The Kenya plume drives rift formation and magmatism along the Kenya Rift, yet its components remain obscured by lithospheric overprinting on mafic volcanics. We present whole-rock geochemical data for mantle-derived silicic rocks from Suswa, sampled at the reworked craton margin of the Kenya Rift where continental lithosphere profoundly influences mafic rock compositions. These rocks exhibit high SiO2 (54.9–60.4wt.%), total alkalis (Na2O + K2O = 9.3–13.8wt.%) and lithophile elements (Rb, Th-U, Nb-Ta and rare earth elements (REE); typically 10–100× chondrite and primitive mantle values), alongside low MgO (< 1.3wt.%). Their isotopic signatures show variable 87Sr/86Sr (0.7036–0.7056), depleted Nd-Hf isotopes (143Nd/144Nd = 0.51282–0.51285, 176Hf/177Hf = 0.28291–0.28293) and moderately radiogenic Pb isotopic ratios (206Pb/204Pb = 19.99–20.20, 207Pb/204Pb = 15.73–15.84 and 208Pb/204Pb = 39.82–39.97). The more depleted Nd isotopes compared to those of mafic rocks ( < ~ 0.51275) in the reworked craton margin and the northern Tanzania Craton, indicate minimal lithospheric incorporation. Low MgO contents, positive correlations of MgO with Al2O3, CaO, TiO2, P2O5, Ba, Sr and Eu, and characteristic REE and trace-element patterns support fractionation involving olivine, pyroxene, plagioclase, Ti oxide and apatite. Low Ba/Nb ratios, constant Rb/Nb and La/Nb ratios, and a negative 87Sr/86Sr–SiO2 correlation preclude significant crustal contamination in most samples. Integrated Sr-Nd-Pb-Hf isotopic and trace-element systematics, combined with regional data, reveal a multi-component mantle source: plume-derived <Emphasis Type="BoldItalic">HIMU</Emphasis> and depleted common mantle (<Emphasis Type="BoldItalic">C</Emphasis>) components, plus a lithosphere-derived <Emphasis Type="BoldItalic">EMII</Emphasis> component. Quantitative two-component mixing models, using the plume end-member (<Emphasis Type="BoldItalic">C</Emphasis> modified by <Emphasis Type="BoldItalic">HIMU</Emphasis>-derived melt) and the lithospheric mantle end-member (<Emphasis Type="BoldItalic">EMII</Emphasis> modified by <Emphasis Type="BoldItalic">HIMU</Emphasis>-derived melt), indicate 10–20% lithospheric contribution to the Suswa source. Critically, the identification of the <Emphasis Type="BoldItalic">C</Emphasis> component within these depleted rocks, supported by literature data, confirms its presence within the Kenya plume. This underscores the imperative of targeting volcanic sequences with minimal lithosphere contamination to resolve the primary plume components beneath continents.

Geochemical fingerprinting of plume in Suswa silicic volcanics, Kenya rift

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2025年11月10日

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Lu-Bing Hong,
Yu Wang,
Liang Ma,
Wei-Liang Huang,
Zhi-Jie Jia,
Jian-Bing Peng

摘要

The Kenya plume drives rift formation and magmatism along the Kenya Rift, yet its components remain obscured by lithospheric overprinting on mafic volcanics. We present whole-rock geochemical data for mantle-derived silicic rocks from Suswa, sampled at the reworked craton margin of the Kenya Rift where continental lithosphere profoundly influences mafic rock compositions. These rocks exhibit high SiO₂ (54.9–60.4wt.%), total alkalis (Na₂O + K₂O = 9.3–13.8wt.%) and lithophile elements (Rb, Th-U, Nb-Ta and rare earth elements (REE); typically 10–100× chondrite and primitive mantle values), alongside low MgO (< 1.3wt.%). Their isotopic signatures show variable ⁸⁷Sr/⁸⁶Sr (0.7036–0.7056), depleted Nd-Hf isotopes (¹⁴³Nd/¹⁴⁴Nd = 0.51282–0.51285, ¹⁷⁶Hf/¹⁷⁷Hf = 0.28291–0.28293) and moderately radiogenic Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 19.99–20.20, ²⁰⁷Pb/²⁰⁴Pb = 15.73–15.84 and ²⁰⁸Pb/²⁰⁴Pb = 39.82–39.97). The more depleted Nd isotopes compared to those of mafic rocks ( < ~ 0.51275) in the reworked craton margin and the northern Tanzania Craton, indicate minimal lithospheric incorporation. Low MgO contents, positive correlations of MgO with Al₂O₃, CaO, TiO₂, P₂O₅, Ba, Sr and Eu, and characteristic REE and trace-element patterns support fractionation involving olivine, pyroxene, plagioclase, Ti oxide and apatite. Low Ba/Nb ratios, constant Rb/Nb and La/Nb ratios, and a negative ⁸⁷Sr/⁸⁶Sr–SiO₂ correlation preclude significant crustal contamination in most samples. Integrated Sr-Nd-Pb-Hf isotopic and trace-element systematics, combined with regional data, reveal a multi-component mantle source: plume-derived HIMU and depleted common mantle (C) components, plus a lithosphere-derived EMII component. Quantitative two-component mixing models, using the plume end-member (C modified by HIMU-derived melt) and the lithospheric mantle end-member (EMII modified by HIMU-derived melt), indicate 10–20% lithospheric contribution to the Suswa source. Critically, the identification of the C component within these depleted rocks, supported by literature data, confirms its presence within the Kenya plume. This underscores the imperative of targeting volcanic sequences with minimal lithosphere contamination to resolve the primary plume components beneath continents.