Rare gas systematics on Mid Atlantic Ridge (37–40°N)

Rare gas concentrations and isotopic compositions in 19 glasses from the North Atlantic ridge have been analyzed. Sixteen of them are located between 36.8 and 39.9°N on the Azores plateau, three others are located between 21.25 and 30.2°N and may represent normal North Atlantic Mid Ocean Ridge basalts. Helium concentrations decrease by three orders of magnitude between 37° and 38.5°N. These low He concentrations (10−8 cm3 STP/g) are attributed to degassing before eruption. The 4He/3He ratios decrease from 90 000 at 37°N to 75 000 at 38.5°N and then increase to 100 000 at 40.5°N. The low 4He/3He ratio measured on the ridge at 38.5°N indicates a plume–ridge interaction with the Terceira–Pico–São Jorge plume, which has a 4He/3He lower than 50 000 (R/Ra>15). More radiogenic 4He/3He ratios (up to 110 000) can also be found in this segment of the ridge and may indicate either in situ production of 4He by radioactive decay or atmospheric contamination of very low helium content samples. The 20Ne/22Ne ratios decrease from a mantle-like value (>12.0) between 20 and 37°N to the atmospheric ratio (9.8) at 38.5°N. The 40Ar/36Ar ratios show the same tendency as the neon isotopic ratios (atmospheric-like at 38.5°N). This may indicate contamination in shallow degassed magma chambers. However, because the maximum measured 20Ne/22Ne ratios are strongly correlated to the 206Pb/204Pb, 87Sr/86Sr and 143Nd/144Nd ratios, an alternative explanation for the atmospheric 20Ne/22Ne is the presence of an atmospheric component in the source. However, due to the large He loss (and therefore Ne and Ar), atmospheric contamination is the more probable explanation.