Low-O18 basalts from Iceland |
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Authors: | Karlis Muehlenbachs Alfred T Anderson Gudmundur E Sigvaldason |
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Institution: | Geophysical Laboratory, Carnegie Institution of Washington, Washington D.C. 20008 U.S.A.;Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637 U.S.A.;Science Institute, University of Iceland, Reykjavik, Iceland |
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Abstract: | The volcanic rocks of Iceland are anomalous in their oxygen isotope content. Recent tholeiitic and transitional alkali basalts from Iceland range in (δO18 from 1·8 to 5δ7%. Most of the tholeiitic basalts and their phenocrysts are at least 1% lower in δO18 than unaltered basalts from other oceanic islands or oceanic ridges. The Icelandic basalts that resemble ridge basalts in δO18 also resemble them in major element chemistry. δO18 values of alkali olivine basalts are closest to those of other oceanic islands. Secondary alteration processes have lowered as well as raised the δO18 values of some Icelandic rocks, but such surface mechanisms cannot account for the distribution of oxygen isotopes in the Recent basalts of Iceland. Three mechanisms that could give rise to the low-O18 magmas are (1) exchange of oxygen between magma and low-O18 hydrothermally altered rock, (2) exchange with low-O18 meteoric water, or (3) an exceptional mantle under Iceland. None of the above models can satisfactorily account for all the observations. |
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