238U 230Th 226Ra disequilibria in volcanics: A new insight into melting conditions

Abstract

Using new mass spectrometry techniques developed for the analysis of Ra isotopes, we present 238U 230Th 226Ra disequilibria data from a variety of volcanic settings, and we compare them with previously published data. Two correlations are observed with alkali volcanic data, one between [ 230 Th 238 U ] and [ 230 Th 226 Ra ] and another between the intensity of the disequilibria and the buoyancy flux of the underlying plume. These two correlations prove that partial melting is the major cause of UThRa fractionations in this volcanic context. The 238U 230Th 226Ra disequilibria then place new constraints on some parameters of the classical melting models (batch melting and dynamic melting). We show, nevertheless, that these estimates remain very speculative. The comparison of 238U 230Th 226Ra disequilibria in alkali volcanics, carbonatites and subduction zones shows a clear parallel between the disequilibria value and the type of volcanic context. Such a parallel reflects the diversity of the conditions of magma generation, and shows that the 238U 230Th 226Ra disequilibria systematics are very dependent on the chemical composition of liquids produced during magmatic processes. A systematic difference is observed between disequilibria in MORB and in alkali volcanics, which could indicate that the melting processes in these two volcanic contexts are very different.