Correlated helium, neon, and melt production on the super-fast spreading East Pacific Rise near 17°S

We report new helium and neon isotopic compositions in dredged basalt glasses from the superfast spreading East Pacific Rise (EPR) between 13° and 23°S. The 3He/4He ratios vary from 8.0 to 11.0 times the atmospheric value (Ra) [4He/3He between 65,700 and 90,300], with the least radiogenic values in samples from near 16.75°S. Atmospheric contamination corrections on neon isotopes are made using step heating and extrapolated 21Ne/22Ne ratios (to a solar 20Ne/22Ne value of 13.8). The lowest corrected 21Ne/22Ne ratios are found near 16.75°S, and are consistent with the helium isotopes in suggesting a less degassed mantle source. The EPR at 17°S is unusual in displaying such isotopic anomalies in the absence of any known hotspot. The minima in 4He/3He and 21Ne/22Ne correspond to extremes in Sr, Nd and Pb isotope ratios, but the length scale of the helium and neon anomalies are shorter along-axis than the peaks for the other isotopic ratios. The minimum in 4He/3He is observed from 16° to 18°S (a distance of ∼220 km), whereas the elevated Sr and Pb values, and lower 143Nd/144Nd, are observed between 16° and 20.7°S (a distance of ∼500 km); neon isotope anomalies are observed on an intermediate length scale along-axis. Unradiogenic helium and neon values correlate with low mantle Bouguer anomalies, and maxima in the axial cross-sectional area, all of which are attributed to higher melt production near 17°S. A conceptual model that fits the observations includes melting of an entrained heterogeneity beneath the EPR at 17 °S. The relationship between helium and neon isotopes and along-axis geophysical characteristics (and by inference melt distribution) is probably related to the extreme incompatibility of helium and neon. High concentrations of 3He and 21Ne (extrap) are found in the glasses near 17°S, which is consistent with higher noble gas concentrations in the anomalous mantle.