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    Trace element partitioning in silica-undersaturated alkaline magmatic systems

    Authors
    Molendijk, S.
    Namur, O.
    Mason, P.R.D
    Dubacq, B.
    Smets, B.
    Neave, D.A.
    Charlier, B.
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    Discipline
    Earth and related Environmental sciences
    Subject
    Natural hazards and Cartography
    Audience
    Scientific
    Date
    2023
    Metadata
    Show full item record
    Description
    Alkaline magmatism is an important chemical end-member of magmatic activity that typically occurs in response to small volume melting of asthenospheric- and/or lithospheric mantle material in intra-continental settings. Understanding trace element partitioning and phase equilibria during alkaline magmatism can therefore provide constraints on intra-continental geodynamic settings. However, the partitioning of trace elements between alkaline melts and their dominant equilibrium mineral phases remains poorly constrained. Feldspathoids in particular have received limited attention with regards to their trace element contents, hampering our ability to interpret geochemical trends in alkaline magmatic systems. In this study, we performed a series of 1 atmosphere experiments in a gas-mixing furnace using a variety of highly alkaline (Na2O + K2O = 4.15 14.97 wt%) and silica-undersaturated (SiO2 = 36.73 45.96 wt%) lava compositions from Nyiragongo, Democratic Republic of Congo, in order to investigate the partitioning behaviour of trace elements in minerals from alkaline magmas. Experimental runs were performed with oxygen fugacity buffered at both QFM (quartz-fayalite-magnetite equilibrium) and QFM + 1 and cover a range of geologically-relevant temperatures (1025 1200 °C). The quenched products of these experiments contained leucite, nepheline, melilite, clinopyroxene, olivine, and rhönite crystals, of which glass-crystal pairs were analysed for rare earth elements, large-ion lithophile elements, and high-field-strength elements. Leucite and nepheline host considerable quantities of large-ion lithophile elements but take up negligible amounts of more highly charged cations. Åkermanitic melilite readily incorporates mono- to trivalent cations with a preference for light over heavy rare earth elements, but incorporates only select divalent cations. Rhönite and clinopyroxene have analogous partitioning behaviours, with a strong preference for heavy over light rare earth elements. Fractionation modelling using the reported partitioning behaviours reproduces the 2021 eruption products of Nyiragongo, with 48% fractionation from an olivine-melilitic parental melt composition. Crystallization of trace-element poor feldspathoid amplifies pre-existing high LREE/MREE ratios of the parental magma and progressively increase trace element abundances for all but monovalent cations.
    Citation
    Molendijk, S.; Namur, O.; Mason, P.R.D; Dubacq, B.; Smets, B.; Neave, D.A.; Charlier, B. (2023). Trace element partitioning in silica-undersaturated alkaline magmatic systems. , Geochimica et Cosmochimica Acta, Vol. 346, 29-53, DOI: 10.1016/j.gca.2023.01.025.
    Identifiers
    uri: https://orfeo.belnet.be/handle/internal/13721
    doi: http://dx.doi.org/10.1016/j.gca.2023.01.025
    url: https://doi.org/10.1016/j.gca.2023.01.025
    Type
    Article
    Peer-Review
    Yes
    Language
    eng
    Links
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