Neon isotope variations in Mid-Atlantic Ridge basalts

The isotopic composition of neon was measured for seventeen samples of submarine basalt glass from the Mid-Atlantic Ridge between 540 and 730N. They include the Reykjanes, Kolbeinsey, and Mohns Ridge segments. Neon isotopic anomalies, relative to the atmospheric ratios, exist in both 20Ne/23Ne and 21Ne/22Ne. A maximum excess 20Ne of 7% was measured in two samples from the Reykjanes Ridge. Samples with lower 20Ne excesses (six samples with sup(ddelta20)Ne between 2 and 4%) from all three ridge segments, appear to result from mixing of a mantle component with a delta20Ne of 7% and atmospheric neon. 21Ne/22Ne ratios are up to 8% above the atmospheric value, with no apparent correlation with the 20Ne excesses. The anomalies in 20Ne/22Ne are difficult to explain by mass fractionation of an atmospheric reservoir since several of the samples have delta20Ne values greater than could be produced by single-stage fractionation. Most likely, the excess 21Ne results from nuclear reactions in the mantle source, although there is no definite correlation between the delta21Ne or the excess 21Ne (cm3 STP/g) and the uranium concentration. Large variations in the observed 4He/20Ne ratio (40-12,000) remain unexplained at this time. (orig.)