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 He-3/He-4 ratio (R) of basalts from the global spreading ridge system is 9.14 +/- 3.59 Pa (n=503), where Pa is the atmospheric ratio and n is the number of data points. The +/- two standard deviation (2 sigma) 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.