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

摘要

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 (~(206)Pb/~(204)Pb) 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.