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

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

Aug 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.126...61c&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 126, no. 1-3, p. 61-74

Physics

Geophysics

22

Magma, Melting, Plumes, Radium Isotopes, Thorium Isotopes, Uranium Isotopes, Volcanoes, Volcanology, Chemical Composition, Geodynamics, Geophysics, Mass Spectroscopy, Planetary Evolution, Subduction (Geology)

Scientific paper

Using new mass spectrometry techniques developed for the analysis of Ra isotopes, we present U-238 - Th-230 - Ra-226 disequilibria data from a variety of volcanic settings, and compare them with previously published data. Two correlations are observed with alkali volcanic data, one between (Th-230/U-238) and (Th-230/Ra-226) 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 U-238 - Th-230 - Ra-226 disequilibria then place new constraints on some parameters of the classical melting models (batch melting and dynamic melting). The comparison of U-238 - Th-230 - Ra-226 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 U-238 - Th-230 - Ra-226 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.

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