The Statistics of Helium Isotopes Along the Global Spreading
Ridge System and the Central Limit Theorem
Don L. Anderson
Seismological Laboratory, California Institute of Technology, Pasadena
Abstract. An unbiased estimate of the arithmetic mean of the 3
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 (2) 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.
In their pioneering work, Craig and Lupton (1976) reported a `strikingly uniform' 3
ratio (R) for oceanic basalt glasses. Kurz and Jenkins (1981) obtained a `remarkably constant'
value of 7 to 9 Ra. A value of R = 8 + 1 Ra has been widely quoted as the 3