The Statistics of Helium Isotopes Along the Global Spreading Ridge System and the Central Limit Theorem

An unbiased estimate of the arithmetic mean of the ³He/ 4 He ratio (R) of basalts from the global spreading ridge system is 9.14 ± 3.59 Ra (n=503), where Ra is the atmospheric ratio and n is the number of data points. The ± two standard deviation (2σ) range covers much of the oceanic island and continental flood basalt data that have been attributed to ‘plume’ or ‘lower mantle’ components. The highest R values along the ridge system are associated with new ridges, backarc basins and near‐ridge seamounts. Low values are associated with long‐lived or abandoned ridges. The median, a more robust measure of the average, is 8.51 Ra.