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 ^(40)Ar/^(36)Ar ratios occur only in samples with low δ^(18)O, consistent with a near-surface origin (i.e., alteration or assimilation of altered volcanics) of both the low δ^(18)O and low ^(40)Ar/^(36)Ar in most samples. However, high ^(40)Ar/^(36)Ar ratios (up to 6500) measured in some samples—the samples that have the highest δ^(18)O—is consistent with little or no alteration of these samples. These high ^(40)Ar/^(36)Ar samples have δ^(18)O ±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^(18)O of the mantle source to these basalts is ±4.6‰. Low δ^(18)O 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.

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