TY - JOUR
T1 - Thermal history of the shock-melted Antarctic LL-chondrites from the Yamato-79 collection
AU - Okano, Osamu
AU - Nakamura, Noboru
AU - Nagao, Keisuke
N1 - Funding Information:
Acknowledgments-ProfessoHr. Honma’sa ssistancwe ith the mass spectrometaern dm icroprobea nalyzear t OkayamaU niversityis very mucha ppreciatedW. e areg ratefutlo Dr. K. Yanai for providingt he meteoritsep ecimenasn df or a loanof thet hins ectionsD, r. K. Misawa for his part in our REE analysisa, nd Drs. H. Kagamia nd H. Sano for theirm aSSs pectrometrayn dEPMA analysesW. e thankP rof. H. Nagasawaf or helpful suggestionsre gardingd iffusion provided throughoutth e courseo f this work. We alsot hankD rs. H. Palme, E. Scott,G . Kurat,a nda n anonymourse viewefro r extremelhy elpful reviewso f the manuscriptT. his researchw ass upportedi,n part,b y Grantsi n Aid for ScientificR esearchfr omt heM inistryo f Education, Sciencea ndC ultureJ,a pan:5 954036t7o N. Nakamuraan d5 9390007 to K. Nakazawa.
PY - 1990/12
Y1 - 1990/12
N2 - The Sr and rare gas isotopic compositions and abundances of lithophile trace elements (K, Rb, Sr, Ba, and REEs) were determined for a series of shock-melted Yamato-79 LL-chondrites to investigate their late thermal history and the chemical features of shock processes. All meteorites show similarities in shock ages (~ 1.2 Ga) as confirmed by Rb-Sr internal isochron and K-Ar dating, rare gas compositions as well as cosmic-ray exposure ages (~28 Ma), petrographie textures, and sampling sites in Antarctica. These results indicate that all of these meteorites are part of the same fall. The 1.2 Ga shock event caused a severe (partial to total) melting followed by recrystallization of olivine and clinopyroxene, vesiculation, shock-induced alkali homogenization, and local isotopic equilibration or perturbation of the Rb-Sr system. The degrees of shock effects are variable from specimen to specimen and from portion to portion, even in a single specimen. Model calculations of Fe diffusion in olivine suggest that hot and cold materials were in close contact in the impact ejecta sheets of the parent body. From these model calculations and the evidence provided by cosmogenic rare gas compositions, it is concluded that an impact melt ejecta pile composed of hot and cold brecciated materials had formed at depth (> 2 m, shielded from cosmic rays) in an impact crater by the 1.2 Ga event. The parent body was fragmented to meter-size stones by an additional collision at ~28 Ma resulting in the formation of the parent material of the Yamato-79 shocked chondrites.
AB - The Sr and rare gas isotopic compositions and abundances of lithophile trace elements (K, Rb, Sr, Ba, and REEs) were determined for a series of shock-melted Yamato-79 LL-chondrites to investigate their late thermal history and the chemical features of shock processes. All meteorites show similarities in shock ages (~ 1.2 Ga) as confirmed by Rb-Sr internal isochron and K-Ar dating, rare gas compositions as well as cosmic-ray exposure ages (~28 Ma), petrographie textures, and sampling sites in Antarctica. These results indicate that all of these meteorites are part of the same fall. The 1.2 Ga shock event caused a severe (partial to total) melting followed by recrystallization of olivine and clinopyroxene, vesiculation, shock-induced alkali homogenization, and local isotopic equilibration or perturbation of the Rb-Sr system. The degrees of shock effects are variable from specimen to specimen and from portion to portion, even in a single specimen. Model calculations of Fe diffusion in olivine suggest that hot and cold materials were in close contact in the impact ejecta sheets of the parent body. From these model calculations and the evidence provided by cosmogenic rare gas compositions, it is concluded that an impact melt ejecta pile composed of hot and cold brecciated materials had formed at depth (> 2 m, shielded from cosmic rays) in an impact crater by the 1.2 Ga event. The parent body was fragmented to meter-size stones by an additional collision at ~28 Ma resulting in the formation of the parent material of the Yamato-79 shocked chondrites.
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U2 - 10.1016/0016-7037(90)90301-Z
DO - 10.1016/0016-7037(90)90301-Z
M3 - Article
AN - SCOPUS:0025600591
VL - 54
SP - 3509
EP - 3523
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
IS - 12
ER -