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

New data from the Reykjanes Peninsula, Iceland, show that 3He/4He 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 3He/4He likely result from addition of crustal 4He to the magma during this assimilation. Ar/36Ar ratios occur only in samples with low δ18O, consistent with a near-surface origin (i.e., alteration or assimilation of altered volcanics) of both the low δ18O and low 40Ar/36Ar in most samples. However, high 40Ar/36Ar ratios (up to 6500) measured in some samples—the samples that have the highest δ18O—is consistent with little or no alteration of these samples. These high 40Ar/36Ar samples have δ18O ±4.6‰, significantly lower than that of the MORB source region (5.2‰). It seems likely that, while alteration by meteoric fluids does influence oxygen isotopic composition, δ18O of the mantle source to these basalts is ±4.6‰. Low δ18O in the Icelandic mantle source could be the result of recycling lithospheric mantle, although this is not likely the source of high 3He/4He ratios in Iceland.