Abstract
New helium and neon data from subglacial Icelandic basalts confirm that the Icelandic mantle source is relatively undegassed. The 3He/4He ratios vary between 14.3 and 25.6 times atmospheric (R/Ra) (4He/3He between 50 500 and 28 000); helium contents vary between 0.3 and 10 μcc STP/g. These values are consistent with the previously published data. The 20Ne contents are highly variable due to atmospheric contamination, and the 20Ne/22Ne ratios vary between atmospheric (9.8) and 12.73±0.04. The most important result comes from the observed 21Ne/22Ne ratios, which are much lower (at a given 20Ne/22Ne) than any other hotspot, despite the relatively low helium isotope ratios. The unique Icelandic helium-neon systematics, as compared to Hawaii, probably reflects different mixing processes related to the location of Iceland on the Mid Atlantic Ridge rather than in a mid-plate setting. If the isotopic variations are caused by mixing, the Icelandic plume end member has a 4He/3He lower than 19 000 (R/Ra>38) and a 21Ne/22Ne ratio lower than 0.035. The 21Ne/22Ne ratio is very close to the solar neon isotope composition and indicates that the plume source has a very low (U+Th)/22Ne ratio that is best explained by a relatively undegassed mantle component.
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