行日 1995-09-20
2巻
1号
開始ページ 91
終了ページ 102
ISSN 1340-7414
言語 Japanese
著作権者 岡山大学地球科学研究報告編集委員会
論文のバージョン publisher
NAID 120002310270