Trace elements in primitive meteorites—VII Antarctic unequilibrated ordinary chondrites

Abstract

We report RNAA results for Co, Au, Sb, Ga, Rb, Cs, Se, Ag, Te, Zn, In, Bi, Tl and Cd (in increasing order of metamorphic mobility) in 22 Antarctic unequilibrated ordinary chondrites (UOC). This brings to 38 the number of UOC for which data for highly volatile elements are known. For elements of lesser mobility (Co to Se, omitting Cs) overall variability in UOC are low, relative standard deviations (one sigma) being no more than a factor of two. For Ag, Te and Zn, relative standard deviations are 2–4×, while for Cs and the four most volatile elements, the variabilities are 8–110×. Elemental abundances do not vary with chemical type (H, L and LL) nor with UOC subtype (3.0–3.9). Contents of all elements reach levels up to, even exceeding, cosmic and all but Cd and the two alkalis, seem unaffected by post-accretionary processes. Contents of highly volatile elements are consistent with the idea that source regions producing contemporary falls and older Antarctic UOC differed in thermal histories. The presence or absence of carbide magnetite assemblages (CMA) generally accords with high or low Cd contents, respectively. This relationship accords with the prior suggestion that CMA formed by alteration of Fe–Ni metal by C–O–H-containing fluids at temperatures <700 K, generated by thermal metamorphism in parent body interiors. The absence of CMA in most UOC (and OC), may indicate that they were subsequently destroyed as metamorphic intensity increased. The high, often supercosmic, Rb and Cs levels in UOC may result from their high solubility in liquid water signalling their redistribution by C–O–H-containing fluid while in the liquid water field. Because of its uniquely high mobility, Cd could have been enriched by the C–O–H fluids and should have been lost from parent regions during later, higher temperature anhydrous metamorphism at temperatures in the 500–600 °C range.