Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox

Hilton, DR, Thirlwall MF, Taylor RN, Murton BJ, Nichols A.  2000.  Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox. Earth and Planetary Science Letters. 183:43-50.

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basaltic liquids, carbon-dioxide solubilities, glasses, hawaii, He-4/He-3, Iceland, island basalts, loihi seamount, mantle, mantle plume, mixing, noble-gases, ocean, Pb-206/Pb-204, Reykjanes Ridge


To consider the He-3 characteristics of plume-related lavas, we report a detailed survey of helium isotope (He-3/He-4) and concentration ([He]) variations 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 most mid-ocean ridge basalts (MORB) - to lows of 4 ncm(3) STP/g. The lowest [He] and intermediate He-3/He-4 ratios occur along the northern RR (closest to Iceland) where eruption depths are shallow (<1000 m) and water contents of 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 susceptible to addition of radiogenic helium thereby lowering He-3/He-4 ratios. Along the southern RR, [He] reaches maximum values, and He-3/He-4 ratios display strong correlations with lead isotopes (Pb-206/Pb-204) consistent with binary mixing. These correlations indicate that the high-He-3/He-4 plume component has higher absolute abundances of the primordial isotope He-3 compared to the source of depleted MORB mantle. This finding implies that the so-called 'helium paradox' - the observation that plume-derived oceanic glasses apparently have lower He-3 contents than MORB glasses - may be an artifact related to considering lavas (e.g. from Loihi seamount, Hawaii) which have not retained their source volatile inventory as well as those erupted along the southern RR. (C) 2000 Elsevier Science B.V. All rights reserved.