Controls on magmatic degassing along the Reykjanes Ridge with implicationsfor the helium paradox

To consider the He-3 characteristics of plume-related lavas, we report a de tailed survey of helium isotope (He-3/He-4) and concentration ([He]) variat ions along an 800-km transect of the Reykjanes Ridge (RR). He-3/He-4 ratios vary from 11.0 to 17.6 R-A (where R-A = air He-3/He-4) whereas [He] ranges over three orders of magnitude from > 5 mu cm(3) STP/g-in the range of mos t mid-ocean ridge basalts (MORB) - to lows of 4 ncm(3) STP/g. The lowest [H e] and intermediate He-3/He-4 ratios occur along the northern RR (closest t o Iceland) where eruption depths are shallow (<1000 m) and water contents o f lavas are high (0.3-0.4 wt%). We suggest that low-pressure, pre-eruptive magmatic degassing is extensive in this region with degassed magmas suscept ible to addition of radiogenic helium thereby lowering He-3/He-4 ratios. Al ong the southern RR, [He] reaches maximum values, and He-3/He-4 ratios disp lay strong correlations with lead isotopes (Pb-206/Pb-204) consistent with binary mixing. These correlations indicate that the high-He-3/He-4 plume co mponent has higher absolute abundances of the primordial isotope He-3 compa red to the source of depleted MORB mantle. This finding implies that the so -called 'helium paradox' - the observation that plume-derived oceanic glass es apparently have lower He-3 contents than MORB glasses - may be an artifa ct related to considering lavas (e.g. from Loihi seamount, Hawaii) which ha ve not retained their source volatile inventory as well as those erupted al ong the southern RR. (C) 2000 Elsevier Science B.V. All rights reserved.