Solar neon in the Icelandic mantle: new evidence for an undegassed lower mantle

New helium and neon data from subglacial Icelandic basalts confirm that the Icelandic mantle source is relatively undegassed. The He-3/He-4 ratios var y between 14.3 and 25.6 times atmospheric (R/Ra) (He-4/He-3 between 50 500 and 28 000); helium contents vary between 0.3 and 10 mu cc STP/g. These val ues are consistent with the previously published data. The Ne-20 contents a re highly variable due to atmospheric contamination, and the Ne-20/Ne-22 ra tios vary between atmospheric (9.8) and 12.73 +/- 0.04. The most important result comes from the observed Ne-21/Ne-22 ratios, which are much lower (at a given Ne-20/Ne-22) than any other hotspot, despite the relatively low he lium isotope ratios. The unique Icelandic helium-neon systematics, as compa red to Hawaii, probably reflects different mixing processes related to the location of Iceland on the Mid Atlantic Ridge rather than in a mid-plate se tting. If the isotopic variations are caused by mixing, the Icelandic plume end member has a He-4/He-3 lower than 19 000 (R/Ra > 38) and a Ne-21/Ne-22 ratio lower than 0.035. The Ne-21/Ne-22 ratio is very close to the solar n eon isotope composition and indicates that the plume source has a very low (U+Th)Ne-22 ratio that is best explained by a relatively undegassed mantle component. (C) 2001 Elsevier Science B.V. All rights reserved.