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Hydrogen and oxygen isotope geochemistry of Ascension Island lavas and granites: variation with crystal fractionation and interaction with sea water

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Abstract

Lavas and pyroclastics on Ascension Island contain plutonic blocks that include fluid-inclusion-bearing peralkaline-granite. 18O/16O ratios, F and Cl have been analysed on whole rocks and/or minerals for lavas and granites, and D/H ratios and H2O+ for comenditic obsidians and granites. Whole rock 18O/16O ratios of fresh alkali-basalt, hawaiite, trachyandesite, trachyte and comendite range from 6.0 to 6.9‰ with 18O tending to increase with increase in SiO2. The δ 18O values of the granites are from 0.0 to 0.3‰ depleted in 18O relative to the comendites. Comenditic obsidians have δD= −80±4‰ and H2O+ ∼0.3 wt.% while amphiboles from the granites have δD= −56±2‰ The O-isotope trend of the lavas is consistent with a crystal fractionation model. Fresh igneous rocks with δ 18O values greater than 7‰ involve processes in addition to crystal fractionation of a basaltic magma. The D/H ratios and Cl contents (∼ 3,000 ppm) of the H2O-poor comenditic obsidians represent undegassed primary magmatic values. The H-isotope compositions and low H2O and Cl (167 ppm) contents of the granites are consistent with the major degassing (loss of >90% of initial H2O) of an H2Osaturated magma derived from the interaction of sea (or possibly meteoric) water with the H2O-undersaturated comenditic melt. It is proposed that, associated with caldera subsidence and stoping, water was sucked in around the residual magma before the system had time to be sealed up. The H2O-undersaturated magma consumed this H2O with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks, at a pressure of about 1.5 kb. The granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites. Granites from oceanic islands may, in general, be a result of generating an H2O-saturated acid melt by such direct or indirect crustal water-magma interaction processes.

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  1. Chris Harris

    Present address: Department of Geochemistry, University of Cape Town, 7700, Rodebosch, South Africa

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  1. Centre de Recherches Pétrographiques et Géochimiques, BP 20, 54501, Vandoeuvre-lès-Nancy, France

    Simon M. F. Sheppard & Chris Harris

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  1. Simon M. F. Sheppard
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  2. Chris Harris
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Contribution No. 603 Centre de Recherches Pétrographiques et Géochimiques

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Sheppard, S.M.F., Harris, C. Hydrogen and oxygen isotope geochemistry of Ascension Island lavas and granites: variation with crystal fractionation and interaction with sea water. Contrib Mineral Petrol 91, 74–81 (1985). https://doi.org/10.1007/BF00429429

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