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

Abstract

To consider the 3He characteristics of plume-related lavas, we report a detailed survey of helium isotope ( 3He/ 4He) and concentration ([He]) variations along an 800-km transect of the Reykjanes Ridge (RR). 3He/ 4He ratios vary from 11.0 to 17.6 R A (where R A=air 3He/ 4He) whereas [He] ranges over three orders of magnitude from >5 μcm 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 3He/ 4He 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 3He/ 4He ratios. Along the southern RR, [He] reaches maximum values, and 3He/ 4He ratios display strong correlations with lead isotopes ( 206Pb/ 204Pb) consistent with binary mixing. These correlations indicate that the high- 3He/ 4He plume component has higher absolute abundances of the primordial isotope 3He 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 3He 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.