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<title lang=en>Extreme radium deficit in the 1957 AD Mugogo lava (Virunga volcanic field, Africa): its bearing on olivine-melilitite genesis</title>
<creator>Condomines, Michel</creator>
<creator>Carpentier, Marion</creator>
<creator>Ongendangenda, Tienge</creator>
<contributor>Géosciences Montpellier ; Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS)</contributor>
<contributor>Risques ; Géosciences Montpellier ; Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS) - Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS)</contributor>
<contributor>Laboratoire Magmas et Volcans (LMV) ; Université Blaise Pascal - Clermont-Ferrand 2 (UBP) - Institut national des sciences de l'Univers (INSU - CNRS) - Université Jean Monnet [Saint-Étienne] (UJM) - Centre National de la Recherche Scientifique (CNRS)</contributor>
<contributor>Université de Kinshasa</contributor>
<description>International audience</description>
<source>ISSN: 0010-7999</source>
<source>EISSN: 1432-0967</source>
<source>Contributions to Mineralogy and Petrology</source>
<publisher>Springer Verlag (Germany)</publisher>
<publisher>Springer Verlag</publisher>
<identifier>hal-01174173</identifier>
<identifier>https://hal.archives-ouvertes.fr/hal-01174173</identifier>
<source>https://hal.archives-ouvertes.fr/hal-01174173</source>
<source>Contributions to Mineralogy and Petrology, Springer Verlag, 2015, 169 (3), pp.29. 〈10.1007/s00410-015-1124-9〉</source>
<identifier>DOI : 10.1007/s00410-015-1124-9</identifier>
<relation>info:eu-repo/semantics/altIdentifier/doi/10.1007/s00410-015-1124-9</relation>
<language>en</language>
<subject lang=en>Radium in magmas</subject>
<subject lang=en>Olivine-melilitite</subject>
<subject lang=en>Metasomatism</subject>
<subject lang=en>Virunga volcanic field</subject>
<subject lang=en>Mugogo volcano</subject>
<subject lang=en>Carbonatite</subject>
<subject lang=en>U-series</subject>
<subject>[SDU.STU.GC] Sciences of the Universe [physics]/Earth Sciences/Geochemistry</subject>
<subject>[SDE.MCG] Environmental Sciences/Global Changes</subject>
<type>info:eu-repo/semantics/article</type>
<type>Journal articles</type>
<description lang=en>We 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.</description>
<date>2015-03</date>
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