Ar-40/ar-39 Geochronology Of Subaerial Ascension Island And A Re-evaluation Of The Temporal Progression Of Basaltic To Rhyolitic Volcanism

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Journal Of Petrology




Ar-40/Ar-39 geochronology of basaltic to rhyolitic lavas, domes, and pyroclastic deposits from Ascension Island indicates that the maximum age of subaerially exposed samples is 1094 ka. Thirty-eight Ar-40/Ar-39 ages, coupled with new and existing geochemical data, constrain the eruptive histories of the four distinct mafic magma types (high Zr/Nb, low Zr/Nb, intermediate Zr/Nb, and Dark Slope Crater) and document temporal variations in magma sources. Lavas from the eastern felsic complex, previously assumed to be as old as or slightly younger than the 602-1094 ka Middleton Ridge complex, are as young as 52 ka. Basaltic to benmoreitic scoria cones and associated flows of the intermediate Zr/Nb magma type have been inferred to be the most recent eruptive products, yet their eruptive histories extend back to 705 ka. These intermediate Zr/Nb magmas are likely to be parental to the abundant trachytic to rhyolitic lavas and domes, which contradicts previous interpretations that call upon a high Zr/Nb parental basalt. Two distinct fractionation trends are observed in the trace element variations of Ascension trachytes and rhyolites. Ar-40/Ar-39 ages of the samples defining the two trends suggest that ilmenite fractionation dominated the Nb budget and thus controlled Zr/Nb ratios in early (> 931 ka) Ascension evolved magmas, whereas zircon-titanite fractionation was predominant in younger felsic magmas. The eruptive sequence and compositions of the subaerial lavas and domes at Ascension Island are unique in comparison with other ocean island volcanoes because of its on-axis location and eruptions of high-SiO2 trachyte and rhyolite.