Low-O 18 basalts from Iceland

Abstract

The volcanic rocks of Iceland are anomalous in their oxygen isotope content. Recent tholeiitic and transitional alkali basalts from Iceland range in (δO 18 from 1·8 to 5δ7%. Most of the tholeiitic basalts and their phenocrysts are at least 1% lower in δO 18 than unaltered basalts from other oceanic islands or oceanic ridges. The Icelandic basalts that resemble ridge basalts in δO 18 also resemble them in major element chemistry. δO 18 values of alkali olivine basalts are closest to those of other oceanic islands. Secondary alteration processes have lowered as well as raised the δO 18 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-O 18 magmas are (1) exchange of oxygen between magma and low-O 18 hydrothermally altered rock, (2) exchange with low-O 18 meteoric water, or (3) an exceptional mantle under Iceland. None of the above models can satisfactorily account for all the observations.