Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland

Abstract

The concentrolion and isotopic composi1ion of helium and hydrogen have been measured in submarine basalt glasses from the Mid-Atlantic Ridge between 52° and 73°N and in recent volcanic rocks from Iceland. Observations or “plume-type” 3 He/ 4 He ratios ( R greater than the mean MORB value of 8 R A ) show that the effect of the Icelandic mantle plume on helium isotope rn1ios is observable along the entire length of the Reykjanes Ridge ( R = 11−16.1 R A ) and to 70°N on the Kolbeinsey Ridge ( R = 10.3−12 R A ). These two ridges are unique among all mid-ocean ridge segments so for studied in possessing high 3 He/ 4 He ra1ios along their entire length. Northof 70°N, basalts associated with the Jan Mayen province (based on major element chemistry) have 3 He/ 4 He ratios somewhat lower than the MORB average ( R = 6.8−8.0 R A ) and similar to other alkali-enriched provinces such as the Azores Platform. In detail, however, the 3 He/ 4 He distribution does not correspond to trace element variations. A maximum ratio of 16.1 R A is present near 60°N on the Reykjanes Ridge: north of this point, toward Iceland, the ratio decreases to a minimum of 12.7 R A at 63°N. The 3 He/ 4 He ratios increase again on Iceland, reaching a maximum in south-central Iceland. In the area between 60° and 63°N, the 3 He/ 4 He ratios tend to be inversely correlated with La/Sm and 87 Sr/ 86 Sr, which suggests that the isotopic heterogeneities beneath Iceland and the Reykjanes Ridge are long lived (> 10 8 years). The concentration and hydrogen isotopic composition of the water in this suite of basalts display a good positive correlation with trace clement data (e.g. La/Sm). Between 61°N and 63°N on the Reykjanes Ridge, the water concentration in the basalt is high (0.22–0.41 wt.%) and its isotopic composition is enriched in deuterium (δD = −61 to −74‰; with one extreme value to −50‰), relative to MORB levels (0.2 wt.% and −77‰). South of 61 °N, both the concentration and isotopic composition of water are normal MORB values (0.11–0.25 wt.% and δD = −71 to −91‰). The Mohns Ridge basalts have elevated water concentrations (0.37–1.23 wt.%) and an isotopic composition (−44 to −60‰) which is unique among all MOR basalts so far studied. It remains to be seen if thesehigh δ D values represent a “primary” mantle water, distinct from the MORB source, or “secondary” water, the result of recycling during subduction.