A primitive plume neon component in MORB: The Shona ridge-anomaly, South Atlantic (51–52°S)

We report on He and Ne isotopes in basaltic glasses from eight dredge stations occupied over the Shona gravity and topographic anomaly high on the Mid-Atlantic Ridge around 51–52°S. The results indicate the presence of a primitive, little degassed, 3He-rich mantle component. 3He4He ratios correlate positively with the bathymetric and gravity anomaly, with values ranging from 12.5 to 6.4 times the atmospheric ratio of 1.38 × 10−6. The highest value is almost identical to that of the Bouvet and Réunion hotspots. Neon isotopic ratios in 20Ne22Ne vs. 21Ne22Ne isotope space indicate recent mixing between a primitive mantle plume component with near-solar Ne and the upper mantle MORB source; and again, the most solar-like Ne found over the Shona ridge-anomaly is similar to that found at the Réunion hotspot. For the first time, a negative trend pointing toward solar values is bridging the commonly observed MORB-air and the Hawaiian L-K-air mixing lines of positive slope. These results suggest that the source of the Shona ridge-anomaly has a similar origin to that of the 3He-rich Bouvet and Réunion plume sources in the deep mantle, which could define a particular noble gas signature in the South Atlantic and the Indian Ocean. It is also evident that the source of the Shona plume is distinct from the 4He-rich source of the Tristan and Gough family of plumes located further north.