LABILE TRACE-ELEMENTS IN CARBONACEOUS CHONDRITES - A SURVEY

We report radiochemical neutron activation analysis data for Co, Au, Ga, Rb, Sb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In (ordered by increasing putative volatility in primary nebular processes) in 42 C2-C6 chondrites, all but three from Antarctica. From these and literature data for 19 additional chondrites, Cl-normalized concentrations of the nine most volatile elements (Ag --> In) are quite constant in most meteorites. Trace element trends in 39 Antarctic and 22 non-Antarctic carbonaceous chondritesare similar. no evidence exists for substantial alteration by weathering of samples in Antarctica, nor do the data reflect modification by open-system, parent body metamorphism at greater-than-or-equal-to 500-degrees-C. Volatile element concentrations and siderophile ratios (Au/Co and Ga/Co) define continua which correlate at statistically significant levels. Carbonaceous chondrites sample not a few, compositionally distinct parents but rather a compositional continuum in which parent materials forming under more oxidizing conditions incorporated lesser complements of volatiles, essentially unfractionated from cosmic composition. This may well reflect the range of formation conditions (temperature, duration, and water/rock ratios) represented by oxygen isotope variations during preterrestrial aqueous alteration of parent materials.