A 'chondritic' eucrite parent body: inference from trace elements

A total of 33 elements (Ag, Al, Au, Bi, Cd, Ce, Co, Cr, Cs, Eu, Fe, Ge, Hf, Ir, Lu, Na, Ni, Os, Pd, Rb, Re, Sb, Se, Si, Sm, Tb, Te, Tl, U, Yb and Zn) were analyzed by radiochemical and instrumental neutron activation in four eucrites; Juvinas (brecciated), Ibitira (vesicular, unbrecciated) and Moore County and Serra de Mage (cumulate, unbrecciated). When arranged in order of volatility, C1-normalized abundance patterns allow nebular and planetary effects to be distinguished. The stepped lithophile pattern reveals the dominance of nebular processes; in Ibitira, refractory elements (Hf, Lu, Tb, Ce, Sm, Yb, U, Eu) are (13.1 +- 0.7) x C1 chondrites; volatile elements (Rb, Cs, Br, Bi) are (6.0 +- 1.5) x 10-2C1. The depletion of Tl seems inherent to the eucrite parent body and is mirrored in the chalcophile elements by the marked deficit of Te relative to Se; apparently volatiles were accreted as a fractionated C3-like component. Consistent but subtle C1-normalized abundance differences between eucrites (Serra de Mage < Moore County < Juvinas < Ibitira) result from crystal/liquid differentiation; Ibitira approximates the composition of an undifferentiated eucrite magna. The siderophile pattern retains little sign of nebular processes, but reflects planetary metal-silicate partition. The bulk composition of the eucrite parent body closely resembles that of H-chondrites, except for two features; moderately volatile elements (e.g. Na, K, Rb) are very much lower, apparently due to the accretion of more chondrule-like material; the metallic Fe-Ni content is only approximately 13%, even though total iron is very similar. (author)