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

Hilton, D.R., Thirlwall, M.F., Taylor, R.N., Murton, B.J. and Nichols, A. (2000) Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox. Earth and Planetary Science Letters, 183 (1/2), 43-50. (doi:10.1016/S0012-821X(00)00253-3).

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 RA (where RA=air 3He/4He) whereas [He] ranges over three orders of magnitude from >5 cm3 STP/g – in the range of most mid-ocean ridge basalts (MORB) – to lows of 4 ncm3 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.

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Contributors

Author: R.N. Taylor

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