TH isotope and U‐series disequilibria in some alkali basalts

The activities of 238 U‐series nuclides and 232 Th have been measured by alpha spectrometry for nine alkali basalts from Heard island, Mt. Cameroon, Nyamuragira, and Nyiragongo. Large 226 Ra enrichments are inferred to be absent from all, suggesting the absence of subsolidus fluids before magma genesis. Th isotope ratios are consistent with deep metasomatism forming carbonates beneath Nyiragongo, versus shallower metasomatism forming amphibole beneath Nyamuragira. In contrast, the Th/U ratio in the Nyamuragira source has increased with time, which is used to confirm that the Th/U ratio of the mantle is about 4.2.