TH, U AND OTHER TRACE-ELEMENTS IN CARBONACEOUS CHONDRITES - IMPLICATIONS FOR THE TERRESTRIAL AND SOLAR-SYSTEM TH/U RATIOS

We have analyzed Th, U and several other trace elements in eleven carbonaceous chondrites (including the two CI chondrites Orgueil and Ivuna) by mass spectrometric isotope dilution techniques. In contrast to concentrations of Th and other refractory trace elements, U concentrations are highly variable, both among and within different meteorites. Th/U ratios range from about 1 to 4 and correlate inversely with U concentrations. Surprisingly, CI chondrites display by far the largest variation in Th/U. It is shown that the negative correlation between Th/U and U cannot be explained by terrestrial contamination or systematic fractionation within the solar nebula; rather it reflects low-temperature mobilization and redistribution of U on the chondrite parent bodies or within the meteorites. The idea of alteration-induced Th/U variations in chondrites is supported by the covariation of U in Orgueil with other elements which are known to be mobile under aqueous conditions (e.g., Sb, Cs, Ba) and the fact that the analytical data of Th, U lie on binary mixing lines. Our approach to estimating the solar-system Th/U ratio is based on analyses of carbonaceous chondrites of all types, the combination of data for Th, U with those for other trace elements and on published Pb isotopes in carbonaceous chondrites. The resulting value of Th/U = 3.9 +/- 0.2 (weight ratio) is higher than previous estimates. The corresponding CI chondritic abundances are Th = 28.9 +/- 1.5 ppb and U = 7.4 +/- 0.5 ppb. This translates into solar-system abundances of Th = 0.0329 and U = 0.0082 (atomic abundances relative to 10(6) Si atoms). Assuming that refractory lithophile elements in the primitive mantle have chondritic relative abundances, but are enriched by a factor of 2.5 relative to CI, we obtain Th = 72.2 ppb and U = 18.5 ppb as primitive mantle abundances. If the early Earth's mantle possessed a Th/U weight ratio of greater than 4.1 as suggested by the lead isotopes of Archean rocks, this would reflect very early terrestrial Th-U fractionation.