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

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

May 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986e%26psl..78....1p&link_type=abstract

Earth and Planetary Science Letters, Volume 78, Issue 1, p. 1-17.

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Scientific paper

The concentration and isotopic composition 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 of ``plume-type'' 3He/4 He ratios (R greater than the mean MORB value of 8 RA) show that the effect of the Icelandic mantle plume on helium isotope ratios is observable along the entire length of the Reykjanes Ridge (R = 11-16.1 RA) and to 70°N on the Kolbeinscy Ridge (R = 10.3-12 RA). These two ridges are unique among all mid-ocean ridge segments so far studied in possessing high 3He/4 He ratios along their entire length. North of 70°N, basalts associated with the Jan Mayen province (based on major element chemistry) have 3He/4 He ratios somewhat lower than the MORB average (R = 6.8-8.0 RA) and similar to other alkali-enriched provinces such as the Azores Platform. In detail, however, the 3He/4 lie distribution does not correspond to trace element variations. A maximum ratio of 16.1 RA is present near G0°N on the Reykjanes Ridge; north of this point, toward Iceland, the ratio decreases to a minimum of 12.7 RA tit G3°N. The 3He/4 He ratios increase again on Iceland, reaching a maximum in south-central Iceland. In the area between 60° and 63°N, the 3He/4 lie ratios tend to be inversely correlated with La/Sm and 87Sr/86Sr, which suggests that the isotopic heterogeneities beneath Iceland and the Reykjanes Ridge are long lived (> 108 years).
The concentration and hydrogen isotopic composition of the water in this suite of basalts display a good positive correlation with trace element 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 (0.2wt.% and -775%). 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 Molms Ridge basalts have elevated water concentrations (0.37-1.23 wt.%) and an isotopic composition (-44 to -60%v) which is unique among all MOR basalts so far studied. It remains to be seen if these high δD values represent a ``primary'' mantle water, distinct from the MORB source, or ``secondary'' water, the result of recycling during subduction.

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