Chemical analyses of howardites, eucrites, and diogenites (HED): Antarctic/non-Antarctic comparisons and evolution of the eucrite parent body
The achondritic howardites, eucrites, and diogenites (HED), along with the stony-iron mesosiderites, comprise an association of closely related meteorites which may come from a single parent body. While howardites appear to be regolith surface breccias, comprised of both eucritic and diogenitic materials, eucrites and diogenites are magmatic in origin, the eucrites being the oldest known magmatic rocks (crystallization ages of 4.3 to 4.6 Gy). Evidence indicates that eucrites and diogenites either crystallized from liquids produced by fractional crystallization of a slowly cooling parent magma, or from liquids produced by varying degrees of partial melting of a common source region. Antarctic and non-Antarctic HED meteorites differ in texture, age, and eucrite:howardite:diogenite proportions. We report compositional data for mobile/volatile trace elements (U, Co, Au, Sb, Ga, Se, Rb, Cs, Te, Bi, Ag, In, Tl, Zn, and Cd) and for major and minor elements (Si, Fe, Mg, Al, Ca, Na, K, Ti, Cr, Mn, and Ni) in Antarctic and non-antarctic eucrites, howardites, and diogenites. Our data reveal that contents of 6-7 mobile trace elements differ statistically between Antarctic and non-Antarctic eucrites, the number of differences depending on whether the two populations are made up of only whole-rock samples or of whole-rock samples plus clasts. Of the elements which differ, 5-6 are depleted in the non-Antarctic samples. U-normalized data indicate that mobile element trends in eucrites were established primarily by volatilization, and that Antarctic and non-Antarctic eucrites sample a common parent body (probably asteroid 4 Vesta). It is therefore probable that non-Antarctic eucrites formed at higher equilibrium temperatures, or had lower closure temperatures than their Antarctic counterparts, and thus that they formed at greater depths within the parent body. Antarctic and non-Antarctic howardites and diogenites also appear to differ in mobile element content, but their trends on Allegre plots are complex, and were probably established by a combination of processes. Observed correlations between mobile trace elements and U for eucrites cannot be explained by simple volatilization. Correlations observed between mobile trace elements and major elements for HED meteorites are consistent with the evolution of these meteorites via partial melting and/or fractional crystallization.
Lipschutz, Purdue University.
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