Isotopic anomalies in high Z elements: Uranium

Uranium in terrestrial volcanic ejecta from mantle-related sources has been analyzed mass spectrometrically. The objective was to seek supporting evidence for or refutation isotopic variations reported by Fried et al. (1985) for some such samples. The possibility that terrestrial U is not of constant isotopic composition is extraordinary. If true, mechanisms for creating the variation must be sought and the lack of homogenization within the earth addressed. Samples of 100 grams or more were processed in order to minimize reagent and environmental (laboratory) blank interference and to permit isolation of large amounts (several to tens of .mu.g) of U for the mass spectrometer (MS) measurements, which utilizes aliquots of (approximately)1 .mu.g. Aliquants from four volcanic samples gave data which indicate enrichments of ₂₃₅U ranging from 0.2% to 5.9% in the 235/238 ratio relative normal uranium ratios. These relative enrichments are consistent with, and in some cases, higher than the 0.18% enrichment reported by Fried et al. (1985) for two volcanic lava samples. However, we were not able to reproduce their results on the Kilauea lava for which they report 0.18% ²³⁵U enrichment. The relative error in our MS ratios is 0.05% -- 0.07%. 1 tab.