UNCORRECTED PROOF Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox D.R. Hilton a; *, M.F. Thirlwall b , R.N. Taylor c , B.J. Murton c , A. Nichols d a Department of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands b Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK c University of Southampton, Southampton Oceanography Centre, Empress Dock, European Way, Southampton SO14 3ZH, UK d Department of Earth Sciences, University of Bristol, Queens Road, Bristol BS8 1RJ, UK Received 12 July 2000; received in revised form 27 August 2000; accepted 27 August 2000 Abstract To consider the 3 He characteristics of plume-related lavas, we report a detailed survey of helium isotope ( 3 He/ 4 He) and concentration ([He]) variations along an 800-km transect of the Reykjanes Ridge (RR). 3 He/ 4 He ratios vary from 11.0 to 17.6 R A (where R A = air 3 He/ 4 He) whereas [He] ranges over three orders of magnitude from s 5 Wcm 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 3 He/ 4 He ratios occur along the northern RR (closest to Iceland) where eruption depths are shallow ( 6 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 3 He/ 4 He ratios. Along the southern RR, [He] reaches maximum values, and 3 He/ 4 He ratios display strong correlations with lead isotopes ( 206 Pb/ 204 Pb) consistent with binary mixing. These correlations indicate that the high- 3 He/ 4 He plume component has higher absolute abundances of the primordial isotope 3 He 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 3 He 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. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: Reykjanes Ridge; helium isotopes; lead isotopes; mantle mixing; helium paradox 1. Introduction Various authors (e.g. [1^3]) have pointed out that the 3 He content of Loihi seamount basalts, and by inference other ocean island basalts (OIB), is generally less than found in mid-ocean ridge basalts (MORB) worldwide. At ¢rst glance, this observation is curious given that Loihi seamount is characterized by high 3 He/ 4 He ratios (e.g. [4^6]) and is thought to sample `plume' helium from that part of the mantle (the lowermost mantle ?) relatively undegassed compared to the source of MORB. A number of con£icting explanations have been o¡ered to explain this apparent para- 0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII:S0012-821X(00)00253-3 * Corresponding author. Present address: Geosciences Re- search Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA.; E-mail: drhilton@ucsd.edu Earth and Planetary Science Letters 5607 (2000) 1^8 www.elsevier.com/locate/epsl