Argon isotope constraints on modification of oxygen isotopes in Iceland Basalts by surficial processes

New data from the Reykjanes Peninsula, Iceland, show that He-3/He-4 ratios of subglacial basaltic glasses are well correlated with CaO/Al2O3. High CaO/Al2O3 ratios are likely due to assimilation of lower crustal clinopyroxene and the large variations in He-3/He-4 likely result from addition of crustal He-4 to the magma during this assimilation. Low Ar-40/Ar-36 ratios occur only in samples with low delta O-18, consistent with a near-surface origin (i.e., alteration or assimilation of altered volcanics) of both the low delta O-18 and low Ar-40/Ar-36 in most samples. However, high Ar-40/Ar-36 ratios (up to 6500) measured in some samples-the samples that have the highest delta O-18-is consistent with little or no alteration of these samples. These high Ar-40/Ar-36 samples have delta O-18 +/- 4.6 parts per thousand, significantly lower than that of the MORB source region (5.2 parts per thousand). It seems likely that, while alteration by meteoric fluids does influence oxygen isotopic composition, delta O-18 of the mantle source to these basalts is +4.6 parts per thousand. Low delta O-18 in the Icelandic mantle source could be the result of recycling lithospheric mantle, although this is not likely the source of high He-3/He-4 ratios in Iceland. (c) 2004 Elsevier B.V. All rights reserved.