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

Abstract

New data from the Reykjanes Peninsula, Iceland, show that 3He/ 4He ratios of subglacial basaltic glasses are well correlated with CaO/Al 2O 3. High CaO/Al 2O 3 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. Low 40Ar/ 36Ar ratios occur only in samples with low δ 18O, consistent with a near-surface origin (i.e., alteration or assimilation of altered volcanics) of both the low δ 18O 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 δ 18O—is consistent with little or no alteration of these samples. These high 40Ar/ 36Ar samples have δ 18O ±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, δ 18O of the mantle source to these basalts is ±4.6‰. Low δ 18O 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.