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Extreme radium deficit in the 1957 AD Mugogo lava (Virunga volcanic field, Africa): its bearing on olivine-melilitite genesis

By Michel Condomines, Marion Carpentier and Tienge Ongendangenda

Abstract

International audienceWe report here U-series and trace element data for an olivine-melilitite lava flow, erupted in 1957 in the Virunga volcanic field and known as the Mugogo eruption. Petrological and geochemical data show that the Mugogo magma represents a primary mantle melt, derived from a low degree of melting of a metasomatised mantle source. It is highly enriched in very incompatible trace elements (e.g. more than 300-fold compared to primitive mantle for Th), with a distinctly lower enrichment in Rb, Ba and Sr. The high Th/U and low (230Th/232Th) ratios (4.4 and 0.750, respectively, with a (230Th/238U) ratio of 1.09) are close to the values found for the Nyamuragira basanites. But the most striking feature is the very low (226Ra/230Th) ratio of 0.48, the lowest ever measured in a mafic volcanic rock. The most probable origin of this Ra deficit is the presence of phlogopite (having a high Ra partition coefficient) in the lithospheric mantle, either as a residual phase during low-degree isobaric melting (in batch-melting or a diffusion-controlled melting models), or in a phlogopite-bearing upper mantle through which the melt migrates and equilibrates (in an equilibrium porous flow model commonly used to describe adiabatic melting). The disequilibrium pattern (230Th) > (238U) > (210Pb) ≫ (226Ra), reversed compared to the pattern observed in the Oldoinyo Lengai natrocarbonatite, suggests that a carbonatite melt might have been involved to explain the Ra deficit. We thus discuss the possibility of an early separation of a Ra-enriched carbonate melt either from the olivine-melilitite melt or from a carbonated mantle source, followed by the production of a low-degree melt of olivine-melilitite composition, but the lack of available experimental, petrological and geochemical evidences makes this process somewhat speculative at present. Further studies of recent (<5 ky) olivine-melilitites are needed to check whether the large Ra deficit is a general characteristic of this type of magma that can be used to constrain models of its formation

Topics: Radium in magmas, Olivine-melilitite, Metasomatism, Virunga volcanic field, Mugogo volcano, Carbonatite, U-series, [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry, [SDE.MCG]Environmental Sciences/Global Changes
Publisher: 'Springer Science and Business Media LLC'
Year: 2015
DOI identifier: 10.1007/s00410-015-1124-9
OAI identifier: oai:HAL:hal-01174173v1
Provided by: HAL-UJM
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