A STUDY OF THE SHOCK EFFECTS ON THE EVOLUTIONARY HISTORY OF L-CHONDRITE METEORITES AND THE ENSTATITE METEORITES: ORIGIN AND EVOLUTION OF A PARENT BODY

THOMAS MICHAEL WALSH, Purdue University

Abstract

We report neutron activation data for Ag, As, Bi, Cd, Co, Cs, Cu, Ga, In, Rb, Se, Te, Tl and Zn in samples of Abee heated at temperatures of 1000-1400(DEGREES)C in a low-pressure environment (initially (TURN)10('-5) atm H(,2)) and in 9 enstatite achondrites (aubrites) and the silicate portion of the unique stony-iron, Mt. Egerton. These data indicate trace element loss above 1000(DEGREES)C via diffusion-controlled processes. These trace element data and those for aubrites, Mt. Egerton and E4-6 chondrites, and mineralogic and isotopic evidence link all enstatite meteorites to a common parent body. Volatile/mobile elements vary inversely with cobalt content in aubrites and Mt. Egerton but directly in E4-6 chondrites; this is inconsistent with all genetic models positing fractionation of such elements during nebular condensation and accretion. However, the data are consistent with the idea that aubrites and Mt. Egerton reflect fractional crystallization of a magma produced from enstatite chondrite-like parent material (probably E6) and late introduction of chalcophiles and mobile elements transported by FeS-Fe eutectic from an E4-6 region experiencing open-system metamorphism. If, as seems likely, volatile/mobile elements reflect secondary processes, they can only be used to establish conditions within the enstatite parent body and not to estimate temperatures during primary nebular condensation and accretion. We also report neutron activation data for Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Se, Te, Tl and Zn in a suite of L-chondrites with well-established shock history. These trace element data show significant differences (many are statistically significant, i.e. >90% confidence level) in mean contents between lightly shocked and heavily shocked L-chondrites. The mean content for all elements (with the exception of Te) in the lightly shocked population exceed those in the more heavily shocked population. Volatile/mobile elements (Ag, Bi, Cd, In, Tl and Zn) exhibit the most dramatic differences in mean contents between the two populations. Lower siderophile mean contents for Au, Co and Ga in heavily shocked L-chondrites containing melt pockets indicate these elements were lost via mobilization of a late shock formed Fe-Ni-S melt. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of school.) UMI

Degree

Ph.D.

Subject Area

Analytical chemistry|Geochemistry

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