Chemical studies of differentiated meteorites: I. Labile trace elements in Antarctic and non-Antarctic eucrites

Abstract

We report data for Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U, and Zn determined by RNAA in 25 Antarctic and 9 non-Antarctic eucrites (including the cumulate eucrites Moore Co. and Serra de Magé). Compositional variations for a given element in a population range from <10% for Ga to ~200× for Ag, Au, Bi, In, or Tl. Highly incompatible, refractory elements like U are well known to exceed Cl levels in eucrites because of their igneous origin. Surprisingly, contents of highly labile elements can also be quite high, sometimes at 0.1 × Cl, I.E., levels more like chondritic. However, a carbonaceous chondrite pattern (the most likely admixture possibility) is not apparent since Cl-normalized trace element data for a given eucrite sample do not match the flat trends known for carbonaceous chondrites. The Unormalized contents of most elements indicate a thermal (vapor/solid) process accompanied in the case of lithophiles Rb and Cs by a geochemical fractionation. For these, or Ag, Bi, Cd, or Tl (elements with ionic radii and charges resembling those of incompatible elements), Antarctic and non-Antarctic noncumulate eucrites lie on the same trend line, indicating that all derive from the same parent. The thermal process responsible for the U-normalized trend could involve devolatilization of eucrite parent material or, as we prefer because of other data, condensation of volcanic emanations to differing degrees on preexisting eucrite (and diogenite and howardite) parent rocks. Antarctic eucrites have different—usually higher—contents of labile elements, particularly Tl, than do non-Antarctic eucrites. The difference seems not to be due to the terrestrial history of the samples but, rather, is preterrestrial in origin. By our preferred scenario, Antarctic eucrites received a greater average complement of mobile elements, hence, were presumably cooler (and nearer the surface of the eucrite parent body) than were non-Antarctic eucrites.