EVOLUTION AND SHOCK HISTORY OF L AND H CHONDRITE METEORITES: MOBILE TRACE ELEMENTS AND ARGON-40/ARGON-39 AGES

TED JAY HUSTON, Purdue University

Abstract

Ages by the ('40)Ar/('39)Ar technique frequently indicate disturbances in temperature release patterns associated with loss of radiogenic ('40)Ar during shock episodes. Previous studies demonstrated that some trace elements are more labile than ('40)Ar in heated chondritic meteorites. To determine the relationship between radiogenic age and trace element contents, we studied 22 L4-6 and 12 H4-6 chondrites whose ('40)Ar/('39)Ar release patterns had already been established. We determined Co, Au, Se, Ga, Rb, Cs, Te, Bi, Ag, In, Tl, Zn and Cd by radiochemical neutron activation analysis. The elements are listed in order of increasing mobility at 1000(DEGREES)C. The data for 16 L4-6 chondrites, reported here, are combined with data from L chondrites showing other evidence of shock-loading. Two populations were adopted: a mildly shocked one, consisting of chondrites with undisturbed, long age plateaus and those in shock facies a-c; and, a strongly shocked one, consisting of chondrites with disturbed ages and those in shock facies d-f. Within each population, the mean contents of the elements do not vary systematically with petrologic grade. This suggests that petrographic features are not a result of primary condensation or of open-system metamorphism. However, the mean contents of every element studied, except Se, are lower in the strongly shocked population relative to the mildly shock one, (many at statistically significant levels, i.e. (GREATERTHEQ) 90%). This indicates the predominance of shock in causing the loss of mobile elements in L chondrites. The mean contents of the more mobile elements in H chondrites with disturbed ages are generally lower than in those with undisturbed ages. This is consistent with shock-induced loss of these elements. Interelement correlations and trends for the L and H chondrite populations indicate a dominant thermal episode consistent with shock heating. Thus, trace element contents in meteorites with evidence of shock cannot be used to resolve primary conditions of formation or secondary effects of thermal metamorphism. Two samples from apparently different chemical and petrographic regions of Rose City, a severely shocked H chondrite, were analyzed for the same 13 trace elements, as noted above. Results indicate a partial metal-silicate fractionation within a closed system. Trace element data for the LL chondrite Y74160, an antarctic find, suggest an open-system metal-silicate fractionation. This is consistent with recrystallization to petrologic grade 7.

Degree

Ph.D.

Subject Area

Chemistry

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